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Information for users ofMETTLER TOLEDO Titration and pH Systems,Density Meters and Refractometers

Benefits of fully automating the titrationprocess

In volumetric analysis, the operator has to perform various steps. Mostautomatic titration automation solutions cover only some of thesesteps, such as the titration itself and the changing of samples. In orderto achieve a significant gain in productivity, it is also important toautomate procedures like sample preparation. Automating the whole

chain of steps makes unattended automatic analysis possible.

H.-J. MuhrContent

DL-Tip• Benefits of fully automating the

titration process 1• Optimum parameters for the

titration III: Termination of the titrationand the choice of thecorrect evaluation procedure 8

Applications• The new Rondo 60 Sample Changer:

Three selected applications 3• X-matePro in the field 6

News• Rondo 60:

A flexible system that cangrow with the users needs 10

• Customer Courses: Hands-ontraining in Switzerland 11

Tips and Tricks• Powerful DL5x/DL7x calculation

formulas - some examples 12• pH electrode troubleshooting 14• Titration applications on the internet 18

The analysis processThe analysis process in titration in-volves various steps of differing com-plexity. At the beginning of an analy-sis, the accurate weighing of a solid orthe precise measurement of the volumeof a liquid sample is crucial for the ac-curate determination of the samplecontent. The sample then has to beproperly prepared, i.e. transformedphysically or chemically into a statesuitable for analysis. It is then trans-ferred to the titrator where the volu-metric determination of the samplecontent takes place. The sample reactswith appropriate reagents and under-goes a fast, single, complete and sto-ichiometric chemical reaction underthe given conditions. Finally, the re-sult, i.e. the content is calculated,documented, and stored.

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Automation of titrationanalysis: better reproducibilityAlmost all the steps of the analysis pro-cess can be automated, i.e. the stepscan be defined as a sequence of indi-vidual operations (titration method)that are performed independently with-out manual intervention. Usually, thesample preparation performed by a ti-trator consists of dissolving the sampleor allowing a chemical reaction to takeplace by stirring at a predefined speedfor a fixed period of time. Afterward,the pH and temperature of the samplesolution can be measured and docu-mented, or used to perform a pre-defined adjustment to the values nec-essary for the titration, e.g. a pH valueadjustment. The titrator controls theentire titration, including the termina-tion of the analysis and the evaluationof the results. In addition, calcula-tion(s) and the documentation, trans-fer and storage of data are done auto-matically. An automated titrator alsoprovides for the activation of externaldevices or auxiliary instruments likepumps, an immersion heater, samplingunits and, of course, a sample changer.

The benefits of automated titration overmanual titration are obvious:• The analysis procedure becomes

much less operator dependent, therebyincreasing the reproducibility.

• Each sample is titrated in exactly thesame way because the control pa-rameters of the titration defined inthe method do not change. This en-hances the repeatability.

• The minimum increment size ismuch smaller than in a manual ti-tration. This leads to a higher reso-lution of the titration curve, therebyimproving interpolation between in-crements. This increases the accu-racy of the results.

• Errors in calculations and documen-tation are minimized.

Automating an automatictitration: the titrator/samplechanger combinationDo the benefits gained by using a ti-trator really save time and allow theuser to do other tasks or analyze moresamples? Do these benefits help himbecome more productive? A closer com-parison shows that a titration per-

formed using a titrator does not neces-sarily take less time than an experi-enced analyst performing the same ti-tration manually. Furthermore, thetime required for the titration is usu-ally not long enough for him to per-form other tasks at the same time. Inaddition, chemists like to observe theprogress of a chemical reaction, i.e.they tend to watch the titrator duringthe titration process. In summary, atitrator definitely increases the preci-sion, accuracy and handling of an in-dividual analysis, but the gain in pro-ductivity is still rather small.The first step toward the complete au-tomation of titration is achieved byconnecting the titrator to a samplechanger. The titrator controls thesample changer via the titrationmethod. This defines all the informa-tion necessary to perform the desiredoperations, e.g. go to next sample, rinsethe sensor, pump an auxiliary reagentor an additional solvent to dilute thesample. The sample changer performs,step by step, all the operations neces-sary for the analysis of the samples onthe tray.

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The new Rondo 60 sample changer:Three selected applications

The new Rondo 60 sample changer is based on a very modular concept - the basicunit can be expanded with a second titration tower, pumps and external devices suchas high speed homogenizers in order to meet the various requirements for developinga titration method. A second titration tower can also be used to perform samplepreparation prior to titration, or for the measurement of multiple parameters that

necessitate a separation of the sensors (e.g. conductivity and pH value in water analysis). Threeapplications are presented below to illustrate the Rondo 60’s flexibility:

Determination of acid and baseNumberAN (Acid Number) and BN (Base Num-ber) determinations are importantanalyses in refineries, petrochemicalindustries, and in power stations. TheAN is also known as the neutraliza-tion number. It is a measure of thequantity of KOH necessary to neutral-ize the acidic constituents in the oil. Itis used to assess the state of gear, hy-draulic or rotary oils. On the otherhand, the BN is an indication of the

concentration of constituents with al-kaline effects (e.g. additives) that canneutralize acid forming impurities.

The exact determination of AN/BN iscrucial for manufacturers and usersof mineral oils and derived productsbecause the presence of acidic constitu-ents can corrode and damage the pro-duction facilities. This can be moni-tored in advance by the AN determina-tion.

200

-200

0

mV

Buffer A

Buffer B

mL

Blank AHCl

Blank BKOH

Solvent

SAN

AN

Sample

Sample

ANAN

AN

Fig. 1: The SAN/AN determination principle.

C. A. De Caro

The analytical procedure is not onlylimited to the titration of the sample.It also consists of sampling and ap-propriate sample preparation beforetitration. After sampling, i.e. collect-ing a representative bulk samplefrom the material to be analyzed, asmaller homogeneous laboratorysample is prepared, e.g. by grindingand dissolving a powder in an ap-propriate solvent (sample prepara-tion). From this, small test portions(aliquots) are transferred to the ti-tration beakers that are then placedon the sample changer tray for indi-vidual analysis.

If sample preparation and sample trans-port are also automated, a number ofdecisive benefits are gained. A samplechanger should be able to do more thanjust “change samples”. Other importanttasks are for example preparing samples,switching external devices, delivering orremoving solvents, cleaning and condi-tioning sensors. In other words, automat-ing a titrator with a sample changer en-ables large batches of samples to be ana-lyzed in unattended operation, and theanalyst is free to do other jobs. The newMETTLER TOLEDO Rondo 60 samplechanger allows the titration analysis tobe completely automated.

The costs of such a system are certainlyhigher than an automated solution with atitrator. However, even a rather conservativecalculation for a titrator/sample changercombination that assumes a time-saving persample of 10-15% at a sample throughputof 20 samples per day proves that the invest-ment pays for itself in less than a year.

In summary, a titrator/sample changer com-bination leads to a large increase in produc-tivity because the periods of unattended op-eration are longer. The analyst can even per-form analyses overnight. The pay back pe-riod is less than a year, even at a moderatesample throughput of 20 samples per day.

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The importance of these analysis tech-niques is evident in the multitude ofnational and international standardsand regulations, e.g. ASTM.

Figure 2 illustrates the principle andbasic steps of an AN analysis:

These steps can be completely auto-mated with a Rondo 60 and a DL7x ti-trator. The methods used to determinethe buffer values, the blank, and theSAN/AN numbers are defined in the ti-trator and run as a list of methods. If-then conditions of the DL7x allow theTitrator to decide whether an analysishas to be stopped in case the electrodeis not in a suitable condition.

Acid Number(ASTM D 664/95)In this analysis, appropriate cleaningof the sensors is a crucial for achiev-ing accurate and reproducible results.Thanks to the magnetic coding of theRondo 60, one or more conditioningbeakers can be defined to rinse andcondition the electrode thoroughly be-fore the next sample. Rinsing is per-formed by additional pumps in com-bination with a new rinsing system,called Power Shower™ that allows thesensor to be cleaned from top to bot-tom.

Automation of water analysisIn water analysis, several parameterssuch as conductivity, pH, alkalinity,chloride content, calcium and mag-nesium content have to be individu-ally determined. The measurementof conductivity is performed sepa-rately from the determination ofother parameters because of the in-terference of the electrolyte flowfrom the electrodes. This can bedone using a Rondo 60 samplechanger with two titration towers (Aand B see Fig. 2 on page 10) posi-tioned next to each other. The con-ductivity is measured at Tower B af-ter which the sample is moved toTower A, where the pH, alkalinity, to-tal hardness, and calcium and mag-nesium content are determined:

1) Conductivity (Tower B)The conductivity of tap water is mea-sured for example with an InLab730Conductivity Sensor mounted on thetitration stand of Tower A using anMPC227 Conductimeter. The mea-surement range of the MPC227 is setto 200 µS/cm. The analog output ofthe MPC227 conductivity meter isconnected to sensor input 4 of a

DL70ES or DL77 ti trator. TheMPC227 is first calibrated (twopoints, air and 1413 µS/cm stan-dard), and then the titrator (onepoint, 1413µS/cm). The slope isstored as an auxiliary value, whilethe offset of the calibration line canbe set to 0 because it is less than theresolution of the titrator (0.1 mV).

2) pH, alkalinity (Tower A)A DG114 electrode (electrolyte:2 M KNO

3) is used as the indication

electrode, and the sample is titratedwith 0.1 M HNO3 to fixed endpointsEP = pH 8.2 or pH 4.3. The electrodeis connected to sensor input 3 of theDL7x titrator.

3) Chloride determination (Tower A)The chloride content is determined bytitration with 0.1M AgNO

3, and indica-

tion using a DM141 electrode (con-nected to sensor input 2).

4) Calcium and magnesium determi-nation (Tower A)Calcium and magnesium are deter-mined separately by complexometrictitration with 0.01 M EDTA (0.1 M)and a DX240 calcium ion-selectiveelectrode DX240 (connected to sen-sor input 1). Before starting the ti-tration, a buffer solution of acetyl-acetone/TRIS (pH 8.5) is added tothe sample to enable the two equiva-lence points to be distinguished (seeApplication Brochure No. 1, ME-724492). The reference half-cell ofthe DG114 is also used as a referencefor the DX240 using the cable ME-51’089’954. This cable allows thereference half-cell of the DG114 tobe connected to the reference inputat sensor input 1 of the DL7x titra-tor by a conventional 4 mm banana-banana cable.

1. Standardization of the titrantKOH (i-PrOH), c = 0.1 mol/L

with potasium hydrogen phthalate

Standardization of the titrantHCl (i-PrOH), c = 0.1 mol/L

with standardized KOH

Measurement of buffer Aacidic buffer, E ≈ +200 mV

Measurement of buffer Balkaline buffer, E ≈ -200 mV

The difference between the 2 poten-tials must be at least 300 mV,

otherwise check electrode!

Blank determination SANConsumption of HCl up to buffer A

Blank determination ANConsumption of KOH down to buffer B

Exact determination of AN

2.

3.

4.

5.

6.

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Fig. 2: Automated Karl Fischer titration by Rondo 60. The special Karl Fischer beakers are clearly visible on the tray.The blue rubber ring is sealing the glass beaker covered by foil.

Automated water determinationby Karl Fischer titrationWhile a second titration tower increasesthe application possibilities of thesample changer, the flexibility can bestill further improved using differenttitration beakers. For instance, the KarlFischer titration for the determinationof water can also be automated bymeans of special beakers and a specialtitration set (see Fig. 2).The Karl Fischer set is based on a verysimple but effective idea - the beakeris sealed with foil and a rubber sealingring. This almost completely elimi-nates the penetration of ambient mois-ture into the titration beaker, whichwould have an adverse affect on the re-sults. The foil is pierced by the elec-trode at the start of the titration.

ConclusionsAutomation has become a very impor-tant part of modern titration. Not onlycan a quantitative analysis be auto-mated by microprocessor-controlledtitrators, but sampling and samplepreparation can also reach a high de-gree of automation with the use ofmodern sample changers of modulardesign. This results in much enhancedflexibility, and means that samplechangers can satisfy the requirementsfor automation for a very wide rangeof different applications.

Together with the METTLER TOLEDODL5x and DL7x series titrators, theRondo 60 is a very flexible, easy-to-usesystem that is ideal for most automa-tion tasks. The modular concept of the

Rondo 60 allows the complete automa-tion of the titration procedure, therebyenabling a high sample throughput tobe achieved.

This frees lab personnel from rou-tine work. The measurements can beperformed more rapidly and more re-liably because operator errors arelargely eliminated, and contact withhazardous chemicals is reduced to aminimum. Unattended operation isalso possible, allowing samples to beanalyzed around the clock by shiftoperators. The longer periods of un-attended operation means that theoperator can now perform othermore demanding tasks, which inturn increases the productivity of theanalytical laboratory.

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X-matePro in the fieldWater - the most precious resource of the 21st century!

Water is essential to all life on earth. Human beings in par-ticular need water that is readily available, not only for con-sumption but also for agriculture and industry. The avail-ability of potable water is, however, limited. As the world’spopulation increases, more and more water is polluted everyyear, but at the same time the need for clean drinking waterincreases. The result is that the regeneration cycle of wateris shortened. Water monitoring has therefore become es-sential. Since water control plays such an important role, itis absolutely essential to have equipment that meets all thespecific measuring requirements.Rivers represent an important part of the regeneration cycle.Because they flow above ground and usually pass over natu-ral or artificial steps and rocks, they contain a lot of dis-solved oxygen. The processes of nitrification and decompo-sition of carbonaceous compounds are therefore enhanced.The concentration of dissolved oxygen and the temperatureof a river are good indications of the river’s regenerationcapacity. Other parameters such as pH, conductivity andoxidation/reduction potential (ORP) are more direct indi-cations of the actual state of a river. This also applies toion- selective measurements such as nitrate measurements.With the new METTLER TOLEDO X-matePro, all these param-eters can be measured on-site within a very short time frame.Its modular design enables a complete and fast spot check tobe made even in systems with rapid fluctuations.

Fig. 1: The River Toess flows through open fields

The River ToessSuch a multiparameter spot check was done along a sec-tion of the river Toess in July of this year. The Toess is asmall river in Switzerland that flows into the River Rhine.It begins at about 1000 m above sea level and flows throughopen fields in a fairly wide valley. Further downstream, theriver is largely covered by trees until an altitude of approxi-mately 490 m is reached where it flows through a forest. Itthen enters a densely populated region before finally join-ing the Rhine at an altitude of about 400 m above sea level.

The parameters shown in Table 1 were measured with theX-matePro. For the purpose of our study, readings were takenat three points along the River Toess, beginning at 700 mand ending at 490 m. All measurements had to be takenwithin a time frame of one hour for the results to be mean-ingful.

The Toess was measured at three points. Automatic tem-perature compensation (ATC) was used for the pH measure-ments. The pH values listed below therefore reflect the ac-tual pH value of the water. The conductivity values werealso measured using ATC. The values were referenced to atemperature of 25 °C. The ion-selective measurements wereperformed using the same temperature as for the pH read-ings. These temperature values were entered manually(MTC).

Fig. 2: Measuring points along the River Toess

K. Sägesser

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A p p l i c a t i o n s

Fig. 4: The modular design of the X-matePro is very convincing.

Fig. 3: The author taking measurements. Note the X-mate field casein the foreground.

Parameter Unit Point 1 Point 2 Point 3

pH (ATC) 7.94 8.23 8.03

Conductivity (ref. 25 °C) µS/cm 428 443 524

Dissolved oxygen % 58.7 81.2 79.2

Ammonium (MTC) ppm 0.6 8.5 2.6

Nitrate (MTC) ppm 8.7 7.4 11.4

ORP mV 251.5 283.1 286.7

Water temperature °C 18.8 18.3 17.2

Air temperature °C 26.8 26.7 24.5

Relative humidity % 47.9 55.9 64.1Table 1. Parameter readings along the River Toess

The pH value does not change signifi-cantly over the stretch of the river ana-lyzed. This is a rough indication thatthe composition of the water does notundergo major changes, e.g. due to theinflow of industrial wastewater.The conductivity value represents thetotal ion content of the water. Since saltcannot be eliminated in purificationplants, it accumulates along the courseof the river.Wherever the river flows over artificialsteps and natural rocks, the percent-age of dissolved oxygen immediatelyincreases, enabling the river to regen-erate faster.

This water contains very little ammo-nium because the elimination processof the purification plant is very efficient.Technically, nitrate is not so easy toeliminate as ammonium in a waterpurification processs. The concentra-tion of nitrate at point 3 is thereforehigher.

All readings were performed using anX-matePro with interchangeable mod-ules. Conductivity was measured usingan InLab®781, pH with an InLab®482,dissolved oxygen with an InLab®681,ORP with an InLab®581 and humiditywith an InLab®986. A BNC module wasused for the ion-selective measure-ments. These components all fit in ahandy portable field case (see Fig. 3),making it easy for a person to measureall the values in a short period of time.The analyses showed that the RiverToess is a very clean river even thoughit flows through populated areas.

Where the river flows through the for-est (measuring point 3), the air tem-perature decreases while the relativehumidity increases.The ammonium values are very low inthe upper part of the river since thereis hardly any human influence. In thelower part of the river between mea-suring points 2 and 3, there is an in-flux of water from a purification plant.

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Optimum parameters for the titration III: Termination ofthe titration and the choice of the correct evaluationprocedure

So far in our series on titration parameters, we have reached the stage of titrating withthe correct control parameters resulting in the best shape curve, and ensuring thecorrect recognition of the equivalence point. What remains is the termination criteria ofthe titration and evaluation of the equivalence point.

Termination of the titrationA powerful feature of the METTLERTOLEDO general-purpose titrators istheir ability to use various terminationcriteria:• to stop as soon as the required num-

ber of equivalence points have beendetected,

• to stop when a fixed potential or pHhas been reached,

• to stop only when the slope of thecurve has dropped below a givenvalue,

• to stop when the total titrant con-sumption reaches a certain factortimes the sample size (or % of thenominal consumption).

In addition, there is one further “emer-gency stop” criterion, namely whenmaximum volume is reached. This isto prevent both overfilling the titrationbeaker and wasting titrant.

These termination criteria can be usedin combination or as single criteria. Ifmultiple criteria are chosen, the titra-tion will be terminated as soon as thefirst condition is reached. By activat-ing the “Combined Termination Cri-teria” parameter, the titration will onlybe terminated once all of the selectedconditions (excluding “Maximum Vol-

ume”) are met. This is particularly use-ful when one would like to stop imme-diately after the first equivalence pointbut would still like to see the overallcurve shape.For example, assume that you havechosen to stop after one equivalencepoint (Fig. 1). By combining this withtermination at slope a better overviewof the titration curve (Fig. 2) is obtained.With the above combination it is also

possible to titrate using “Steepest jumponly” (see UserCom 4) without havingto titrate to a maximum volume. Todo this, “Steepest jump only” and com-bined termination criteria of “Stop af-ter n EQPs” together with “Terminateat slope” are chosen.

Evaluation of the titration curveThe choice of the correct evaluationprocedure depends primarily on theshape of the titration curve. For theclassical S-shaped curve, many proce-dures have been described to determinethe equivalence point. These includeapproximation, interpolation andmathematical procedures such as non-linear regression and curve fitting. Theapproximation and interpolation pro-

cedures do not take the nature of thetitration reaction and indication intoaccount, and therefore do not deter-mine the “true” equivalence point butonly an approximation thereof. Withthe “Standard” evaluation procedurein the METTLER TOLEDO titrators, a

Fig. 1: Termination after 1st equivalence point Fig. 2: Combined termination criteria

C. Gordon

E

V

EQP

E

V

EQP

Slope

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nonlinear regression procedure is usedthat yields the “true” equivalencepoint. This procedure is ideal for acid/base, argentometric, complexometricand some REDOX titrations.

When curves are extremely asymmet-ric, the above procedures no longeryield the best evaluation of the equiva-lence point. This is true for someREDOX titrations with very suddenbreaks, as well as for some photo-metrically indicated titrations. It hasbeen found empirically, that in suchcases, the true equivalence point liesbetween the inflection point and thesection of the curve with the greatestcurvature (smallest radius ofcurvature). Here, it is necessary to usean approximation procedure. One suchmethod is the Tubbs procedure, wherecircles of radii to match the curvatureof the two sections are drawn. The

Fig. 3: Tubbs procedure

intersection of the line joining the centersof the circles with the titration curve istaken as the equivalence point (Fig. 3).

By choosing the “Asymmetric”evaluation procedure in the METTLERTOLEDO titrators, a mathematicalvariation of the Tubbs procedure(according to Ebel) is used. Thisprocedure uses hyperbolae rather thancircles and joins the foci rather thanthe centers of the circles. For thisprocedure to work, the titrator needsat least six measured points in theregions of greatest curvature bothbefore and after the inflection point. Ifinsufficient data points are present, thetitrator will switch back to a standardevaluation automatically and will notethis in the printout of the raw results.

The last two types of curves that theMETTLER TOLEDO titrators are able to

Fig. 4: Segmented Evaluation Fig. 5: Min/Max Evaluation

evaluate are both special cases due tothe indication technique used. The firstis the “Segmented” evaluation of typi-cal conductimetric curves (Fig. 4) andthe other is the “Min/max” evaluationof turbidimetric curves (Fig. 5).

Lastly, the METTLER TOLEDO titratorsallow the determination of two inter-polated endpoints in addition to anyequivalence points. These are specifiedas “Buffer potential 1” and “Buffer po-tential 2”, with raw results QP1 andQP2.

All that is left to do in order to obtainthe final result is to enter or select thecalculation to use, and to specify exactlywhat should be sent to a printer or com-puter. This is covered in detail in therespective titrator operating instruc-tions. Some tips on using advanced for-mula are given in this UserCom.

E

V

EQP E

V

EQP

E

V

EQP

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Rondo 60: a flexible system that can growwith the user’s needs

METTLER TOLEDO recently introduced the Rondo 60 samplechanger. Combined with a METTLER TOLEDO Titrator of the DL5xor DL7x series, the Rondo 60 allows the titration of up to 60samples without any user interaction. The most impressivefeature of the Rondo 60 is its great flexibility. Starting from astandard configuration, the user can build the system that bestfits his needs.To learn more on the application possibilities of the Rondo 60system, see pages 1 to 5 of this UserCom. The following articlegives an overview of the various Rondo 60 accessories.

The Rondo 60 can be equipped with asecond titration tower. If the secondtower is placed opposite the first one(Fig. 1), the titration throughput canbe doubled with a DL77 titrator. If thetowers are placed side by side (Fig. 2),the user has more possibilities forsample preparation and can performconductivity measurements and titra-tions at the same time. The towers canbe placed on the left or the right sideof the instrument depending on theuser’s preference and the availableworkspace.

The Rondo 60 can be used with dif-ferent sample racks, thereby allowingthe measurement of different samplesizes. Apart from the rack for 20 stan-dard METTLER TOLEDO beakers, rackscan be added that allow the use of stan-dard 150 ml, 250 ml and 400 ml glassbeakers or any other beakers that fit(Fig. 3). Together with the optionalSU24 sampling unit and the test tuberack, up to 60 samples can be analyzedautomatically.For sample preparation, a secondmembrane pump can be attached di-rectly to the back of the tower. If a peri-staltic pump is required, the METTLERTOLEDO SP250 can be connected. Eachtower can be equipped with 1 or 2pumps.

Fig. 3: 12, 15, 20 and 60 position racks forRondo 60.

Fig. 4: SP250 peristaltic pump

Fig. 5: SU24 sampling unit

Fig.1: Rondo 60 with two towers for parallel titration

Fig. 2: Rondo 60 with two towers for sample preparation andcombined measurements.

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For moisture determination, the Karl Fischer Set is addedto the standard Rondo 60 configuration (Fig. 6).In both cases, the instrument can be changed back to astandard instrument within minutes.

With the COD set, the Rondo 60 is easily converted into aspecialist system for COD titrations (Fig. 7).

If aggressive chemicals are used, the Rondo 60 can be combined with the CoverUp™ Lid handling system. Becausesample beakers are covered with a lid before and after titration, it is no longer necessary to perform titrations in a fumehood.

Customer Courses: Hands-on training in Switzerland

As one of our users you are in possession of a highly versatile instrument that allows you toperform the most complicated applications simply by pressing the RUN key. Do you constantlydevelop new methods, adapt parameters and calculations to suit your needs, know the pros andcons of all the titration methods and indication techniques - are you in command?

... or do you get the feeling that the instrument is sometimes doing what it wants?

• And have you ever wondered how you could get the titrator to perform other analyses?• Do you want to know the ultimate truth about optimal parameter settings?• In short: are you interested in learning more about the theory of titration and the use of our

titrators?

Then why not visit one of our customer courses and spend 4 to 5 days of intensive titration trainingwith our specialists.For more information and registration please contact your local METTLER TOLEDOorganisation.

Fig. 6: Rondo equipped with KF set Fig. 7: Rondo equipped with COD set

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T i p s a n d T r i c k s

Powerful DL5x/DL7x calculation formulas - someexamples

METTLER TOLEDO titrators have extremely versatile calculation func-tions. The flexible formulas allow much more than just simple calcula-tions. Three more advanced examples that demonstrate the power ofthese calculation functions are presented below. Basic information onthe use of formulas is given in the titrator operating instructions.

1. Using calculations to select the correctequivalence pointIn mixtures of two or three different acids or in the case ofpolyprotic acids such as phosphoric acid, the selection ofthe correct equivalence is important for the calculation ofthe content of each of the constituents. For example, in or-der to determine the content of three different acids of in-creasing pKa values, a method is used that ensures the un-ambiguous assignment of the equivalence points to each ofthe acids, and that also takes into account the fact that aconstituent might sometimes be missing. In this case, thetitration is performed to a predefined maximum volume orpH, and the contents are calculated using the consumptionidentified by the condition:

R1 = Q(3.0<EPOT<6.0) • C/m (or U for DL7x)

In this way, the consumption to the equivalence point whosepotential lies between pH 3 and 6 is used to calculate thecontent. For the second acid with a higher pKa, anotherrange at higher pH values is used and so on.

2. Using the calculations to calculate statisticsin periodic analysisIn a water laboratory, the m-value (methyl orange value,pH 4.3) of a water sample is determined. Three samples aretitrated every hour (8 hours per day) with daily statistics.To do this, a method is run on a DL7x/Rondo combinationwith one sample using “List continuous”. Because of this,the programmed statistic function cannot be used and themethod has to contain the formulas for the rolling meanand standard deviation in the calculation functions:

H.-J. Muhr

xi

= result of a samplex = mean valuen = number of samples

Since the mean value changes aftereach sample, it is not possible to usethe standard equation without storingevery result up to the point where therolling mean and standard deviationhave to be calculated.Rearrangement of the above equationyields the following:

xi = result of sample ix = mean valuen = number of samples

In the new equation, the mean valueis separated from the sum so that thesum of the squares can be stored sepa-rately from the mean value, as an aux-iliary value. This enables rolling sta-tistics to be calculated.

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T i p s a n d T r i c k s

TitleSampleAuxiliary value ID text . . . . . . . . . . . . . . .Counter Formula . . . . . . . . . . . . . . H1=H1+1(counts the number of samples)StirTitrationCalculation Result name . . . . . . . . . . . . m-Value Formula . . . . . . . . . . . . . . R=QP1*C/U Constant . . . . . . . . . . . . . . C=1000 Result unit . . . . . . . . . . . . mmol/L Decimal places . . . . . . . . . . . 2Auxiliary value ID text . . . . . . . . . . . . . . .Sum m-Value Formula . . . . . . . . . . . . . . H2=H2+R1 (∑x

i)

Auxiliary value ID text . . . . . . . . . . . . . . .Sum m-Value squared Formula . . . . . . . . . . . . . . H3=H3+R1*R1 (∑x

i2)

Calculation Result name . . . . . . . . . . . . Mean m-Value Formula . . . . . . . . . . . . . . R2=H2/H1 (∑x

i/n)

Constant . . . . . . . . . . . . . . Result unit . . . . . . . . . . . . mmol/L Decimal places . . . . . . . . . . . 2Calculation Result name . . . . . . . . . . . . s (m-Value) Formula . . . . . . . . . . . . . . R3=sr((H3-C2)/(H1-1)) Constant . . . . . . . . . . . . . . C2=H1*R2*R2 Result unit . . . . . . . . . . . . mmol/L Decimal places . . . . . . . . . . . 5Calculation Result name . . . . . . . . . . . . %RSD m-Value Formula . . . . . . . . . . . . . . R4=R3*100/R2 Constant . . . . . . . . . . . . . . Result unit . . . . . . . . . . . . % Decimal places . . . . . . . . . . . 3RecordAuxiliary value ID text . . . . . . . . . . . . . . .Reset Sum Formula . . . . . . . . . . . . . . H2=0 Condition . . . . . . . . . . . . . .Yes Condition. . . . . . . . . . . . .H1=24 (3 samples per hour for 8 hours)Auxiliary value ID text . . . . . . . . . . . . . . .Reset n Formula . . . . . . . . . . . . . . H1=0 Condition . . . . . . . . . . . . . .Yes Condition. . . . . . . . . . . . .H1=24 (3 samples per hour for 8 hours)Auxiliary value ID text . . . . . . . . . . . . . . .Reset Sum squares Formula . . . . . . . . . . . . . . H3=0 Condition . . . . . . . . . . . . . .Yes Condition. . . . . . . . . . . . .H1=24 (3 samples per hour for 8 hours)

A method for the determination of the m-value

14

T i p s a n d T r i c k s

The method shows the relevant functions for the calcula-tion of rolling statistics. An auxiliary value function is in-troduced before the actual titration to serve as a samplecounter. After each sample, the rolling mean value and thestatistics are documented. After 8 hours or 24 samples, thecounter and the sums are reset to zero, thus reinitializingthe statistics. The process is controlled by three auxiliaryvalue functions at the end of the method that are only ex-ecuted when the condition “n = 24” is fulfilled.

3. The use of log and anti-log functionsto calculate results in direct measurementwith an ISEThe calibration of a pH or ISE electrode is a logarithmicfunction where the pX value (-log[X]) is plotted againstthe measured mV signal. When performing a direct mea-surement with an ISE, the mV signal is automatically con-verted into a pX value by the titrator. In order to convert thepX value to the desired unit, it is necessary to use an anti-

pH electrode troubleshooting

The electrode is the key to successful analysis. Since the electrode is the only part ofthe analytical instrument that is in direct contact with the sample, its selection andmaintenance (and therefore sensitivity) has the strongest influence on precise andaccurate measurements. However, an electrode that has been correctly selected andthat has been working properly may nevertheless suddenly start performing badly. Inthis article we will help you to identify possible reasons for this and suggest a numberof procedures for restoring the electrode to its original performance.

Before beginning the diagnostic procedureBefore testing an electrode, make sure that the electrodecable and the instrument are working properly. Then ex-amine the sensor closely. Visual inspection can very oftenprovide important clues about the cause of the problem, e.g. aclogged diaphragm or an air bubble in the tip of the electrode.

Glass membraneDiagnosisOne symptom may very often have various causes. The fol-lowing table will help you to find out what caused yourelectrode to fail with regard to the pH-sensitive glass mem-brane:

log function. METTLER TOLEDO titrators include this anti-log function, denoted by “pw”, resulting in the calculationgiven below:

Name Content ISER = pw(-E) • CC = 1000 (if the standards concentration was given in g/L)Unit = ppm

These three examples demonstrate the versatility of the cal-culation functions in METTLER TOLEDO titrators, enablingadvanced calculations that involve square roots, logarith-mic or exponential functions, etc.to be made. These specialfunctions are available in titrators of both the DL5x andDL7x series. Together with the conditions included in mostof the functions of the DL7x, the calculation functions pro-vide an invaluable tool for the control and evaluation of atitration and thereby ensure that correct results are obtainedevery time.

K. Sägesser

15

T i p s a n d T r i c k s

Table 1: pH-Sensitive Glass Membrane: Symptoms, Causes and Procedures

fogniegAssalg

sehctarcSno

enarbmem

nekorBenarbmem

tfahsro

reyalleGrodeyortsed

detardyhed

egarotsyrDedortcelefo

nomuiclaCssalg

enarbmem)mlifhsitihw(

rotaf,liOseudiserrat

)?elbisiv(

fostisopeDnwonknu

secnatsbus)?elbisiv(

decudeRepols

-%08>()%09<

llamsyreVepols

)%08<(

wolSesnopser

noitautculFgnidaerfo

tnioporeZtfihs

gnidaeRpuspmujnwoddna

hgiH,erutarepmet

foegarosnes

,noisarbAdilos

,selcitraptcerrocni

gninaelc

lacinahceMro

erutarepmetkcohs

icientfednoI-non,aidem

suoeuqasnoitacilppa

gnorWegarots

-erusaeMaidemtnem

-erusaeMaidemtnem

-erusaeM,aidemtnem

gninaelcon

noitarenegeRtxenees(

)egap

rosneSebtonnac

deriaper

rosneSebtonnac

deriaper

noitardyheRhtiw

roetylortceletap water

noitardyheRhtiw

roetylortceletap water

rosnespiD.cnocni

dicacitecastisopedlitnu

era.devlossidsdrawretfA

.noitareneger

rosnesnaelChtiw

gnisaergedneht,tnega

htiwesnirfI.retaw

,yrassecen.noitareneger

rosnesnaelCelbatiushtiw

gninaelcfI.tnega

,yrassecen.noitareneger

Explanations: Highly probable PossibleProbable

Cause

Symptom

Due to

Proceduresfor restoring the electrode

16

T i p s a n d T r i c k s

Regeneration procedureA reduced calibration slope as a resultof changes in the gel-layer on the glassmembrane can very often be observedwith older electrodes or electrodes thathave been stored dry. Similar effects arenoticeable when an electrode is usedfor non-aqueous applications be-cause the gel-layer is dehydrated.The pH-sensitive glass membrane isreactivated using a regeneration so-lution (METTLER TOLEDO order num-ber 51340073). This solution is a mix-ture of hydrochloric (HCl) and hydrof-luoric acids (HF).

Please noteSince these acids are extremely aggres-sive, make sure that you observe all thenecessary safety precautions, e.g. wearprotective goggles, a laboratory coatand chemical-resistant gloves. Keep thevolume of solution to a minimum byusing a small acid-resistant vessel.Dip the tip of the electrode into the re-generation solution for 5 to 15 min-utes. The maximum immersion pointis level with the upper shoulder of thepH-sensitive glass (see Fig. 2). Neverdip the sensor shaft into the solutionbecause the hydrofluoric acid will dam-age it. Then rinse the electrode thor-oughly with water and condition it forapproximately an hour in a buffer so-

Figure 1:What can be fixed?

lution of pH7. Finally, place the sensorovernight in the reference electrolytespecific to that sensor.

Reference diaphragmDiagnosisFaulty electrodes often have a cloggeddiaphragm. In most cases, the dia-phragm is visibly discolored. Some areblack, others are gray or just an off-white color. Sometimes the discolora-tion is so faint that it is only visiblewhen the ceramic tube is viewed fromthe side through the glass. A gradualchange from white on the inside to off-white or gray on the outside can beseen. Silver sulfide makes the dia-phragm appear black. Proteins cancause a whitish discoloration that ishardly visible. This form of contami-nation rarely occurs just on the sur-face. It usually penetrates into the poresof the diaphragm. Lipophilic sub-stances, such as fats and oils, can cre-ate a thin, non-permeable layer overthe diaphragm that is often invisible.

In general, three procedures can befollowed to restore an electrode toits normal working state. First of all,the glass membrane can be regen-erated, secondly, the reference dia-phragm may have to be cleaned andthirdly, it may be necessary to re-place the electrolyte. This latterpoint will not be discussed in thisarticle because it forms part of thenormal electrode maintenance pro-cedure.

Reference electrolyte

Reference diaphragm

pH-sensitiveglass membrane

Figure 2:Regeneration Procedure

Maximimumimmersions level

17

T i p s a n d T r i c k s

noitanimatnocfoepyT tnegagninaelC emitnoitcaeR skrameR

ediflusrevliS aeruoihT07004315-EM

.snim06–5 noitarolocsidlitnuevaeL.sraeppasid

fosepytelbissopllAtsriF.noitanimatnoc

gnivomerrofnoitadnemmocer.secnatsbusnwonknu

L/lom1.0lCH .srh21.xorppA lanretnirofdesueboslanaC.gninaelc

fosepytelbissopllAdnoceS.noitanimatnoc

gnivomerrofnoitadnemmocer.secnatsbusnwonknu

erutximdicaciruflus-cimorhC .snim03.xorppA ehtnostisopedsnaelcoslA.llewyrevenarbmem rosneS

retfadetarenegerebtsum.erudecorpsiht

snietorP noitulosgninaelclCH/nispeP86004315-EM

.rh1>

snietorP %2HOaN .snim02.xorppA

secnatsbuscilihpopiL enoteca,lonahtE .snim03.xorppA .slioelbiderofelbatiusylhgiHtfosafotroppushtiwylbissoP

.hsurb

elacs,muiclaC dicacitecA .snim03.xorppA detarenegerebtsumrosneS.erudecorpsihtretfa

sedisnet,spaoS )C°08(retawtoH .srh21.xorppA tohhtiwllewrosnesesniRniesremmi,sdrawretfA.retaw

,loocotevaeldnaretawtoh.sruoh21yletamixorppa ylnO

retawpatesu rodellitsidton,.retawdesinoi-ed

Table 2: Clogged Reference Diaphragm: Causes and Cleaning Procedures

Cleaning proceduresThe following table gives tips and hints on how to clean ceramic diaphragms. Some of these measures can be used on the outside as well as onthe inside of the electrode. The same applies of course to Redox or separate reference electrodes which have a contaminated ceramic dia-phragm.

Special notes for InLab® electrodes:• Since InLab® electrodes can be used for field measurements in conjunction with portable meters, these electrodes are very often submerged

in the sample. MultiPin and S7 are not rated according to ingress protection IP67. This is a problem particularly when the sample containssalt, which enhances corrosion. Always check the connectors for signs of corrosion when the electrode has been submerged.

• Gel-filled InLab® sensors, e.g. InLab® 417, should not be stored horizontally. Bubbles that form around the lead-off elements and in the tipof the electrode cause erroneous readings. However, since the gel is highly viscous, the bubbles cannot be removed by shaking. Always storesuch electrodes vertically, with the tip of the electrode facing downward.

Despite all the corrective procedures mentioned above, it is not always possible to repair a faulty electrode. Damage that has occurred to thewiring or casing of an electrode during use, is irreparable. In such cases the electrode must be replaced.

18

T i p s a n d T r i c k s

Titration applications on the internet:

…fast, up-to-date and free!

METTLER TOLEDO titration literature contains a wide range of well-proven applications. Most of this information can now be downloadeddirectly from the Internet or, as previously, ordered as individualbrochures.

The internet offers several advantagescompared with ordering hardcopies. Bytyping in a keyword, you can search forspecific applications very quickly; bychoosing an industry, all the relevantapplications are displayed together. Andbecause the application database is

constantly being updated, the user isalways sure that he has the very latestinformation. By the time this UserComgoes to press, the application databasewill contain well over 200 applications,including all the applications in theDL5x and DL7x titrator brochures.

How to navigate to the application database

1. Direct your internet browser to “www.titration.net” andclick on any picture on the page. The titration mainpage appears. Click “Applications”.

The step-by-step description below ex-plains how to use the application da-tabase. All you need is a computer withInternet access and a Web browser likethe Microsoft Internet Explorer orNetscape Navigator.

2. On the “Applications” page, click “Find your applica-tion”.

3. The database input form appears. Bookmark it forlater use (click “Add to Favorites” in the InternetExplorer)

Ch. Bircher

19

T i p s a n d T r i c k s

How to use the database

1. Type the name of the topic,substance or material you arelooking for in the keyword box.You can combine/limit yoursearch by selecting an industryfrom the pull-down menu andlinking it with “and/or” logicaloperators.

2. A list of search results including ashort summary appears. Select theapplication you want to see indetail by clicking its title. You arerequested to enter your usernameand password. If you are not yetregistered on titration.net, you cando this by clicking “register”.

3. The application screen containsall the information in theapplication brochures (except thecurves). You can print it out ifrequired.

20

P u b l i c a t i o n s

The application chemists of theAnaChem market support group haveprepared several publications and aseries of application brochures to sup-port customers in their routine work

in the laboratory. Each brochure isdedicated either to a particular branchof industry (such as paper and pulp,petroleum and beverages), a particu-lar titrator or a specific analysis tech-

nique. The following list shows all thepublications together with their ordernumbers. They are available from yourlocal METTLER TOLEDO marketingorganization.

Publications, reprints and applications German English

Titration in routine and process investigations 51724658 51724659Basics of Titration 51725007 51725008Fundamentals of Titration 704152 704153

Applications Brochure 1 Customer Methods 724491 724492Applications Brochure DL70 Gold and Silver 724613Applications Brochure 2 Various Methods 724556 724557Applications Brochure 3 TAN/TBN 724558 724559Applications Brochure 5 Determination in Water 51724633 51724634Applications Brochure 6 Direct measurement with ISE 51724645 51724646Applications Brochure 7 Incremental Techniques with ISEs 51724647 51724648Applications Brochure 8 Standardization of titrants I 51724649 51724650Applications Brochure 9 Standardization of titrants II 51724651 51724652Applications Brochure 11 Gran evaluation DL7x 51724676 51724677Applications Brochure 12 Selected Applications DL50 51724764 51724765Applications Brochure 13 Nitrogen Determination by Kjeldahl 51724768 51724769Applications Brochure 14 GLP in the Titration Lab 51724907 51724908Applications Brochure 15 Guidelines for Result Check 51724909 51724910Applications Brochure 16 Validation of Titration Methods 51724911 51724912Applications Brochure 17 Memory card “Pulp and paper” 51724915Applications Brochure 18 Memory card “Standardization of titrants” 51724916 51724917Applications Brochure 19 Memory card “Determination in Beverages” 51725012 51725013Applications Brochure 20 Petroleum 51725020Applications Brochure 22 Surfactant Titration 51725014 51725015Applications Brochure 23 Edible oils and fats 51725054Applications Brochure 24 KF Titration with DL5x 51725023Applications Brochure 26 METTLER TOLEDO Titrators DL31/38 * 51709854 51709855Applikationsbroschüre 27 KF Titration with Homogenizer 51725053Applications Brochure KF Chemical 724353 724354Applications Brochure KF Food, Beverage, Cosmetics 724477 724478Applications Brochure KF 10 DL35 Applications 724325 724326Applications Brochure DL18 724589 724590Applications Brochure DL12 724521Applications Brochure DL25 724105 724106Applications Brochure DL25 Food 51724624 51724625Applications Brochure DL25 Petro / Galva 51724626 51724627Applications Brochure DL25 Chemical 51724628 51724629

* Also available in French (51709856), Spanish (51709857) and Italian (51709858)

Editorial officeMettler-Toledo GmbH, AnalyticalSonnenbergstrasse 74CH-8603 Schwerzenbach, SwitzerlandTel. ++41 1 806 7711Fax ++41 1 806 7240E-Mail: msganachem@mt.comInternet: http://www.mt.com

Dr. Ch. Bircher, C. Gordon, Dr. C. A. De Caro,C. Iserland, Dr. H.-J. Muhr, K. Sägesser

Layout and productionPromotion & Documentation, Walter Hanselmann© 9/2000 Mettler-Toledo GmbH

Printed in Switzerland ME-51710043

Printed on 100% chlorine-free paper, for the sake ofour environment.