3 Gravimetric Analysis

Gravimetric Analysis

1

Gravimetric Analysis- What is It?

• Definition:a precipitation or volatilization method based on the determination of weight of a substance of known composition that is chemically related to the analyte

• analyte - chemical element or compound of interest

Gravimetric Analysis- What is It?• Reaction:

aA + rR -----> AaRr ppt

where:–a is # of moles of analyte A– r is # of moles of reagent R–AaRr is a pure, insoluble

precipitatewhich we can dry and weigh or ignite to convert to something we can weigh

–ppt=precipitate

T.W.Richards• 1914 Nobel Prize to

T.W.Richards (Harvard University) for the atomic weights of Ag, Cl, and N

• Richards and his group determined atomic weights of 55 of the 92 known elements using gravimetry

T.W.Richards

• “Every substance must be assumed to be impure, every reaction must be assumed to be incomplete, every method of measurement must be assumed to contain some constant error, until proof to the contrary can be obtained. As little as possible must be taken for granted.”

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How to Perform a Successful Gravimetric Analysis

• What steps are needed?1. Sampled dried, triplicate portions weighed2. Preparation of the solution3. Precipitation4. Digestion5. Filtration6. Washing7. Drying or igniting8. Weighing9. Calculation

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Gravimetric Analysis • Gravimetric Analysis – one of the most accurate

and precise methods of macro-quantitative analysis.

• Analyte selectively converted to an insoluble form.

• Measurement of mass of material • Correlate with chemical composition• Why?• Simple• Often required for high precision

8

Determination of mass

– Direct – By difference

NaHCO3 + H2SO4→CO2 + H2O +NaHSO3

Determination of NaHCO3

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Desirable properties of analytical precipitates

– Readily filtered and purified– Low solubility, preventing losses during

filtration and washing– Stable final form (unreactive)– Known composition after drying or ignition

Suction Filtration

• Filter flask• Buchner funnel• Filter paper• Glass frit• Filter adapter• Heavy-walled rubber

tubing• Water aspirator

Suction Filtration

• Mother liquor

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Kinds of Precipitating reagents:

• Selective • Ag+ + Halides (X-) AgX(s)

• Ag+ + CNS- AgCNS(s)

• Specific• Dimethylglyoxime (DMG)• 2 DMG + Ni2+ Ni(DMG)2(s) + 2 H+

14

Mechanism of Precipitation

15Fig. 10.2. Representation of silver chloride colloidal particleand adsorptive layers when Cl- is in excess.

Cl- adsorbs on the particles when in excess (primary layer).

A counter layer of cations forms. The neutral double layer causes the colloidal particles to coagulate.

Washing with water will dilute the counter layer and the primary layer charge causes the particles to revert to the colloidal state (peptization). So we wash with an electrolyte that can be volatilized on heating (HNO3).

Cl- adsorbs on the particles when in excess (primary layer).

A counter layer of cations forms. The neutral double layer causes the colloidal particles to coagulate.

Washing with water will dilute the counter layer and the primary layer charge causes the particles to revert to the colloidal state (peptization). So we wash with an electrolyte that can be volatilized on heating (HNO3).

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

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Filterability of Precipitates

• Colloidal suspensions– 10-7 to 10-4 cm diameter– Normally remain suspended– Very difficult to filter

• Crystalline precipitates– > tenths of mm diameter– Normally settle out spontaneously– Readily filterable

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R.S.S = (Q-S)/S

Precipitate formation affected by:Solubility of PrecipitateTemperature Concentration of reagensRate of mixing

-Relative Super Saturation(R.S.S) R.S.S = (Q-S)/S

Q = Instantaneous Concentrations of the mixed reagents

S = Equilibrium Solubility of Precipitate

Smaller R.S.S leads to crystalline precipitates.Q S

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Conditions for Analytical Precipitation

• Precipitation from hot solution – The molar solubility (S) of precipitates increases with

an increase in temperature– An increase in S decreases the supersaturation and

increases the size of the particle.

• Precipitation from dilute solution – This keeps the molar concentration of the mixed

reagents low. Slow addition of precipitating reagent and thorough stirring keeps Q low. (Uniform stirring prevents high local concentrations of the precipitating agent.)

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Conditions for Analytical Precipitation

• Precipitation at a pH near the acidic end of the pH range in which the precipitate is quantitative. – Many precipitates are more soluble at the lower (more

acidic) pH values and so the rate of precipitation is slower.

• Digestion of the precipitate. – The digestion period can lead to improvements in the

organization of atoms within the crystalline nuclei, such as expulsion of foreign atoms (or other impurities).

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Important Factors for Gravimetric Analysis

Nucleation ≈ (RSS)n

Individual ions/atoms/molecules coalesce to form “nuclei”

Particle Growth ≈ (RSS)×nCondensation of ions/atoms/molecules with

existing “nuclei” forming larger particles which settle out

Competition

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Colloidal SuspensionColloidal particles remain suspended due to:

small size

adsorbed ions giving a net + or – charge

(Brownian motion)

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Coagulation, agglomerationSuspended colloidal particles coalesce to form larger

filterable particles by:

Heating

stirring

adding inert electrolyte

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24

25

AgCl (s) → AgCl (colloid)

Peptization

Re-dissolution of coagulated colloids by : washing and removing inert electrolyte

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DigestionHeating the precipitate within the mother liquor (or solution

from which it precipitated) for a certain period of time to encourage densification of nuclei.

– During digestion, small particles dissolve and larger ones grow. This process helps produce larger crystals that are more easily filtered from solution

DT

27Fig. 10.1. Ostwald ripening.

During digestion at elevated temperature:

Small particles tend to dissolve and reprecipitate on larger ones.

Individual particles agglomerate.

Adsorbed impurities tend to go into solution.

During digestion at elevated temperature:

Small particles tend to dissolve and reprecipitate on larger ones.

Individual particles agglomerate.

Adsorbed impurities tend to go into solution.


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Ideal Analytical Precipitation• In an ideal world, an analytical precipitate for gravimetric

analysis should consist of perfect crystals large enough to be easily washed and filtered.

– The perfect crystal would be free from impurities and be large enough so that it presented a minimum surface area onto which foreign ions could be adsorbed.

• The precipitate should also be "insoluble" (i.e., low solubility such that loses from dissolution would be minimal).

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Increasing Purity• Re-precipitation

– a procedure including washing away the mother liquor, redisolving the precipitate, and precipitating the product again

Structure of a Colloid

• Why do two colloid particles resist aggregating to form a crystal?

• If two colloid particles, each with a negative charge, come close to one another, they will repel!

• So colloid particles are stable and resist crystal formation.

Coagulating a Colloid

• What can be done to overcome this colloid stability and force crystals to form?

• High heat, stirring, only a slight excess of the excess reagent, and the addition of an electrolyte can force a colloid to coagulate into crystals.


• High heat, initially with stirring, is thought to lower the thickness of the double layer, thus making it easier for two colloid particles to collide and coagulate.

• The higher kinetic energy will also helpthem gain enough energy to overcome

the repulsion.


• If too much of the excess reagent is added, then the double layer increases in volume as more of the excess solute ions will be adsorbed to the surface, which in turn requires a larger counter-ion layer.


• So it is important to make sure that there is only a slight excess of the excess reagent.

• Thus the diameter of the double layer will be minimized, enabling neighboring colloids to coagulate.

Coagulating a Colloid• On the other hand, the addition of a suitable

electrolyte like nitric acid or hydrochloric acid may also lower the diameter of the double layer.

• Now the high concentration of the appropriate ion will make it easier to form the counter-ion layer and its thickness will be reduced.

• Again, two neighboring colloids can get closer together, making it easier to coagulate.

Digesting

• Once a colloid starts to coagulate, it is best to digest the solution.

• Digestion is when the heated solution with the coagulating crystals sits undisturbed for an hour or more.

Digesting• Typically, the colloidal suspension is stirred with

heating until crystals start to coagulate. • Then stirring is stopped, and the solution is heated

to almost boiling for at least 10 minutes. • Finally, the solution is allowed to cool slowly and

sit undisturbed for several hours.• Digestion results in larger, purer crystals which are

easier to filter.

Filtration

• Once the crystals have formed and digested, they need to be filtered.

• The washing step can be a problem, as peptization of the coagulated colloid may occur.

• This means that the coagulated colloid reverts to a smaller colloidal particle.

Filtration

• Washing with pure water often causes this problem as this lowers the concentration of counter-ions, which then causes the double layer to increase in volume, and the coagulated solid may break back into smaller colloids.

• These colloids will then go right through the filter, and the filtrate may look cloudy.

Filtration• Typically, the wash solvent is a dilute

solution of the electrolyte. • This keeps the double layer intact,

minimizing peptization. • This electrolyte will then volatilize during

the drying step.• The filtered and washed crystals are then

dried to constant mass.

Coprecipitation of Impurities• During the precipitation process, other soluble

compounds may also be removed from the solution phase.

• These other compounds are carried out of solution by the desired crystals.

• They are impurities and they are said to have coprecipitated.

• These are NOT other insoluble compounds, but by several mechanisms, have been carried out of solution.

Coprecipitation of Impurities

• Coprecipitation occurs in several ways:– adsorption onto the surface of the crystals,– inclusions (absorption into crystal)– occlusions (absorption)

• Inclusions occur when ions of the impurity occupy lattice sites in the crystal, while occlusions are just particles which are physically trapped inside the crystal

Reprecipitation

• If coprecipitation occurs or is known to be a common occurrence with this solute, then

reprecipitation of the solute should be conducted.

• In reprecipitation, the filtered precipitate containing impurities is redissolved and then the crystals are reprecipitated.

Reprecipitation

• This technique effectively lowers the concentration of impurities, so the second

precipitation will contain fewer impurities.

• This is a common technique for iron and aluminum hydroxides which coprecipitate other more soluble hydroxides.

45

Co-precipitation

– Normally soluble compounds carried down with insoluble precipitate.

– 1)Equilibrium process– 2)Kinetic of growth process

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Co-precipitation– 1)Equilibrium process

– surface adsorption, Is improved by:– Digestion, washing, re-precipitationi

Co-precipitation

• mixed crystals – A type of coprecipitation in which the

impurities occupy the crystal lattice sites

– mixed crystal– (MgKPO4 & Mg NH4PO4)

Is improved by:– Primary separation

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Co-precipitation

– 2)Kinetic of growth process– occlusion, Rapid growth (Foreign ions)

– Slow rate, digestion & reprecipitation

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Co-precipitation

– Entrapment, (portion of solution)

Is improved by:

– Slow rate, digestion & reprecipitation

Gathering Agents• Occasionally, reprecipitation is intentionally used

to gather a trace component that coprecipitates. • When the precipitate is redissolved in a very small

amount of solvent, the trace component has been effectively concentrated.

• In this case, the precipitate used to gather the trace component is called the gathering agent.

Masking Agents

• Masking agents can also be used to prevent coprecipitation.

• The masking agents react with the impurities to from highly soluble complexes to keep them in solution.

Homogeneous Precipitation• In homogeneous precipitation, the

precipitate is formed through a second chemical reaction.

• First, a reagent is treated in a manner so that it forms what is called a precipitating agent or reagent.

• The precipitating reagent then reacts with the solute ion to form the desired solid precipitate.

Homogeneous Precipitation• As the precipitating reagent is generated in

the solution gradually, this limits the relative supersaturation of the precipitate.

• So crystals are more likely to form, be larger, and be more pure.

• This is relatively common for the precipitation of hydroxide salts where urea is used to generate the precipitating agent hydroxide.

Drying a Precipitate• Drying a precipitate seems easy. • Many compounds can be easily dried at around

110°C to remove any water which is adsorbed.• Other compounds need much higher heat to

remove water.• The temperature must be carefully decided as

many compounds will decompose if the heat is too high.

Common Desiccants

Mechanism of Action

Hydration ANHYDRONE® (Magnesium Perchlorate anhydrous), CaCl2, MgO, MgSO4, K2CO3, KOH, Drierite, Na2SO4 (anhydrous), H2SO4, ZnCl2

Absorption and/ or Adsorption

BaO, CaSO4, Molecular Sieve, H3PO4, NaOH Pellets

Chemisorption CaO, P2O5

• Silica gel goes from blue to pink as it absorbs moisture Can be regenerated in oven

• Anhydrous sodium sulfate gets clumpy as it absorbs water

More Information about desiccants including common interferents and regeneration temperature can be found at:http://www.jtbaker.com/techlib/documents/3045.html

http://www.jtbaker.com/techlib/documents/3045.html

Igniting a Precipitate• Yet other precipitates have a variable composition

and must be further treated to form a compound of uniform composition.

• One common way to treat variable composition compounds is through ignition: high heating.

• This is common with iron analysis. Variable composition ferric bicarbonate hydrates are ignited at around 850°C to produce anhydrous ferric oxide.

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Alternative Gravimetry Technique• Homogeneous Precipitation• What?

– Precipitating agent generated slowly by chemical reaction in analyte solution

• Why?– Precipitant appears gradually throughout– Keeps relative supersaturation low– Larger, less-contaminated particles

• How?– (OH-) by urea decomposition– (NH2)2CO 2 OH- + CO2 + 2 NH4

+

– (S=) by thioacetamide decomposition– CH3CSNH2 H2S + CH3CONH2

– (DMG) from biacetyl + hydroxylamine– CH3C(=0)-C(=0)CH3 + 2 H2NOH DMG + 2 H2O

Fig 12-5, p.32460

Species analyzed

Precipitated form

Form weighed Some interfering species

K+ KB(C6H5)4 NH4+, Ag+, Hg2+, Tl+,

Rb+, Cs+

Mg2+ Mg(NH4)PO4.6H2O Mg2P2O7 Many metals except Na+ and K+

Ca2+ CaC2O4.H2O CaCO3 or CaO Many metals except Mg2+, Na+, or K+

Ba2+ BaSO4 BaSO4 Na+, K+, Li+, Ca2+, Al3+, Cr3+, Fe3+, Sr2+, Pb2+, NO3

-

Ti4+ TiO(5,7-dibromo-8-hydroxyquinoline)2

same Fe3+, Zr4+, Cu2+, C2O42-,

citrate, HF

VO43- Hg3VO4 V2O5 Cl-, Br-, I-, SO4

2-, CrO42-,

AsO43-, PO4

3-

Cr3+ PbCrO4 Ag+, NH4+

Mn2+ Mn(NH4)PO4.H2O Mn2P2O7 Many metals

Fe3+ Fe(HCO2)3 Fe2O3 Many metals

Ni2+ Ni(dimethylglyoximate)2

same Pd2+, Pt2+, Bi3+, Au3+

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Organic precipitating agents are chelating agents.

They form insoluble metal chelates.

Organic precipitating agents are chelating agents.

They form insoluble metal chelates.


Table 12-2, p.330

Table 12-3, p.330

67

68

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• 8-Hydroxyquinoline


Mg2+ + 2

N

OH

+ 2H+N

O

Mg

O

N

Selectivity through pH control

• 8-Hydroxyquinoline Examples


Metal pH

Initial Ppt.

pH

Complete

Ppt

Metal pH

Initial Ppt.

pH

Complete

Ppt

Aluminium 2.9 4.7 – 9.8 Manganese 4.3 5.9 – 9.5

Bismuth 3.7 5.2 – 9.4 Molybdenum 2.0 3.6 – 7.3

Cadmium 4.5 5.5 – 13.2 Nickel 3.5 4.6 – 10.0

Calcium 6.8 9.2 – 12.7 Thorium 3.9 4.4 – 8.8

Cobalt 3.6 4.9 – 11.6 Titanium 3.6 4.8 – 8.6

Copper 3.0 >3.3 Tungsten 3.5 5.0 – 5.7

Iron(III) 2.5 4.1 – 11.2 Uranium 3.7 4.9 – 9.3

Lead 4.8 8.4 – 12.3 Vanadium 1.4 2.7 – 6.1

Magnesium 7.0 >8.7 Zinc 3.3 >4.4

• Dimethylglyoxine


Weakly alkaline conditions

Nickel salt bright red

Ni2+ + 2 CH3 C C CH3

N NHO OH

+

CH3 C C CH3

N N

Ni

OO

O ONN

CH3CCCH3

HH 2H+

73

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Conversion

• Removal of volatile reagents & solvent• Extended heating at 110 to 115 OC• Thermal Conversion to Measurable Form• Chemical conversion to known stable form• CaC2O4(s) CaO(s) + CO(g) + CO2(g)

• Volatilization & trapping of component• NaHCO3(aq)+ H2SO4(aq) CO2(g)+ H2O + NaHSO4(aq)

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Combustion Analysis• Combustion analysis is still used to determine the

amount of C, H, N, O, S, and halogens in an unknown sample.

• In the classic freshman combustion problem, a hydrocarbon is combusted in excess oxygen gas to produce water vapor and carbon dioxide gas.

• The water and carbon dioxide are trapped and the mass of these products is obtained.

• Then calculations begin.

Combustion Analysis

• Today, elemental combustion analyzers measure C, N, H, and S at the same time.

• Oxygen analysis is done through pyrolysis with no oxygen gas and halogen analysis occurs through an automated titration.

Other Gravimetric Techniques

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Combustion Analysis, or Elemental Analysis

(g)SO(g)SO(g)NO(g)H(g)COSN,H,C,32222

C 1050

Determines the C, H, N, and S content in a single operation by using GC and thermal conductivity measurements


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Combustion Analysis, or Elemental Analysis cont.


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Thermal Gravimetric Analysis (TGA)

• Precisely monitoring weight loss of a sample in a given atmosphere as a function of temperature and/or time

• Atmospheres: N2, O2, air, or He • Temperature: ambient to 1000 °C• Records the first derivative of the

mass loss


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TGA cont. • Evaluate the thermal decomposition and stability of

materials– Polymers, resins, rubbers, explosives

• Information on bulk composition of compounds– Thermal oxidation, heat resistance– Residual water or solvents – Compositional analysis – Ash content in a sample– Quantity of inorganic filler in a polymer


87

The percent weight loss of a test sample is recorded while the sample is being heated at a uniform rate in an appropriate environment.

The loss in weight over specific temperature ranges provides an indication of the composition of the sample, including volatiles and inert filler, as well as indications of thermal stability.

The gas environment is pre-selected for either a thermal decomposition (inert – He or N2 gas), an oxidative decomposition (air or O2), or a combination therein.

TGA cont.

TGA Instrumentation

88http://radchem.nevada.edu/chem455/lecture_22__thermal_methods.htm

1. A sample (0.1 to 15 mg) is placed into a tared TGA sample pan, which is attached to a sensitive microbalance.

2. The sample holder (connected to the balance) is subsequently placed into a high temperature furnace.

3. Balance assembly measures the initial sample weight at room temperature and continuously monitors changes in sample weight (losses or gains) as heat is applied to the sample (up to 1500 °C).

TGA tests may be run in a heating mode at some

controlled heating rate, or isothermally. Typical weight loss profiles are analyzed for

the amount or percent of weight loss at any given

temperature, the amount or percent of noncombusted

residue at some final temperature, and the

temperatures of various sample degradation processes.

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TGA cont.

90

H2O, C7H3O6, C6H4, CO2 are consecutively lost.

TGA cont.

91

Gravimetric Calculations

• Gravimetric Factor (GF):• A→2B(s)

• GF = g A/g B• GF = (FW A /FW B)x(a moles A/b moles B)• BaCl2 +2AgNO3→Ba(NO3)2 + 2AgCl(s)

1)(

2)(

2

BaClFW

AgClFWGF

The Gravimetric Factor

• G.F. = a FW[analyte] b FW[precipitate]

• Analyte ppt G.F.CaO CaCO3

FeS BaSO4

UO2(NO3)2.6H2O U3O8

Cr2O3 Ag2CrO4

Gravimetric Factor

• Analyte ppt G.F.CaO CaCO3 CaO/CaCO3

FeS BaSO4 FeS/BaSO4

UO2(NO3)2U3O8 3UO2(NO3)2/U3O8

Cr2O3 Ag2CrO4 Cr2O3/2Ag2CrO4

• Naming is critically important (next class)

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Gravimetric ErrorsCo-precipitation: (with AgCl)

Co-precipitant Error Rationale

NaF Positive All NaF is excess

NaCl Negative Fwt Na<Ag

AgI Positive All AgI is excess

PbCl2 (fwt 278.1) Negative Gravimetric Factors decreases

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Calculating Results from Gravimetric Data

• The calcium in a 200.0 ml sample of a natural water was determined by precipitating the cation as CaC2O4. The precipitate was filtered, washed, and ignited in a crucible with an empty mass of 26.6002 g. The mass of the crucible plus CaO (fwt 56.077 g/mol) was 26.7134 g. Calculate the concentration of Ca (fwt 40.078 g/mol) in the water in units of grams per 100 mL.

96

Calculating Results from Gravimetric Data• An iron ore was analyzed by dissolving a

1.1324 g sample in concentrated HCl. The resulting solution was diluted with water, and the iron(III) was precipitated as the hydrous oxide Fe2O3

.xH2O by addition of NH3. After filtration and washing, the residue was ignited at high temperature to give 0.5394 g pure Fe2O3 (fwt 159.69 g/mol). Calculate (a) the % Fe (fwt 55.847 g/mol) and (b) % Fe3O4 (fwt 231.54 g/mol) in the sample.

97

Calculating Results from Gravimetric Data

• A 0.2356 g sample containing only NaCl (fwt 58.44 g/mol) and BaCl2 (fwt 208.23 g/mol) yielded 0.4637 g of dried AgCl (fwt 143.32 g/mol). Calculate the percent of each halogen compound in the sample.

Vial containing the unknown

Two 400 mL beakers and two stir rods.

60 mL of 0.55 M CaCl2

Crucible tongs

Beaker tongs

Desicooler Crucibles

Drying agent

Obtain two crucibles from the oven, put the lid on and let them

cool on the bench top for 20 min. Then weigh them accurately on

the analytical balance.

Use crucible tongs to transfer crucible to analytical balance.

Record the mass in your notebook.

Accurately determine the mass of the vial, contents and lid. Record this mass and unknown #.

Tap out about half of the unknown into one

of the 400 mL beakers. (Label this beaker)

Reweigh accurately the vial with the lid and record this mass.

Tap the remaining unknown into the other 400 mL

beaker (label it).

Reweigh accurately the vial.

Pour about 20 mL of distilled water into each of the 400 mL beakers. Swirl (gently) to dissolve the unknown sample, and warm the beakers a little if you’re having trouble dissolving the sample.

Once the unknown samples have

dissolved, add about 80 mL more distilled water to each beaker

and cover each beaker with a watch

glass and warm them to steaming.

Steaming

Once solutions have started steaming, reduce the heat to medium- low and add 30 mL of 0.55 M

CaCl2 to each solution.

Stir each solution with its own stir rod for a few

seconds, then leave the stir rod in the solutions and allow them to heat

for another 30 min.

When about 30 minutes is up and the precipitated has settled, add one or two drops of CaCl2 to the supernatant liquid. If no additional precipitate forms take the beakers and put them in an ice water bath to cool.

Ice water bath.

AspiratorSuction cone

Utility clamp

Vacuum filter flask

Stand

Black vacuum (rubber) tubing

Crucible

Filter the solution through one of the

pre-weighed crucibles.

Rinse and filter the precipitate in the

beaker several times with a small quantity of

cold distilled water.

Place the crucibles in the desicooler.

And in the oven overnight to dry.

The next day come and weigh the

crucibles. Record their masses.

• The figure on the right shows the reaction of Ba(NO3)2 with K2CrO4 forming the yellow BaCrO4 precipitate.

• The BaCrO4 precipitate is being filtered in the figure on the right. It can then be dried and weighed.

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3 Gravimetric Analysis