Instr 12421 Analyze pH

8/13/2019 Instr 12421 Analyze pH

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--------------------Analyze pHModule 12421


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Instrument Trainee Task Module 12421

ANALYZE pH-

Objectives

Upon completion of this chapter, the student should be able to:

1. Define pH.2. State the pH ranges of acids and bases.3. Nae an iportant identif!ing factor of an acid4. Discuss various techni"ues of pH easureent.

Prerequisites

Successful copletion of the follo#ing $as% Modules is re"uired

before beginning stud! of this tas% odule& N''() $as% Modules124*1 through 1242*.

)e"uired Student Materials

1. Student Module2. )e"uired safet! e"uipent

+nstruent $rainee $as% Module 12421 2


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$/-( O 'ON$(N$S

Section Topic .Page

1.*.* +ntroduction.. 52.*.* pH

.. 53.*.* pH Measureent



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64.*.* 'alibration of pH ,robes. 124.1.* Saple 'alibration of a pH ,robe.. 12

4.2.* Operational 'hec%.. 134.3.* 'opletion.134.4.* S!ste 'alibration. 134.5.* Operational 'hec%.. 144.7.* 'opletion.14

$rade $ers +ntroduced +n $his Module

Alkalinity: / substance #ith a ,H of greater that 6 is said to be al%aline8thereb! having a high al%alinit!.

Logarit!: / atheatical ter #hich indicates the po#er to #hich anuber ust be raised to get a given nuber.

"ole: /dvadrogo9s nuber8 7.*3 : 1* 23.$his nuber is used to e:pressver! large "uantities8 such as the nuber of olecules in a gra ofaterial. ;ie.. 1 ole of olecules in a gra of carbon

$he "uantitative easure of the acidit! or alkalinity of a solution is thepH of the solution. Sulfuric acid and boric acid are considered acids justas caustic soda and lie are recogni0ed as al%alies or bases. Ho#ever8these copounds var! ar%edl! in their strength8 or activit!. / 5? b!#eight solution of boric acid can be used as an e!e #ash #ith no danger.

Siilar use of a 5? b! #eight solution of sulfuric acid #ould bedisastrous.

$hus8 #e ust consider soething else besides concentration of an acidor a base to understand its "ualit! or effectiveness for a given function.+n the follo#ing discussion8 reference to strong or #ea% acids could alsoappl! to strong or #ea% bases.

One of the ost iportant identif!ing factors of an acid is the activit! ofthe h!drogen it contains. Sulfuric acid and boric acid both containh!drogen8 but the h!drogen in sulfuric acid dissociates in the presence of#ater to becoe free h!drogen ions. @hen boric acid is added to #ater8ver! little of the h!drogen is liberated as free h!drogen ions. +nstead8 itreains relativel! inert in the undissociated olecules of the acid. rothis discussion8 it a! no# be reali0ed that the true easure of acidit!concerns the easure of the dissociated or free h!drogen ionconcentration of a given solution. $he e:pression of such a value #ouldbe ver! a#%#ard if stated in ters of nuber of h!drogen ions per cubiccentieter ounce8 or soe unit of finite #eight or volue. $he Danishcheist8 Sorenson8 in 1A*A proposed that the initial letter of the rench#ord Bpoten0B8 eaning potenc! or po#er8 be called the h!drogen ione:ponent and e:pressed as pH. $echnicall!8 the pH of a solution isdefined as the negative logarithm of the h!drogen ion concentration8

#ritten ;HClog ;HCpH

,ure #ater is neither an acid or a base. +t is neutral. /t 25E'8 anequilibrium e:ists in pure #ater such that the h!drogen ionconcentration e"uals the h!dro:!l ion concentration and both have valuesof 1.* : 1*>6 gras oles per liter. =sing the definition of pH8 this eansthat the pH of pure #ater at 25E' is 6.

+f the pH of an! solution is 68 the solution is neutral. +f the pH is less than 6the solution is acidic8 and if the pH is greater than 6 the solution is basic. /



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solution #ith a pH of 4 to 7 is a #ea% acidF #ith a pH less than 4 thesolution is a strong acidF #ith a pH G to 1* it is a #ea% baseF and #ith a pHgreater than 1* it is a strong base.

/n iportant relationship in #ater solutions is the relationship bet#een

the h!drogen or h!droniu ion concentration and the h!dro:!l ionconcentration. $he ter used to e:press the h!dro:!l ion concentration ofa solution is pOH. $he pOH of a solution is defined as the negativelogarith of the h!dro:!l ion concentration8 #ritten ;OH >log ;OH>pOH

or #ater solutions8 the product of the h!drogen ion concentration ;HC14. $his eansthat the su of the pH and the pOH is al#a!s 14.;HC< : ;OH14

pH C pOH14

pH is a easure of the acidit! or basicit! of #ater solutionsF it is aeasure of the strength of acids and bases. Table 1 give pH>pOHrelationships. Table 2 sho#s the pH of various coon substances.)eferring to the above e:aple8 a 5? b! #eight sulfuric acid solution hasa pH of appro:iatel! *.3. $he 5? b! #eight boric acid solution #ill havea pH value of about 5.*. $hus8 these t#o acids having appro:iatel! thesae h!drogen concentration have pH values #idel! apart8 due to thedegree of dissociation of the h!drogen present in each.

Hydrogen Ion

Conentration

!H"#!gm moles$ #

Hydro%yl Ion

Conentration

!&H-# !gm moles$ #

pH p&H 'esription

1

1*>1

1*>3

1*>4

1*>6

1*>A

1*>11

1*>13

1*>14

1*>14

1*>13

1*>11

1*>A

1*>6

1*>5

1*>3

1*>1

1

*

1

3

5

6

A

11

13

14

14

13

11

A

6

5

3

1

*

Strong /cid

@ea% /cid

Neutral

@ea% ase

Strong ase

$able 1. pH pOH )elationship

/nal!0e pH Module 12421 (


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$able 2. $he pH of arious 'oon Materials and /cids

3.*.* pH M(/S=)(M(N$>

$he easureent of pH involves deterining the concentration ofh!drogen ions ;HC< or h!droniu ions ;H3*C< irrespective of the presenceof an! other ions. +t is accoplished using an electrode that generates anef proportional to the h!drogen ion ;HC< concentration and easured#ith a ver! high input ipedance volteter. or this easureent s!steto operate8 a coplete path for current ust e:ist. $his necessitates thee:istence of a second electrode iersed in the saple. $he secondelectrode is coonl! referred to as the reference electrode.

/ nuber of different electrodes for easuring pH are available forlaborator! use8 but onl! one8 the pH glass electrode8 is accurate enoughand universal enough for industrial use. $he glass electrode has a bulboustip ade of a special t!pe of glass that is sensitive to h!drogen ionconcentration outside to the glass in the solution of un%no#n pH. +nsidethe electrode is a solution of precisel! %no#n pH8 usuall! a H'1 solution#ith a pH of 6.*Figure 1 is a siple s%etch of a glass electrode. /n ef isdeveloped across the glass barrier proportional to the difference inh!drogen ion concentrations of the un%no#n and internal solutions. +f theun%no#n pH #ere 6.*, the (M generated #ould be O.OF this is %no#n asthe isopotential point of the electrode. /s the un%no#n pH decreases8 theef generated across the glass barrier #ill be positive inside the electrode

and negative outside. 'onversel!8 as the un%no#n pH increases8 thepotential inside the electrode becoes negative #ith respect to the

+nstruent $rainee $as% Module 12421 )

*a&H 4+ Causti ,oda 14 Higly

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*a3/&4 Milk o0 ime12

*H4&H

11Milk o0 Magnesia

1

.ora%

gg ite*aHC&3 )

/ure ater( *eutral

Milk6

Amerian Ceese5

H3.&3 .eer4

&range uieHC2H3&2 3

H3C6H5&7 7astri 8luids 2

emon uieHC 1

Higly

H2,&4 5+ ,ul0uri Aid Aidi


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e:ternal potential. $he actual value of the ef generated b! the glasselectrode is predicted b! the Nernst equation ;an anal!sis of the Nernste"uation is be!ond the scope of the course< and is 5A.2 per pH unitchange at 25E'. $he Nernst e"uation also predicts that a significantchange in output #ill occur #ith changes in saple teperature. or this

reason soe for of teperature copensation is re"uired. /ctualethods of teperature copensation are addressed later in this chapter.$he ipedance of the glass electrode is on the order of 1*AohsF thisnecessitates a ver! high input ipedance easureent instruent. $hepotential inside the electrode can be connected directl! to the instruent.

$o coplete the easureent the potential outside the glass electrodeust also be deterined.

igure 1. (leentar! Ilass (lectrode

$he reference electrode is used to coplete the path for current flo# frothe outside of the glass electrode to the instruent. +t ust provide aconstant reference potential that is independent of changes in saplecoposition or concentration. $here are t#o reference electrodes incoon useF the! are the caloel and silver>silver chloride electrodes. /t!pical silver>silver chloride reference electrode is sho#n in Figure 2.$heelectrode lead is connected to a silver e:tension that is ebedded in acolun of silver chloride. $he silver chloride is in contact #ith theelectrol!tic solution through a cotton plug. $he electrol!te is a highl!concentrated potassiu chloride ;1M J'1 or 4M J'1 are t!pical< solution.Silver chloride ;/g 'l< is soluble in the J'1 solution8 so the electrol!tecontains JC8 /gC and '1>ions. $he silver etalKsilver ion solution fors ahalf cell ;half of an o:idation>reduction reaction< that generates an ef of235.G #hen copared to a standard h!drogen electrode. $he efproduced is constant because the concentration of the electrol!te is heldconstant. (lectrical continuit! to the saple solution is ade through ali"uid junction. $his ef #ill be influenced b! saple characteristics8ho#ever8 it is a ver! sall ef and is ignored in all but the ost accuratelaborator! instruents. $he silver>silver chloride electrode eets there"uireents of providing a stable8 independent reference potential. $hecaloel electrode replaces the silver #ith ercur! and the silver chloride#ith caloel>ercurous chloride. +ts operation is essentiall! identical to

the silver>silver chloride electrode. +t produces an ef of 244.4 . oththe caloel and silver>silver chloride electrodes produce an ef that ust

/nal!0e pH Module 12421


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$he offset voltage cancellation and teperature copensation can beaccoplished autoaticall! using the odified glass electrode sho#n inFigure 3. $he glass electrode is odified in that the silver #ire is no#coated #ith silver chloride and the H' 1 solution is replaced #ith a J' 1solution. No#8 an o:idation>reduction half cell e:ists in the glass electrode

and reference electrode. )egardless of the direction of current flo#;direction is a function of saple pHis caused b! a s!etrical charge distribution on the pH sensitive glassand varies #ith the age of the glass. $his relativel! constant offset voltagere"uires the cell to be standardi0ed. During standardi0ation the cell ise:posed to a solution of %no#n pH and the easuring circuit is adjusted toobtain the desired reading.

igure 3. Ilass and )eference (lectrode

Despite all this8 the output of the pH cell is still dependent on teperature./s stated previousl!8 the Nernst e"uation predicts a dependence of theglass electrode output on teperature. $he agnitude of this change isillustrated in Table 3. Since the error caused b! teperature is linear itcan be corrected using a variable resistance to adjust the instruents9sgain. $his can be done anuall! b! easuring teperature and setting

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the corresponding value of a potentioeter. +t is done anuall! b! theaddition of a therocopensating probe8 #hich is nothing ore than aproperl! selected )$D. Note that the teperature copensator correctsfor the change in slope of the glass electrodes pH>voltage curve and in no#a! copensates for changes in the pH of an actual solution #ith changes

in teperature.

TM/9AT:9

;8 ;C

/&T*TIA /9 / :*IT

Milliline pH eter #ith three probes. ,robes!stes that use onl! a single penetration into the process are available8but the three>probe s!stes are ore coon. 'are should be ta%en#hen selecting the location for pH probes to ensure the! are e:posed to ahoogeneous i:ture of the solution under test. $he glass electrodepresents an e:treel! high ipedance to current flo#8 so to avoid circuitloading8 ost pH instruents have an input ipedance of 1*12 ohs.ollo# anufacturer9s directions closel! #hen handling the probes orinstruent since oisture8 dust8 or fingerprints can reduce the ipedanceto ground causing decreased accurac! and response tie.



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igure 4. +n>-ine pH Meter

4.*.*'/-+)/$+ON O pH ,)O(S>

,h probes should be calibrated periodicall!8 depending on the fre"uenc! ofuse and the accurac! desired. +ncluded in the ne:t section is a saplecalibration procedure.

4.1.* Saple 'alibration of a pH ,robe

Disconnect the output leads of the transitter.

'onnect the ultieter to the output of the transitter and set on

/ scale.

)eove the probe fro the s!ste and rinse #ith purified #ater.

+erse the probe in the 4.* pH buffer solution and allo# the

indication to stabili0e.

)ecord the follo#ing indications on the chec%list&

Multieter indication ;a

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Instr 12421 Analyze pH