Calibration of Ph Meters

8/11/2019 Calibration of Ph Meters

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Calibration of pH meters and glass electrodes at the National Physical

Laboratory of Israel

Elena Kardash and Ilya Kuselman

The National Physical Laboratory of Israel (INPL), Givat Ram, Jerusalem 919!, Israel,

tel"# $9%&'&'*'*!+ fa# $9%&'&'&'%9%+ e'mail# lena. [email protected]

-. is one of the most fre/uently measure0 -hysico'chemical -ro-erties in fiel0s of

health care, safety, -harmaceutical in0ustry, environmental control an0 others 12" The

-. value is 0efine0 as the ne3ative 0ecimal lo3arithm of the hy0ro3en ion activity a H #

pH 4 'l3 a H (1)

5nly in hi3hly 0ilute (so'calle0 6i0eal7) solutions, the activity is e/ual to the hy0ro3en

ion concentration" 8in3le ion activity is not measurable by any thermo0ynamically vali0

metho0 an0 re/uires a convention for its evaluation" or this reason, a -. scale is base0

on the -. values of a series of stan0ar0 buffer solutions 0etermine0 by the

-otentiometric techni/ue &2" Routine -otentiometric measurements of -. are carrie0

out by means of a -. meter, accom-anie0 by a tem-erature sensor, a -.'in0icator 3lass

electro0e an0 a reference electro0e" The follo:in3 cell is re/uire0 for the -.

measurements#

3lass electro0e;test solution<salt bri03e solution;reference electro0e, (&)

:here the sin3le vertical bar is the electro0e'solution interface, an0 the 0ouble vertical

bar is the li/ui0 =unction 0enotin3 an interface bet:een the test solution an0 the salt

bri03e solution" Glass electro0e may be 0esi3ne0 in combination :ith an eternal

reference electro0e or as a sin3le -. electro0e" Glass electro0e contains an internal

mailto:[email protected]

mailto:[email protected]



reference electro0e (usually >3?>3@l), an internal reference buffer (usually :ith -. %)

an0 a 3lass membrane"

-. meter is basically a hi3h im-e0ance voltmeter :ith facility for convertin3 the

out-ut volta3e into -., at measurin3 tem-erature, or at other chosen tem-erature, by

tem-erature com-ensation"

Airect -. rea0in3s are -ossible after a0=ustment of a -. meter" >ccor0in3 to the

International Bocabulary of Casic an0 General Terms in Detrolo3y *2, a0=ustment is an

o-eration of brin3in3 a measurin3 instrument into a state of -erformance suitable for its

use" or a -. meter a0=ustment, at least t:o so'calle0 technical buffers are necessary

&2"

If the electromotive force (emf) values corres-on0in3 to the t:o technical buffer

solutions 81 an0 8& :ith -.81 an0 -.8& are E1 an0 E&, res-ectively, the -. value of the

unFno:n (test) solution can be calculate0 from its measure0 E by the follo:in3

formula#

GF

EE -.

EE

-. -.)EE( -. -. 1

8

&1

88

18 1

&1

1

−−=

−

−−−= " (*)

:here F is the -ractical slo-e of the 3lass electro0e !2#

&1 88

&1

-. -.

EE

-.

EGF

−

−=

∆

∆=

(!)

&



The -ractical slo-e F usually 0iffers from the theoretical slo-e (Nernst coefficient) F 4

&"*& RT? 2 (R is the universal 3as constant, H"*1!1 Jmol '1 '1+ T is the elvin

tem-erature, is the ara0ay constant, 9!H"*9 @oulombsmol'1)"

The -. value to be 0etermine0 shoul0 lie bet:een the -. values of t:o a0=acent

buffer solutions, i"e" the -. value of one buffer solution -.81 shall be 3reater than that

of the sam-le to be measure0, :hile the -. value of the other buffer solution (-. 8&)

shall be less"

8imilar to e-erimental 0etermination of other -hysical an0 chemical -ro-erties, -.

measurement is affecte0 by the limite0 accuracy of the measurement -rocess" 8ome

factors, such as li/ui0 =unction -otential of cell (&), asymmetry -otential of 3lass

electro0e, clo33in3 of 3lass electro0e 0ia-hra3m etc", lea0 to systematic errors in

measure0 results, :hich can not be com-ensate0 by the a0=ustment -roce0ure"

Therefore, the accurate -. measurement re/uires a calibration of the -. meter'

electro0es assembly !2"

@alibration is a set of o-erations that establish, un0er s-ecifie0 con0itions, the

relationshi- bet:een values of /uantities in0icate0 by a measurin3 system, or values

re-resente0 by reference materials, an0 the corres-on0in3 values realiKe0 by stan0ar0s

*2"

To achieve international com-arability IP>@ recommen0e0 in && to a--ly thetraceability conce-t to -. measurements that :ill allo: to connect -. measurement

results from the national metrolo3ical institution level 0o:n to fiel0, laboratory an0

in0ustry measurements 2" Traceability is a -ro-erty of the result of a measurement or

the value of a stan0ar0, :hereby it can be relate0 to state0 references ' usually national

or international stan0ar0s ' throu3h an unbroFen chain of com-arisons, all havin3 state0

*



uncertainties" The unbroFen chain of the com-arisons (calibrations) is calle0 a

traceability chain *2"

In Israel, the National Physical Laboratory (INPL) is the national metrolo3ical

institute" The national -rimary stan0ar0 of -. value 0evelo-e0 at INPL consists of a

-recise -.?ion'meter an0 the follo:in3 electrochemical cell#

>3?>3@l;* D @l<* D @l<Test solution;.& Pt?Pt, ()

:here the -.'in0icator electro0e is the Pt?Pt hy0ro3en 3as electro0e+ the reference

electro0e is >3?>3@l;* D @l+ the bri03e solution is * D @l" The -rimary stan0ar0

is the stan0ar0 that is 0esi3nate0 or :i0ely acFno:le03e0 as havin3 the hi3hest

metrolo3ical /ualities, an0 :hose value is acce-te0 :ithout reference to other stan0ar0s

of the same /uantity *2" The emf of cell () 0e-en0s on the activity of hy0ro3en ions

accor0in3 to the Nernst e/uation#

E 4 E>3?>3@l ' Fl3 a., ()

:here E>3?>3@l is -otential of the >3?>3@l electro0e, B"

@alibration of cell () is carrie0 out usin3 si -rimary -. buffers at &M@ %2" These buffers are -re-are0 from the NI8T stan0ar0 reference materials (RDs) :ith e-an0e0

uncertainties of " -., :hich are minimally Fno:n to0ay"

The national stan0ar0 of -. value has successfully -artici-ate0 in t:o

interlaboratory international Fey com-arisons of -. measurement results in && an0

!



&! or3aniKe0 by the @onsultative @ommittee for >mount of 8ubstance (@@OD) at

the Cureau International 0es Poi0s et Desures (CIPD)"

The 0escribe0 national stan0ar0 of -. value is use0 for calibration of -. meter'

electro0e assemblies" The calibration consists of the follo:in3 ste-s#

1) estimation of electrical resistance R E of the teste0 3lass electro0e+

&) calibration of the teste0 3lass electro0e+

*) calibration of the -. meter+

!) calibration of the tem-erature com-ensator"

Electrical resistance of the 3lass electro0e is 0etermine0 as the slo-e of the

voltammo3ram (current vs" -otential) :ithin the ran3e of " '1""B" The R E shoul0 be

not hi3her than 1⋅19 5hm"

@alibration of the 3lass electro0e is -erforme0 by measurin3 emf values :ith this

electro0e simultaneously :ith the reference hy0ro3en electro0e in the same cell an0

buffer solutions" The e-erimental slo-e 8l. of the linear calibration curve 6E'-.7 for

buffers solutions measure0 :ith hy0ro3en electro0e, an0 the similar slo-e 8lG of the

teste0 3lass electro0e, are 0etermine0 by the re3ression analysis of all results of the

measurements for each electro0e" The 8l. an0 8lG values shoul0 be not less than *

mB?-. an0 not 3reater than 9" mB?-. at &o @ H2" The ratio (8G ? 8.)×1Q shoul0

be not less than 9Q"

In or0er to maFe a -. meter calibration in0e-en0ent of the va3aries of electro0e

-erformance, the calibration is con0ucte0 by the -. meter com-arison :ith Fno:n 0"c"

-otentiometer !2 traceable to the national stan0ar0 of volt" Potentiostat?3alvanostat is

usin3 as 0"c" volta3e source in the ran3e of a--lie0 -otentials 0ifference & mB"



@alibration of a tem-erature com-ensator is 0one by com-arin3 tem-erature values

measure0 usin3 this com-ensator :ith the rea0in3s of the reference thermometer

traceable to the national tem-erature stan0ar0"

The national -rimary stan0ar0 of -. is use0 also for certification of buffers for -.

measurements 92"

Thus, :e are offerin3 the follo:in3 services base0 on the national -rimary stan0ar0

of -. value an0 other INPL stan0ar0s#

@alibration of a -.'meter (as a voltmeter)"

@alibration of an automatic tem-erature com-ensator (as a thermometer)"

@alibration of a 3lass electro0e"

@ertification of buffers for -.' measurements"

These calibrations ensure the shortest unbroFen traceability chain to the national

stan0ar0 of -. value an0 other INPL stan0ar0s as re/uire0 by I85 1%& 12 for

calibrations of any measurement instrument"

References

1" P" 8-itKer, C" erner" Im-rove0 reliability of -. measurements" >nalyt" an0

Cioanalyt" @hem" &&, v" *%!, --" %H%'%9"

&" Deasurement of -. value of clear a/ueous solutions" German 8tan0ar0 AIN

19&H, 19H"

*" International Bocabulary of Casic an0 General Terms in Detrolo3y" &n0 e0",

International 5r3aniKation for 8tan0ar0iKation, 199*, 8:itKerlan0"



!" 8-ecification for Laboratory -. meters" Critish 8tan0ar0 *1!#19%H, 19H%"

" "G"" CaucFe" Thermo0ynamic 5ri3in of the 8ub'Nernstian Res-onse of Glass

Electro0es" >nal" @hem" 199!, v", --"!19'!&!"

" R"P" CucF, 8" Ron0inini, >"" @ovin3ton, "G"" CaucFe an0 the others" The

Deasurement of -. S Aefinition, 8tan0ar0s an0 Proce0ures" Re-ort of the

orFin3 Party on -." IP>@ Provisional Recommen0ation" 8ee

htt-#??:::"iu-ac"or3?re-orts?-rovisional?archives"html" or Pure >--l" @hem"

&*, in -ress"

%" I" EFeltchiF, E" ar0ash'8trochFova, 5" AreaKen, an0 I" uselman" Influence of

Cuffers Ouality on the -. Deasurement ncertainty# Pre0iction an0

E-erimental Evaluation" >ccre0itation an0 Ouality >ssurance" &&, v"%, --"

!1&'!1"

H" -.'Deasurement+ 8u--lementary -.'measurin3 >--aratus+ Technical

Re/uirements" German 8tan0ar0 AIN 19&, 199!"

9" I" EFeltchiF, E" ar0ash'8trochFova, an0 I" uselman" In'house -. Reference

Daterials" Dicrochim" >cta" &*, in -ress"

1" I85?IE@ 1%&" General Re/uirement for the @om-etence of Testin3 an0

@alibration Laboratories" 1999"

%

http://www.iupac.org/reports/provisional/archives.html

http://www.iupac.org/reports/provisional/archives.html

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Calibration of Ph Meters