Measurement Techniques, Vol. 41, No. 6, 1998

STATE STANDARDS

STATE PRIMARY STANDARD FOR THE pH SCALE

UDC 621.753.38

N. N. Zdorikov, O. V. Karpov, I. I. Maksimov, E. E. Seiku, and V. V. Sobol'

Information is given on the state primary standard for the pH scale that has been set up at VNIIFTRL The measurement system is described and certification results are given. Thefuture development of p H measurement in Russia is considered.

Measurement of pH is one of the commonest methods of monitoring composition for aqueous solutions and other liquid media. Means of measuring pH, or pH meters, are among the leading instruments for analyzing liquids, and there are some millions of them in the Russian Federation. They are used in virtually all branches of industry, as well as in agriculture, ecology, medicine, and scientific research. There are some dozens of standards, recommendations, and other standardization documents dealing with pH measurement. Measurements on pH are made in certifying food products such as meat, milk, milk products, plant oil, grain, wine, and beer as well as in the pharmaceutical and chemical industries. Recent developments in medicine and biotechnology and increased demand for ecological monitoring and the protection of nature have imposed elevated requirements for accuracy in determining pH, and these require reliable standard facilities. The All-Russia Technical Physics and Electronic Measurement Research Institute (VNIIFrR~ has developed a standard for the pH scale that has been confirmed by the Russian State Standard Commi~ion as the state primary standard (GET 54-98). The state primary standard for the pH scale is intended to reproduce, store, and transmit the pH scale in the range from 3.547 to 10.317 in the temperature range from 0 to 95"C. The standard has been confirmed and new editions of the All-Russia state standards have been prepared: "The State System of Measurements: Aqueous Solution pH Scale" (sent for confirmation to the Intergovermnental Council on Standardization, Metrology, and Certification in Minks) and also "The State System of Measurements: State Test Scheme for Means of Measuring pH" (in the stage of agreement with the CIS countries), and this provides for synchronized and more accurate reproduction of the pH scale, while a state test scheme will provide methodological unity and reliability in checking and testing means of measuring pH and will raise the reliability in monitoring product quality. The state primary standard for the pH scale heads the test scheme (see Scheme 1), which governs metrological support in pH measurement and extends to the following means of measurement: pH means with pH measurement ranges from 0 to 14, measurement electrodes with pH measurement ranges from 0 to 14, auxiliary electrodes, and measurement converters for pH meters. Scheme 1 incorporates the current state and future development prospects for pH meters in Russia. It has been formulated in such a way that for each measuring instrument there is established a sequence and method for use in the unified state test scheme. The transfer to the pH scale recommended by the International Union of Pure and Applied Chemistry (IUPAC) and the provision for reproducing the scale at the contemporary international level will facilitate the entry of Russia into the world market by matching the technologies, measurement and test facilitates, and adoption of the results from mutual certification and attestation. During metrological research on the standard in 1995, comparisons were made with the standard at the German National Metrological Institute, which confirmed that our standard has a high technical level and that the chemical substances Translated from Izmeritel'naya Tekhnika, No. 6, pp. 5-8, June, 1998. 0543-1972/9814106-0499520.00

o1999 Kluwer Academic/Plenum Publishers

499

State Test Schemefor Meansof MeasuringpH (draft); State primarystandardfor pH Measurementapparatus withset of nontransDortelectrochemicalcells Referencebuffer Set of standard solution buffer solutions pH 3,547 + 10.317 t= 25"C

e = 0.002 S = 0,001

t=0...60*C t=60...95"C

E)=0.003 S = 0 . 0 0 2 e=0.005 S=0.003

0+95"C

e=0.005

$=0.003

Iodircct measurement I

Work~ pH = = ~

Apparatuswith electrochemical cell and liquidconnectton 16 buffer solutions

r

~o

ffi 0... 6 0 " C t=60...95"C

(

D,=----

)

I

& = 0.002

I

--

6=0.005 ,~=0.009

(

I .

I

r

0 + 95 "C

t = 25 ~

t =0...60"C

=0.006 8=0.010

t = 60... 9 5 " C

~ = 0,004

Z

i [I

O r K)

I pH-meters pH 0 + 1 4 6 = 0.01

I

r

+

,

e=0

{

= 0.005 I

I I 9 16 buffer solutions

[

~'

II

_1__ ~

~

A:

. . . .

0.03

16 buffer solutions I~_ 1 + 1 4 & = 0.03

I

Directmeasurcn~nt~_ method & = 0.03

9 ....

!

6=0.010

A

: 0.005 I

I Referenceelectrodes fi= O.SmV

~, Matching A a b y means ~ -

4 o,,=~===r

)

!

pH-meters pH 0 + 1 4 S = 0,02 ... 0,05

~ Directmeasuremcm method & = 0.03

..........

E

5

500

electrodes pH 0 + 1 4

& =0.01... 0.2

I

I

~ ~dmCasurcat~ & = 0.03

,~ = 0,006

I [

pH 1 + 14 ~ = 0.01

I

~Direct~~_ -

r

&

~ = 0.004

t= 0...60"C

I

(

<

.

Hydrogen-eJectrode pH meters

t =P~251(~14 t = 60... 9 5 " C

.

I

Differentialpotenfiometriccell 16 buffersolutions

ca=

.

mcthod~ = 0.002

electrode &=3 mV

sensors .~ =0.06... m V

-

Fig. 1. Measuring equipment for the pH scale standard. used are of food quality [1]. Technology has been developed for synthesizing, purifying, and quality checking on the chemical substances or making the standard solutions. Short-series production has begun for standard solutions for making the working standards of pH in the second and third divisions [2]. Measurement Procedure. Difficulties in pH measurement follow from the definition [3, 4], which is not based on rigorous thermodynamic relationships. The formal definition of pH includes the activity of one species of ion, which cannot be measured directly: pH= - l g a H = - l g ( r n H "/H /" m*),

in which a H is the hydrogen-ion activity in the solution, m H the molarity of the hydrogen ions, ~'rt the hydrogen-ion activity coefficient, and m ~ is the molarity of the hydrogen ions in the standard state, 1 mole-kg-:. The potentiometric method of pH determination is the most convenient as regards apparatus and also the most accurate and reproducible. The activity of the hydroxonium ions is measured reversibly with respect to protons in the hydrogen electrode in an electrochemical cell of transport type: comparisonelectrode }KCl(m __ 3.5 mole-kg-1) ][solution S or XlPt, H2,

(1)

in which X is the test solution for which the pH(X) is determined and S is a standard solution with an assigned value of pH(S). Then the value of pH(X) is given by pH(X) = pH (S)

E ( X ) - E(S)

(-R"T7F) l-ff'~ - pH (S) -

E ( X ) - E(S)

k

'

in which E(X) and E(S) are the measured emfs in the cell of (1); R is the universal gas constant; F is Faraday's constant, T is the thermodynamic temperature; and (RT/F) In 10 = k is the slope. To make pH measurements, one must have available standard values of the pH(S) for various temperatures for a series of standard solutions. The standard at VNIIFTRI determines the pH value for a reference buffer solution RBS (potassium hydrop.hthalate of molality 0.05 mole.kg-1) together with seven standard buffer solutions SBS [1]. The determination procedure is described in detail in [1, 4, 5] and is performed in electrochemical cells without transport: 501

Fig. 2. Transport-free electrochemical cell.

Ag I Ag C I (sol) I RaS (or SBS), C 1- (me I ) I Pt, H2

(2)

p H ( S ) = - l g (aH"fCl)mc 1._>0 + Ig(Tcl)mcl .--,0'

(3)

with pH(S) defined as

in which me1 is the chloride ion molarity and 3'cl the chloride ion activity coefficient. The first term in the sum in (3) is derived by extrapolating the acidity function found by experiment from emf measurements in the (2) cell to zero chloride ion concentration, while the second is derived by calculation from the Bates-Guggenlaeim condition [1, 4]. Measurement Equipment. The standard contains the following: the measuring equipment (Fig. 1); set of buffer solutions: RBS and SBS. The measuring equipment includes a set of 12 electrochemical cells (Fig. 2) without transfer, which contain hydrogen and silver chloride electrodes, a TVP-6 water thermostat with a capacity of 200 liter, a gas supply system, a section for purifying and checking water, an MN-2 solid-state working de voltage standard, a standard digital platinum thermometer, and a V7-34A universal digital voltmeter. A pentium-133 personal computer processes the measurements and stores the data. Metrological Characteristics. The state primary standard for the pH scale provides for reproducing values of pH with a residual systematic error 0 of not more than 0.002 at 25~ 0.003 between 0 and 60~ (apart from 25~ and 0.005 from 60 to 95~ The standard deviation S in a pH measurement based on nine independent measurements is not more than 0.001 for 25~ 0.002 in the range 0 to 60~ (apart from 25~ and 0.003 in the range from 60 to 95~

502

The confidence limits ApH for the total error of measurement with fiducial probability P = 0.99 are 0.003 at 250C, 0.005 in the range from 0 to 60"C (apart from 25"C), and 0.009 in the range from 60 to 95"C. The metrological characteristics correspond to the current international level [5, 6]. The electrochemical measurement laboratory at VNIIFTRI has received an invitation from the IUPAC working group to participate in an intergovernmental comparison for the determination of the equivalence of initial materials (standard reference materials). Development Prospects. The plan for developing the metrological support to pH measurement envisages improving the pH scale standard by automating the measurements, devising pH scales for mixed water-organic solutions, participation in international studies on harmonizing the multistandard NIST scale (USA) and the one-standard BSI scale (Britain), together with the creation and implementation of a scale for the analog of the hydrogen parameter for aqueous solutions at high pressures and temperatures. It is planned also to revise some out-of-date standards and methodological recommendations on pH measurement. The scientist responsible for the state primary standard for the pH scale has been confirmed by the head of the electrochemical measurement laboratory at VNIIFTRI as Dr. Seiku Elena Evgen'evna.

REFERENCES .

2. 3. 4. 5. 6.

N. N. Zdorikov et al., Izmerit. Tekhn., No. 12, 51 (1997). Yu. M. Abramenko et al., Zakonodatel'naya Metrologiya, No. 5, 28 (1997). S. R. L. Soreusen and K. Linderstrom-Lang, Compt. Rend. Tray. Lab. Carlsberg, 15, No. 6, 40 (1924). R. Bates, pH Determination: Theory and Practice [Russian translation], Khimiya, Leningrad (1968). A. K. Covington, R. G. Bates, and R. A. Durst, Pure and Appl. Chem., 57, No. 3, 531 (1985). H. B. Kristensen, A. Salomon, and G. Kokholm, Analytical Chem., 63, No. 18, 885 (1991).

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