IEC 60746-2 pdf download – Expression of performance of electrochemical analyzers – Part 2: pH value

IEC 60746-2 pdf download – Expression of performance of electrochemical analyzers – Part 2: pH value

IEC 60746-2 pdf download – Expression of performance of electrochemical analyzers – Part 2: pH value
1 scope
This lnternational Standard is intended:
to specify terminology,definitions and requirements for statements by manufacturers foranalyzers,sensor units and electronic units used for the determination of the pH ofaqueous solutions;
to establish performance tests for such analyzers,sensor units and electronic units;
to provide basic documents to support the applications of quality assurance standards lSo9oo1, Iso 90o2 and lso 90o3.
2Normative reference
The following referenced documents are indispensable for the application of this document.For dated references,only the edition cited applies. For undated references, the latest editionof the referenced document (including any amendments) applies.
IEC 60746-1:2002,Expression of performance of electrochemical analyzers – Part 1: GeneralIso 9001,Quality management systems – Requirements
Iso 9002,Quality systems – Model for quality assurance in production,installation andservicing
Iso 9003,Quality systems – Model for quality assurance in final inspection and test
3Terms, definitions, symbols and abbreviations
3.1 Terms and definitions
For the purposes of this part of lEC 60746, the definitions given in Clause 3 of lEC 60746-1,as well as the following apply.
pH value
A measure of the conventional hydrogen ion activity a日+ (see equation (1)), in an aqueoussolution given by the expression:
pH =-log aH+
lt is measured with respect to pH values assigned to certain reference pH buffer solutions.The measurement is performed by determining the e.m.f., E, between a pair of electrodesimmersed in the sample to be measured, according to the cell scheme:
Reference electrode l Sample l pH electrode
and a measurement with the same electrode pair at the same temperature in a referencebuffer solution of pH(S1) according to
The e.m.f.s E(S1),etc. are defined as the difference of the potential of the right-hand (pH)electrode minus the potential of the left-hand (reference) electrode.
The pH of the sample is then given ideally by:
pH = pH(S,)-B一E(S
where k = 2,3026 R.T/F, the theoretical,Nernstian,slope (see 3.1.2) .
Numerical values for k, the theoretical slope factor,at temperatures from 0 °C to 95 °C, aregiven in Annex A.
NOTEMeasurements in non or partially aqueous media are beyond the scope of this document; the reader shouldrefer to specialist texts.
practical slope factor and percentage theoretical slopePrs
performance of the electrode pair may fall below the theoretical slope k exhibiting the practicalslope k which may be determined by replacing the sample with a second reference buffersolution of pH value pH(s2) with an e.m.f.E(S2), then:
E(S2)-E(s,)k=ES2)- s1)
NOTEThe difference in pH value between the two reference buffer solutions should be as large as possible,however,solutions above pH 10 and below pH 3 should not generally be used (see Annex B).
The percentage theoretical slope (PTS) is given by:
PTs -100 k’
Equations (1) and (2) can be combined by substituting k for k in equation (1) where:
pH = pH(s,)_ [E-E(S)]-lbpH(Sz)-pH(sz)
and the two reference buffers are usually chosen to bracket the pH of the sample.
pH sensor
the most commonly used pH sensor is the glass electrode,other potentiometric sensors, forexample, the antimony electrode only being adopted when its use is precluded.The pH isfet(ion selective field effect transistor) sensor is an alternative to potentiometric sensors,necessitating manufacturer-specific instrumentation.
reference electrode
appropriate half-cell providing a stable potential at constant temperature against which thepotential of the pH sensor is measured.Electrical contact with the sample is made at a liquid-junction with the reference electrolyte or an interposed bridge solution.
temperature compensator
electrical sensor in thermal contact with the sample providing the means for temperaturecompensation
sensor unit
insertion or flow-through housing into which pH and reference sensors, as well as usually, atemperature compensator (see 4.3.4) and possibly auxiliary devices (see 4.3.5) are fitted.
zero point pH
pH value at which the e.m.f. of the electrode pair (sensor unit) is 0 v at a given temperature,unless otherwise stated, understood to be 25Rc.
isopotential pH, pH, , of the electrode pair (sensor unit)
pH,pH , at which the e.m.f.,E, of the electrode pair is temperature invariant. lt is a functionof the temperature coefficients of the individual electrodes and provides temperaturecompensation for the electrode pair zero shift with appropriate instrumentation.
alkaline (or sodium) error of the glass electrode
error of the e.m.f. caused by sensitivity of pH glass electrodes to alkali ions at high pHresulting in apparent low pH values. Major interferences are Na* > Li* > K+ > Ba2+.Errorsincrease with increasing alkali concentration,pH and temperature. The magnitude isdependent on the glass membrane composition.
reference buffer solution
aqueous solution prepared according to a specific formula using recognized analytical gradechemicals and water having a conductivity no greater than 2 uS·cm’ at 25 °C(see Annex B)