Calibration of pH meters, glass electrodes Calibration of pH meters, glass electrodes and conductometers at INPLand conductometers at INPL

Maruthupandi M Indian TN MDU

pHpH is defined as the negative decimal logarithm of the hydrogen ion activity aH.

pH scale is based on the pH values of a series of standard buffer solutions determined by the potentiometric technique.

Routine potentiometric measurements of pH are carried out by means of a pH meterpH meter equipped by a pH-indicator pH-indicator glass electrode,glass electrode, a reference electrodereference electrode and an automatic automatic temperature compensatortemperature compensator.

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The following cell is required for the pH measurements: 

glass electrode│test solution║bridge solution│ref. electrode, (1) 

where │ is the electrode-solution interface, and ║ is the liquid junction denoting an interface between the test solution and the salt bridge solution.

pH meterpH meter is a high impedance voltmeter. Direct pH readings are possible after adjustmentadjustment of a pH meter.

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If emf corresponding to two buffer solutions S1 and S2 with pHS1 and pHS2 are E1 and E2, respectively, the pH pH valuevalue of the unknown (test) solution can be calculated from its measured emf value E by the following formula:

1 ,2+2 1

pH pHS S E E1pH pH (E E ) pHS 1 SE E k'1 1

where k' is the practical slope of the glass electrode.

(2)

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The practical slope k' usually differs from the theoretical slope - Nernst coefficient - k = 2.3026 RT/F (R is the universal gas constant 8.31451 J·mol-1·K-1, T is the Kelvin temperature, F is the Faraday constant, 96485.309 Coulombs·mol-1).

The pH value to be determined should lie betweenbetween the pH values of the two adjacent buffer solutions.

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Some factors lead to systematic errorssystematic errors in pH measurement results, which can not be compensated by the adjustment procedure:

asymmetry potentialasymmetry potential of glass electrode,

liquid junction potentialliquid junction potential of cell (1),

clogging ofclogging of glass electrode diaphragmdiaphragm etc.

This problem can be solved by a calibration of the pH a calibration of the pH meter–electrodes assemblymeter–electrodes assembly.

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The IUPAC recommendations of the year 2002 require at establishing a traceability chaintraceability chain of the pH measurement results from the national metrological from the national metrological institutioninstitution level down to field, laboratory and industry measurements.

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The national primary standard of pH valuenational primary standard of pH value developed at INPL consists of a precise pH/ion-meter and the following electrochemical cell:

Ag/AgCl│3M KCl║3M KCl║Test sol-n│H2,Pt/Pt. (3)

The emf of cell (3) depends on the activity of hydrogen ions according to the Nernst equation:

E = EAg/AgCl - k·lg aH (4)

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Calibration of cell (3) is carried out using six primary six primary pH bufferspH buffers at 25°C. These buffers are prepared from the NIST standard reference materials with expanded uncertainties of ± 0.005 pH.

The national pH value standard have participated successfully in two interlaboratory international key interlaboratory international key comparisonscomparisons in the years 2002 and 2004 organized by the CCQM.

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Calibration of a pH-meterpH-meter (as a voltmeter).

Calibration of an automatic temperature automatic temperature compensatorcompensator (as a thermometer).

  Calibration of a glass electrodeglass electrode.

  Certification of buffers for pH- measurementsbuffers for pH- measurements..

These calibrations assure the shortest unbroken shortest unbroken traceability chaintraceability chain to the national measurement standards of Israel.

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Electrolytic conductivity () is the measure of a solution’s ability to conduct current. Reliable conductivity measurements require traceability to national or international measurement standards.

INPL has developed the such Israeli standard. 

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The standard is developed using the direct currentdirect current method and includes conductivity cell. This standard allows to determine values from 5ּ10-4 up to 10 S·m-1.

Calibration of the conductivity cell at fixed temperature is carried out using five standard conductivity five standard conductivity solutionssolutions from NIST.

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Conductometer calibrationConductometer calibration is based on the comparisoncomparison of the measuring results obtained by the tested conductometer and by INPL electrolytic conductivity standard.

In the year 2003 INPL took part in the iinterlaboratory nterlaboratory key comparisonkey comparison organized by the CCQM for solutions with low conductivity levels. 0.005 S·m-1. The results obtained were in good agreementgood agreement with the assigned values.

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Calibration of a conductometeconductometerr.

Calibration of a conductivity cellconductivity cell.

These calibrations assure the shortest unbroken shortest unbroken

traceability chaintraceability chain to the national measurement

standards of Israel.

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The national standards of pH and electrolytic

conductivity values have been developed and passed

successfully interlaboratory key comparisons..

INPL proposes calibration of corresponding

equipment based on these standards.

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Calibration of a glass electrodeCalibration of a glass electrode is performed by measuring of emf values with this electrode simultaneously with the reference hydrogen electrode in the same cell and buffer solutions.

The experimental slope SlHE of the linear calibration curve “E-pH”, measured with hydrogen electrode, and the similar slope SlGE of the tested glass electrode should be not less than 53 mV/pH53 mV/pH and not greater than 59.5 mV/pH59.5 mV/pH at 25oC. The ratio (SlGE / SlHE)100, % should be not less than 90%.

Dr. Elena Kardash - Calibration of pH meters, glass electrodes and conductometers at INPLmaruthupandi

In order to make a pH meter calibrationpH meter calibration independent of the vagaries of electrode performance, the calibration is conducted by the pH meter comparison with known d.c. potentiometer traceable to the national standard of volt.

Calibration of a automatic temperature compensatorCalibration of a automatic temperature compensator is done by comparison of temperature values measured using this compensator with simultaneous readings of the reference glass thermometer, traceable to the national temperature standard.

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