What is the theoretical background of conductometry?

2. BASICS OF CONDUCTOMETRY2.2. Conductivity measurement in water forpharmaceutical use according to USP andPharm. Europe (EP)There are special requirements for the conductivitymeasurement in water for pharmaceutical use («waterfor injections») according to USP 645, EP 2.2.38resp. the latest EP -4.8-07/2004:0169. Besides a precisionconductometer whose temperature compensationcan be disabled, a conductivity measuringcell and a conductivity standard are to be used thatallow to determine the cell constant with a maximummeasuring error of 2%. To prevent the uptake of carbondioxide, the measurement should be carried underexclusion of air and/or in a flow cell. The samplefulfills the specification if one of the following threeconditions is met:Stage 1:The sample is measured directly without further pretreatmentand without temperature compensation. Ifthe water fulfills the specification indicated in table17, the test is considered as passed.Stage 2:If the conditions are not fulfilled by stage 1, continueas follows: The conductivity of at least 100 mL sampleis measured under strong agitation at 25 °C ±1 °C assoon as the drift caused by the uptake of carbondioxide is smaller than 0.1 μS/cm per five minutes. Ifthe measured value is smaller than 2.1 μS/cm, the testis considered as passed.Stage 3:However, if the sample does not fulfill the specificationof stage 2, a sample of exactly 100 mL is mixedwith 0.3 mL saturated KCl solution. Then the pH valueof this solution is measured exactly to 0.1 pHunits. Only if the conductivity fulfills the conditionsspecified in table 18 the test is considered as passed.Otherwise the water cannot be used for pharmaceuticalpurposes.Table 17: First step of the conductivity measurement according to USP 645 and EP -4.8-07/2004:0169Temperature Conductivity not Temperature Conductivity notlarger than (μS/cm)larger than (μS/cm)0 0.6 55 2.15 0.8 60 2.210 0.9 65 2.415 1.0 70 2.520 1.1 75 2.725 1.3 80 2.730 1.4 85 2.735 1.5 90 2.740 1.7 95 2.945 1.8 100 3.150 1.9Table 18: pH and conductivity criteria for stage 3pH Conductivity not pH Conductivity notlarger than (μS/cm)larger than (μS/cm)5.0 4.7 6.1 2.45.1 4.1 6.2 2.55.2 3.6 6.3 2.45.3 3.3 6.4 2.35.4 3.0 6.5 2.25.5 2.8 6.6 2.15.6 2.6 6.7 2.65.7 2.5 6.8 3.15.8 2.4 6.9 3.85.9 2.4 7.0 4.66.0 2.492