What is the theoretical background of conductometry?

2. BASICS OF CONDUCTOMETRYTable 14: Conductivity Κ of various substances and solutionsConductorMetallic copperPotassium hydroxidesolution (c = 1 mol/L)KCl solution(c = 0.1 mol/L)Brackish waterAcetic acid(c = 1 mol/L)Drinking waterGraphiteDistilled waterUltrapure waterPure benzeneT (K)273291293273291298273273273273Conductivity caused byElectronic conductionIonic conduction resulting fromthe complete dissociation of KOHIonic conduction resulting fromthe complete dissociation of KClIonic conduction resulting fromthe dissociation of salts and carbonic acidIonic conduction resulting fromthe partial dissociation of CH 3 CH 2 COOHIonic conduction resulting fromthe dissociation of salts and carbonic acidElectronic conductionIonic conduction resulting from contaminationby salts, dissociation of water and carbonic acidIonic conduction resulting fromlow self-dissociationIonic conduction resulting fromthe dissociation of traces of waterConductivity Κ (μS cm –1 )645’000’000’000184’00011’66020’000...1’000’0001’30010...2’0001’2000.06...100.0560.00000005Conductometry is used for direct measurements andin titration. The theory is identical for both methods.Whereas in direct measurements it is the absolutevalue that is of interest, in titrations it is the change inthe measured value. Direct measurement is oftenused for monitoring surface waters, waterworks, waterdesalination plants and in the preparation of ultrapurewater, where particular limits must not beexceeded. Conductivity detection is mostly used forprecipitation titrations, where the equivalent point isrecognized by the conductivity reaching a minimumvalue. The absolute value is of secondary importance.Selecting the right cell constantThe cell constant c is defined for conductometricmeasuring cells. A measuring cell with two parallelelectrodes at a distance of 1 cm and each with anarea of 1 cm 2 theoretically has a cell constantc = l · A -1 = 1 cm -1 . The cell constant is never exactlyl · A -1 , as the electric field is not strictly homogeneous.The rules of thumb given in Table 15 are usedfor selecting the correct measuring cell:Conductivity measurementsWhereas the instruments used for potentiometryhave been standardized (input impedance >10 12 Ω,zero point at pH 7), this is not the case with conductometers.The influence of the cable capacity, themeasuring frequency level, the conductivity rangeand the adjustable cell constant, the method used forconductivity measurements (phase-sensitive, frequency-dependent,bipolar pulse, etc.) vary and dependon the type of instrument. This means that theinstrument must be taken into account for solvingapplication problems. Important parameters are:• Platinizing quality (platinum black) ➝ high seriescapacity C S• Electrode area A ➝ high series capacity C S• Cell constant c• Measuring frequency f• Cable capacity C P• Cable resistance R C• Instrument measuring range (resistance range)Figure 13:Cell constants and recommended conductivity intervals.90