Preparing Accurate Activity Calibration Solutions

Since this topic lies at the basis of all measurements with ion-selective electrodes and it is discussed too little in the literature, a pragmatic introduction to these mean activity coefficients will be supplied in some detail below. 

Let us start with a simple example and consider the chemical potential of a 1 1 valent electrolyte such as KCl. This is composed of the chemical potentials of the anions and cations. If we relate the mol ratios with and Xq-, then we obtain  

This sum conesponds to the work done in transferring 2 moles of ions (one mole of K and one mole of Cl ) from infinity into the solution. In order to refer to the chemical potential of 1 mole of transferred ions, one is talking about a mean chemical potential and mean activity coefficients/+, and Eq. (A7) must be divided by two  

For electrolytes which dissociate into v cations and v anions, in general  

If this equation is substituted into the theoretically derived Expression (A3), then 

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Several overall conclusions can be drawn based on the statistical evaluation of the data submitted by the participants of the DR CALUX intra-and interlaboratory validation study. First, differences in expertise between the laboratories are apparent based on the results for the calibration curves (both for the curves as provided by the coordinator and for the curves that were prepared by the participants) and on the differences in individual measurement variability. Second, the average results, over all participants, are very close to the true concentration, expressed in DR CALUX 2,3,7,8-TCDD TEQs for the analytical samples. Furthermore, the interlaboratory variation for the different sample types can be regarded as estimates for the method variability. The analytical method variability is estimated to be 10.5% for analytical samples and 22.0% for sediment extracts. Finally, responses appear dependent on the dilution of the final solution to be measured. This is hypothesized to be due to differences in dose-effect curves for different dioxin responsive element-active substances. For 2,3,7,8-TCDD, this effect is not observed. Overall, based on bioassay characteristics presented here and harmonized quality criteria published elsewhere (Behnisch et al., 2001a), the DR CALUX bioassay is regarded as an accurate and reliable tool for intensive monitoring of coastal sediments. 

This method is generally significantly less precise than the use of a calibration plot, but may often be more accurate if the composition of the sample matrix is unknown or too variable. It is in practice a calibration plot prepared in the individual sample solution itself and therefore under the exact, if unknown, conditions of the sample and with the same concentration of surface active agents etc. 

Calibration. Standard filters can be prepared by making a Fe(OH)3 precipitate of a U standard solution. In a small Teflon beaker add 50/d of 50mg/mL FeQs solution to a weighed uranium spike containing an accurately known activity Av of approximately 100 dpm (135 fig U), mix, add dilute (1 mol/L) NH3 solution while mixing until the pre-... 

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