Choosing an EMF Measuring Device

For the sake of clarity, the detailed equivalent circuit shown in Fig. 32 can be more simply represented. In Fig. 35, R represents the sum of all test circuit resistances and is designated source resistance. 

Ri represents the input impedance of the ion meter used. According to this simplified circuit, the electrode EMF is short-circuited by the input impedance if this impedance is substantially smaller than the source resistance. The error resulting from poorly matched resistances can be calculated with the following formula  

If anions were measured, the indicator would be deflected in the opposite direction. A few newer instruments have been designed specifically for use with ion-selective electrodes. These have controls which enable the operator to specify the type (anion or cation) and valency (mono- or divalent) of the indicated ion, so that the pH scale can be used directly without further calibration. Often a logarithmic scale is provided as well to allow direct indication of activities and concentrations (Fig. 36). An ion meter can 

Which of these possibilities one should choose depends upon the particular type of application. The analog-indicating amplifiers are the least expensive. This type of 

For greater precision, a better instrument must be used. Meters with analog display, but which have additional expanded scales belong to the next higher price range. With such devices the full scale can be used to represent 1.4 or 2.0 pH units. The zero point of the expanded pH or mV scale can be dialed in with a calibrated step switch on some instruments. Such scale expansions are operationally partial compensation processes. 

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