Slope and calculations

Several methods leading to the determination of the sample concentration Q of an ionic species i exist. In the presence of a TISAB solution (see section 19.1), that can fix the pH, these methods are based on application of relation 19.3. 

For a monovalent ion at 298 K, the slope factor S, that represents the term 2.303 RT/F is equal to 0.0591V (the theoretical slope). This is also the value of the gradient factor per pH unit for a glass electrode. 

In practice the calibration curve = f (Q) using a set of standard solutions, leads to the real slope (i.e. the experimental value of the slope factor), which is an indicator of the performance of the electrode system used. 

The following technique may be useful to eliminate the effects of the matrix. It requires only one standard solution and two measurements of potential. However, the volumes must be known with a great precision. First, the PD E is measured 

The calculation, starting from expression 19.4, leads to a new expression giving Cx  

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Given are various values of the slope and calculated amplification efficiencies. The optimal amplification efficiency is highlighted in boldface type. 

This is a better way, because by doing several replicate measurements, very reliable values can be obtained for both E and the slope, and calculation eliminates errors in reading off the concentration for a given potential. 

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