Method of Standard Addition with Subsequent Dilution

2 Method of Standard Addition with Subsequent Dilution 

With the double standard addition method it is not the electrode slope precisely at the sample concentration which is determined, but rather the slope in a somewhat more concentrated region (between Cx + AC and Cx + 2 AC), By diluting the solution after the first addition, a slope can be obtained much nearer to the sample concentration. In this procedure, as usual, a known amount of standard measured ion solution is added and the EMF change AEad is noted. For this a concentration change AC approximately that of the sample solution is chosen. To position the working point precisely in the middle, the solution is then diluted 1 1. The EMF change due to this dilution AEd is also noted. The slope of the electrode used can then be calculated with the following equation  

In Appendix 6 the Cxi AC values corresponding to various to-be-determined R values are tabulated. The evaluation is even easier if the expression (10 0-301 i)-i = A is tabulated as an increment factor depending on E ad and (see Appendix 7). 

The main advantage of this technique lies with quick survey measurements near the detection limit Using this technique 10 M fluoride solutions could be analyzed with 30% accuracy. This is quite noteworthy in view of the fact that the calibration curve begins to derivatefrom linearity with the fluoride electrode at 10 M - two orders of magnitude higher [228  

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Method of standard addition with subsequent dilution (according to Orion). 

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