Method of Double Standard Addition

To derive the evaluating formula we again start with Eq. (60). An ion-selective electrode and a reference electrode dipping in a solution of unknown measured ion concentration Cx will exhibit an EMF according to  

Now a precisely known volume of a standard solution containing an accurately established measured ion concentration is added. Due to the concentration rise AC, the EMF reaches a new value E2  

So far this resembles the addition processes described above. Here, however, after noting this new EMF, a second equal portion of standard measured ion solution is added to the sample solution. If volume changes can be neglected, the final EMF is given by  

Naturally, with this double standard addition procedure the same precautions (constant ionic strength, constant degree of complexation, AE = const.) must be observed 

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Method of double standard addition (according co Orion). 

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