Alkalinity and Acidity Endpoints

We have been careful in the preceding discussion of alkalinity and acidity titrations and calculations to refer to the pH of the various endpoints as approximate values. The actual values that correspond to pHcoj, pHhcos-. and pHcOj - are not truly fixed values, rather they vary with theCj.coj in solution. If we treat the titrations as closed systems the Ct.coj at the endpoint will be the same as theCr.coj in the initial solution. This appears to be a reasonable approach if the solution is not shaken vigorously and if the titration is conducted rapidly. The variation in equivalence point pH values is shown in Fig. 4-20. 

The reader should keep in mind the difference between endpoint pH and equivalence point pH. The equivalence point pH is the theoretical pH at which a titration is complete while the endpoint pH is an approximation of the equivalence point pH. 

The equivalence point pH values can be calculated exactly using previously presented methods, especially the one employing the pC-pH diagram (e.g., see Section 4-12-2). Here we will examine approximate methods lor their calculation. For pHcog we will check the rationale behind the Standard Methods statement quoted previously. 

At the total alkalinity (and mineral acidity) equivalence point, we assume that there is present an H2CO3 solution of concentration Ct.coj which is equal to Ct.coj the sample prior to titration. Therefore, at the endpoint, 

For a total alkalinity of 50 mgAiter as CaCOs (1 meq/liter) and an original sample pH between 7.5 and 9, 

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