Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 115

determinations of alkalinity, especially in neutral waters where there is no hydroxide
present, the only result required is 'total alkalinity'. Both types of alkalinity are usually
expressed in units of ppm (or mg L −1 ) calcium carbonate.
Sulphuric or hydrochloric acids are the usual acids used as titrants. Sulphuric is often
preferred because chloride can be titrated with silver nitrate in the same sample solution
after the alkalinity has been determined.
Several approaches to carrying out alkalinity titrations are possible.
(i) Preset endpoint
To measure 'total alkalinity' titrate to pH 4.5. If the sample pH at the start of analysis is
greater than pH 8.3, two methods can be used if desired. In the first method, titrate to pH
8.3. The result from the second titration is added to the first result to give the 'total
alkalinity'. The two methods can be linked together in a 'sequence'.
Nearly all standard methods for alkalinity titration are based on preset endpoint
techniques. The greatest advantage of the preset technique occurs when the samples are
complex, and buffered; in such samples it can be difficult to obtain clearly defined
inflexion points. The pH electrode is calibrated in buffer solutions before using the preset
endpoint technique.
(ii) First derivative, second derivative
First or second derivative techniques can be used for alkalinity titration if clearly defined,
reproductible inflexion points can be obtained. Evaluate the sample before finalising a
method. A good method of evaluation is to programme a maximum volume of 25ml of
titrant, and 10mV constant increment, terminate the titration when the pH value of the
sample solution has fallen to below pH 3.5. If two endpoints have been found, they
probably correspond to 'P and M alkalinity' as defined above. Confirm this by checking
the calculated pH values of each endpoint. If only one endpoint has been found, and the
pH of the solution at the start of the titration was below pH 8.3, the endpoint probably
corresponds to the total alkalinity.
(iii) Gran function
This is a powerful technique for measurement of alkalinity, but it has only recently
become possible to perform automatic titrations based on Gran functions. The weak
carbonate base is titrated with a strong acid. The result of the titration should indicate two
endpoints; the first corresponds to the disappearance of hydroxide plus the first half of the
carbonate as they are titrated with acid, and it represents the P alkalinity. The second
corresponds to the appearance of strong acid after all the alkalinity has been titrated, and
it represents the total alkalinity. The Gran technique is especially useful for resolving the
two endpoints when carbonate levels are low compared with hydroxide, and for titrating
low levels of total alkalinity. Non-continuous stirring is recommended if readings are
required in a quiescent solution; this tends to maximise stability of electrode reading.
Hillbom et al. [151] describe a probe which allows potentiometric titration of total
alkalinity to be automatically carried out in situ in non saline waters.
Automated methods of measuring alkalinity are typically based on the change of pH of
a buffer containing methyl orange. The methyl orange is buffered at a pH at the acid end
of the transition range of methyl orange. Additions of alkalinity cause the colour of the