Lachat Method: Alkalinity 10-303-31-1-D
Methyl orange is used as a color reagent for this method because its pH range is the same as the pH of the equivalence point for a total alkalinity titration. The methyl orange indicator is in a dilute pH 3.1 buffer which is just below its color change pH. When an alkaline sample is injected, the poorly buffered methyl orange changes color in proportion to the change in pH of the weak buffer, and thus in proportion to the alkalinity of the sample.
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