BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
Measurement of the End Point.
The end point of titration is usually amplified by adding a suitable indicator to the titrating flask.
An indicator is a chemical that changes its color when its chemical environment changes. For the
indicator to be effective this color change must be abrupt and must occur over a very small range
or chemical change.
Different indicators are used for different titrations, depending on the chemicals involved. For
acid-base titrations, phenolphthalein is commonly used. It is pink in alkaline solutions, but at pH
8 - 8.3, it becomes colorless. The titration is stopped at this point as virtually all the alkali will
have been neutralized at pH 8 – 8.3. The common indicators and the titrations for which they are
used are shown below.
Table 3-9
Commonly Used Indicators
Indicator Titration Color Change
Phenolphthalein Acid/base Pink @ pH>8 – 8.3, Colorless @ pH4, Yellow @ pH 8, Red @ pH 4, Blue @ pH