Volumetric analysis of drugs - Essentials of Pharmaceutical Chemistry - page 144

Indicator

pH 1 2 3 4 5 6 7 8 9 10 11 12 13

Red
Red
Red

Yellow Pink

Grey

Red

Cresol Red
Metacresol Purple

Orange

Yellow Violet

Orange

Yellow Tropaeolin OO Orange

Grey

Red Yellow Violet-blue Thymol Blue

Mentanil Yellow

Orange

Red Yellow Orange

Pale
Yellow

Colourless Yellow

Naphthol Yellow

Yellow

Bromophenol Blue Grey Blue-violet

Red Yellow

Methyl Orange Orange

Methyl Orange-Xylene
Cyanol FF
Dimethyl Yellow

Violet Green Grey

Red
Blue

Yellow

Orange

Red

Congo Red
Bromocresol Green

Violet

Yellow Blue

Green

Methyl Red

Red Yellow

Orange

Red Blue Litmus Violet

Grey Yellow Blue-violet

Bromocresol Purple
Bromothymol Blue Yellow Blue

Orange

Green
Orange-
red

Neutral Red

Red
Yellow Red Pink

Phenol Red

Pale red Blue Green

1- Naphtholphthalein
Phenolphthalein Colourless

Colourless
Colourless

Red Pink

Pale
blue

Blue Thymolphthalein

Yellow

Pale yellow

Alizarin Yellow GG
Tropaeolin O

Yellow
-orange

Yellow Orange

Yellow Red

Titan yellow

Orange

Figure 6.3 A table of the pH ranges of indicators.

Back and blank titrations

In the example above, a reaction was chosen that was quick to carry out
and was quantitative, i.e. it went to completion. In many pharmaceutical
analyses this is not the case and a back titration has to be carried out.
Back titrations are often combined with blank titrations , particularly
if there is some loss of reagent during the assay (e.g. as a result of splashing
or vigorous boiling) or the concentration of a volumetric reagent changes
during the assay. A back titration involves addition of a known excess of
reagent to the sample (this drives the reaction to completion) and titration
of the unreacted excess of reagent with a suitable titrant. The volume that