120
Exercise 8
When phenolphthalein is added to water and no pink reaction appears, only
bicarbonate alkalinity is present. Thus, the phenolphthalein alkalinity measures all the
hydroxide alkalinity and one-half of the carbonate alkalinity. To determine the other half
of the carbonate alkalinity and the bicarbonate alkalinity, more acid is added until a pH
change indicates that the bicarbonate has been dissociated and released as CO 2 to the
atmosphere during titration.
Pipet carefully SO ml ofthe sample into a Erlenmeyer flask over a white titration surface.
Add four to five drops of phenolphthalein indicator. If pink, add N/SO (0.02N) sulfuric acid
slowly until the pink color disappears upon stirring. Note the volume of acid used; this
volume corresponds to the equivalence point of pH 8.3 and is required to calculate
phenolphthalein alkalinity.
Then add three to four drops of mixed bromcresol green-methyl red indicator (or
methyl orange) to the same sample. Continue to slowly add N/SO sulfuric acid to the
appropriate equivalence point.
It has been shown that methyl orange usually is unsuitable for the determination of low
alkalinities. Errors arise largely from the fact that the first perceptible color change with
methyl orange occurs at a pH of 4.6. Many individuals possess imperfect color perception,
rendering it almost impossible to properly identify the faint orange color characteristic of
methyl orange at a pH of 4.6. Often the sample is overtitrated to a deeper orange or faint
pink, representing a pH as low as 4.2S, with a consequent sacrifice in accuracy.
For most fresh waters the pH of the end point can be estimated from:
(II)
[see Stumm and Morgan (1981)]. The following pH values are suggested as the
equivalence points for the corresponding alkalinity concentrations as calcium carbonate
(American Public Health Association et aI., 1989):
Alkalinity (as mg/I CaCO J )
< 30-30
1S0
SOO
pH
5.1
4.8
4.5
Thus, methyl orange under the most favorable circumstances is justified only for
solutions of alkalinity in excess of 150 ppm. A mixed indicator prepared from bromcresol
green and methyl red is suitable for the high pH end points, while methyl orange can be
used for those below 4.6. It is advisable to prepare buffer solutions of the applicable pH,
add the proper volume of indicator, and use these solutions as standards for color
comparison for the various indicator color transitions.
Sample volumes requiring less than 50 ml of titrant yield the sharpest color change at
the end point and are, therefore, recommended.
The mixed indicator yields the following color responses:
pH
Color
> S.2
Greenish blue
5.0
Light blue with lavender gray
4.8
Light pink gray with bluish cast
4.6
Light pink
Note the total volume of acid used; this is required for the calculations of total alkalinity.
Phenolphthalein alkalinity as peq/I =
(
volume of standard acid ) x (normality of acid x 10 6 )
used to first end point (ml)
volume of sample (ml)
Exercise 8
When phenolphthalein is added to water and no pink reaction appears, only
bicarbonate alkalinity is present. Thus, the phenolphthalein alkalinity measures all the
hydroxide alkalinity and one-half of the carbonate alkalinity. To determine the other half
of the carbonate alkalinity and the bicarbonate alkalinity, more acid is added until a pH
change indicates that the bicarbonate has been dissociated and released as CO 2 to the
atmosphere during titration.
Pipet carefully SO ml ofthe sample into a Erlenmeyer flask over a white titration surface.
Add four to five drops of phenolphthalein indicator. If pink, add N/SO (0.02N) sulfuric acid
slowly until the pink color disappears upon stirring. Note the volume of acid used; this
volume corresponds to the equivalence point of pH 8.3 and is required to calculate
phenolphthalein alkalinity.
Then add three to four drops of mixed bromcresol green-methyl red indicator (or
methyl orange) to the same sample. Continue to slowly add N/SO sulfuric acid to the
appropriate equivalence point.
It has been shown that methyl orange usually is unsuitable for the determination of low
alkalinities. Errors arise largely from the fact that the first perceptible color change with
methyl orange occurs at a pH of 4.6. Many individuals possess imperfect color perception,
rendering it almost impossible to properly identify the faint orange color characteristic of
methyl orange at a pH of 4.6. Often the sample is overtitrated to a deeper orange or faint
pink, representing a pH as low as 4.2S, with a consequent sacrifice in accuracy.
For most fresh waters the pH of the end point can be estimated from:
(II)
[see Stumm and Morgan (1981)]. The following pH values are suggested as the
equivalence points for the corresponding alkalinity concentrations as calcium carbonate
(American Public Health Association et aI., 1989):
Alkalinity (as mg/I CaCO J )
< 30-30
1S0
SOO
pH
5.1
4.8
4.5
Thus, methyl orange under the most favorable circumstances is justified only for
solutions of alkalinity in excess of 150 ppm. A mixed indicator prepared from bromcresol
green and methyl red is suitable for the high pH end points, while methyl orange can be
used for those below 4.6. It is advisable to prepare buffer solutions of the applicable pH,
add the proper volume of indicator, and use these solutions as standards for color
comparison for the various indicator color transitions.
Sample volumes requiring less than 50 ml of titrant yield the sharpest color change at
the end point and are, therefore, recommended.
The mixed indicator yields the following color responses:
pH
Color
> S.2
Greenish blue
5.0
Light blue with lavender gray
4.8
Light pink gray with bluish cast
4.6
Light pink
Note the total volume of acid used; this is required for the calculations of total alkalinity.
Phenolphthalein alkalinity as peq/I =
(
volume of standard acid ) x (normality of acid x 10 6 )
used to first end point (ml)
volume of sample (ml)