The Inorganic Carbon Complex
119
(b) To standardize the pH meter, place the electrodes in fresh buffer solution for 5 min and
replace in fresh buffer. Standardize by manufacturer's instructions, allow 2 min, and
restandardize. Repeat until no drift is apparent. Rinse copiously with distilled water,
followed by a rinse or two with the sample, and then place in some sample water for 5 min.
(c) Rinse the electrodes with sample water several times, replace with fresh sample water,
allow sufficient time for drift to stop, and make the reading. Remove the electrodes. Mixing
of the sample during measurements should not be done, or done cautiously, as a moderate
amount of stirring will reduce time for equilibration, but excessive mixing may generate
spurious electrical potentials, or enough CO 2 may be exchanged with the air to
significantly alter the pH.
2. Measurement of alkalinity: Using a number of water samples collected from natural waters,
e.g., a vertical profile in a stratified lake, determine the alkalinity of each sample. Replicate the
analyses from each sample and evaluate the variance (see Appendix 2). Compare your results
with those of other members of your class. What are the sources of error?
(a) Method 1: Gran alkalinity
(i) Pipet carefully 50 ml of the sample into a glass beaker. Add a small Teflon-coated stir
bar.
(ii) Measure the pH of the sample as described previously.
(iii) Add 0.025 ml ofO.1 N HCI. (An automatic pipet is recommended.) Mix the solution at
slow speed. Stop mixing and record the pH after it has stabilized.
(iv) Repeat step (iii) until the pH drops below 3.5. Depending on the volumes used, at
least five data points should lie between pH 4 and 3.5.
(v) Calculate the first Gran function Fl from:
Fl = (volume of sample + volume of acid added) * lO-pH
Example: the tenth pair of data in Table 8.1 were calculated after nine increments of
0.025 ml of acid were added. After converting to liters, the Gran function was
calculated from:
F 1 = (0.050 I + 0.0002251) x 10- 3.698
Fl = 1.007 x lO-5 equivalents
(vii) Using only the data where pH is less than 4, regress Fl as the independent y variable
versus the volume of acid added as the x variable. The correlation coefficient r should
equal or exceed 0.999, and the slope should be within lO% of the normality of the
acid used. The X intercept can be determined from the Y intercept and slope
from:
Y intercept
X intercept = - 1 * ----"slope
(viii) Calculate alkalinity from:
Alk = (normality of acid)(X intercept, liters)/(sample volume, liters)
Multiply by 1 x lO6 to convert units to Jleqjl.
(b) Method 2: Titration with color indicator solutions. Hydroxide and carbonate ions cause
high pH in water and result in values above 8.3, while bicarbonate ions in solution cause
pH values below 8.3. Hydroxide and bicarbonate ions do not exist together in the same
solution since they will react to form carbonate ions. Carbonate and hydroxide ions can
exist together, and bicarbonate and carbonate ions often are found together.
The organic indicator phenolphthalein changes color at about pH 8.3. When
phenolphthalein is added to a solution that contains carbonate or hydroxide alkalinity,
the solution turns pink. When acid then is added, the alkalinity will be reduced and the
pink color will disappear as the pH falls below 8.3. At this point the hydroxide is
neutralized.
119
(b) To standardize the pH meter, place the electrodes in fresh buffer solution for 5 min and
replace in fresh buffer. Standardize by manufacturer's instructions, allow 2 min, and
restandardize. Repeat until no drift is apparent. Rinse copiously with distilled water,
followed by a rinse or two with the sample, and then place in some sample water for 5 min.
(c) Rinse the electrodes with sample water several times, replace with fresh sample water,
allow sufficient time for drift to stop, and make the reading. Remove the electrodes. Mixing
of the sample during measurements should not be done, or done cautiously, as a moderate
amount of stirring will reduce time for equilibration, but excessive mixing may generate
spurious electrical potentials, or enough CO 2 may be exchanged with the air to
significantly alter the pH.
2. Measurement of alkalinity: Using a number of water samples collected from natural waters,
e.g., a vertical profile in a stratified lake, determine the alkalinity of each sample. Replicate the
analyses from each sample and evaluate the variance (see Appendix 2). Compare your results
with those of other members of your class. What are the sources of error?
(a) Method 1: Gran alkalinity
(i) Pipet carefully 50 ml of the sample into a glass beaker. Add a small Teflon-coated stir
bar.
(ii) Measure the pH of the sample as described previously.
(iii) Add 0.025 ml ofO.1 N HCI. (An automatic pipet is recommended.) Mix the solution at
slow speed. Stop mixing and record the pH after it has stabilized.
(iv) Repeat step (iii) until the pH drops below 3.5. Depending on the volumes used, at
least five data points should lie between pH 4 and 3.5.
(v) Calculate the first Gran function Fl from:
Fl = (volume of sample + volume of acid added) * lO-pH
Example: the tenth pair of data in Table 8.1 were calculated after nine increments of
0.025 ml of acid were added. After converting to liters, the Gran function was
calculated from:
F 1 = (0.050 I + 0.0002251) x 10- 3.698
Fl = 1.007 x lO-5 equivalents
(vii) Using only the data where pH is less than 4, regress Fl as the independent y variable
versus the volume of acid added as the x variable. The correlation coefficient r should
equal or exceed 0.999, and the slope should be within lO% of the normality of the
acid used. The X intercept can be determined from the Y intercept and slope
from:
Y intercept
X intercept = - 1 * ----"slope
(viii) Calculate alkalinity from:
Alk = (normality of acid)(X intercept, liters)/(sample volume, liters)
Multiply by 1 x lO6 to convert units to Jleqjl.
(b) Method 2: Titration with color indicator solutions. Hydroxide and carbonate ions cause
high pH in water and result in values above 8.3, while bicarbonate ions in solution cause
pH values below 8.3. Hydroxide and bicarbonate ions do not exist together in the same
solution since they will react to form carbonate ions. Carbonate and hydroxide ions can
exist together, and bicarbonate and carbonate ions often are found together.
The organic indicator phenolphthalein changes color at about pH 8.3. When
phenolphthalein is added to a solution that contains carbonate or hydroxide alkalinity,
the solution turns pink. When acid then is added, the alkalinity will be reduced and the
pink color will disappear as the pH falls below 8.3. At this point the hydroxide is
neutralized.