Purpose: To calculate acid neutralizing capacity (ANC) by titrating weak bases with a strong acid. To calculate the various forms of alkalinity based on different titration conditions.
Materials:
Procedure:
1. Set up the titration equipment for a titration volumetric analysis and fill the burette with the standard 0.1 M HCl.
2. Obtain 20 ml samples of the 0.1 M Na2CO3 and 0.1 M NaHCO3 prepared solutions, and 500 ml of tap water or lake water.
3. Titration with indicators:
a. Add one or two pillows of phenolphthalein indicator to the sample with constant stirring
i) if the color of the solution remains clear, record a phenolphtalein alkalinity (P) of zero,
ii) if the color of the solution becomes red-pink, titrate with 0.1 M HCl until the solution becomes clear,
iii) record the ml of titrant consumed.
b. Add one or two pillows of brom cresol green methyl red indicator
i) if the clolor of the solution becomes red-pink, record the total alkalinity (T) as being equal to the P alkalinity,
ii) if the color of the solution becomes blue-green, continue titrating until the solution becomes red-pink,
iii) record the ml of titrant consumed.
Titration with a pH meter (You should use the color indicators and the pH meter simultaneously):
4. Set up the titration equipment and determine the starting pH.
5. Titrate by adding one ml increments of 0.1 M HCl, equilibrate by gentle stirring and record the pH.
6. Prepare a titration curve with ml of HCl consumed on the x-axis and pH on the y-axis.
7. Determine the inflection points. The first inflection point should occur at a pH of 8.3, which corresponds to the P alkalinity. The second inflection point will occur at a pH close to 4.5, which corresponds to the T alkalinity.
8. Record the ml of titrant consumed to reach the respective P and T alkalinities including that consumed in determining the P alkalinity.
9. Calculate alkalinity as follows:
alkalinity, mg CaCO3/l = (ml standard acid used X normality of standard acid X 50000)/ml sample.
10. Calculate hydroxide alkalinity, carbonate alkalinity, bicarbonate concentration as mg CaCO3/l using the following table:
ALKALINITY RELATIONSHIPS*
| Result of Titration |
Hydroxide Alkalinity as CaCO3 |
Carbonate Bicarbonate as CaCO3 |
Bicarbonate Concentration as CaCO3 |
| P = 0 | 0 | 0 | T |
| P < 1/2T | 0 | 2P | T - 2P |
| P = 1/2T | 0 | 2P | 0 |
| P > 1/2T | 2P - T | 2(T - P) | 0 |
| P = T | T | 0 | 0 |
*Key: P-phenolphthalein alkalinity; T-total alkalinity
Questions:
1. (10 points) If you are concerned about the effects of acid deposition on the health of a lake would you be more interested in the lake's acidity, pH, or alkalinity? Why?
2. (10 points) Convert your total alkalinity values from mg CaCO3/L to the proportions coming from hydroxide (mg OH-/L), bicarbonate (mg HCO3-/L), and carbonate (mg CO3-2/L) alkalinity.
3. (10 points) Does the pH of a solution effect its alkalinity? In other words, do solutions with higher pH's have greater buffering capacity?
4. (10 points) Why do you need a larger sample of French Creek water (500 ml vs. 20 ml for the carbonate solutions) when doing the titration?
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