Alkalinity can be defined as buffering capacity of water to neutralize acid. In other words, it is a measure of how much the amount of acid can be added to water so that buffering materials can help to minimize drastic changes in pH. [1] Alkalinity in natural water is generally due to the presence of buffering materials such as carbonate ions (CO32-), bicarbonate ions (HCO3-) and hydroxide (OH-) which acts as buffer system in maintaining and resisting sudden changes in pH. [2]
[Alkalinity] = 2[CO32-] + [HCO3-] + [OH-] - [H3O+]
Buffers are able to absorb the excess amount of H+ present in the water and thus balancing the water pH. Maintaining an ideal level of alkalinity is pretty essential to avoid any harmful effects to human being and also aquatic life. For aquatic life, they may die if the pH drops below 5 or rises above 10. When the alkalinity is too low, anything that is encountered to water is readily and susceptibly from sudden shifts in pH which might fluctuate from 6 or lower to as high as 10 or above. On the other hand, waters with high alkalinity are more readily against changes in water pH which might fluctuate from about 7.5 to 8.5. Alkalinity is expressed in units of milligrams per liter (mg/l) of calcium carbonate (CaCO3) because the carbonate ion, CO32-, is its primary constituent.
When CO2 dissolves in water it forms a weak acid. This acid and its salts in solution constitute the carbonic system which forms a buffer system that resists pH change. The pH of water is controlled by the following reaction:
CO2 (aq) + H2O (l) ==> H2CO3 (aq)
H2CO3 (aq) ==> H+ (aq) + H3O+ (aq)
HCO3- (aq) ==> H+ (aq) + CO32- (aq)
If base is introduced, this will eventually uses up some OH- that was present in the buffer, causing the pH does not raise so much as compared with without buffer. If strong acid is added, the process will definitely uses up some H+ present, causing the pH does not drop so much as compared with without buffer. This can be illustrated using the equation shown below:
HCO3- (aq) + OH- (aq) ==> CO32- (aq) + H2O (l)
CO32- (aq) + H3O+ (aq) ==> HCO3- (aq) + H2O (l)
Phenolphtalein may be used for alkalinity titration to pH 8.3. However, methyl orange is used as an indicator for pH pf 4.3. Both of these can be indicated by the change of color from pink to colorless for Phenolphtalein and change of color from orange to red color for methyl orange.
3.1 Preparing 0.1 M sodium hydroxide solution
Apparatus and chemicals
Sodium hydroxide pellets (NaOH), 250ml beaker, and spoon spatula
Procedure
About 2± 0.05g of sodium hydroxide pellets were weighed quickly using 4 decimal points analytical balance in a 250mL beaker and transferred it to a 500mL volumetric flask. The sodium hydroxide pellets were diluted in the 500mL volumetric flask with 50mL of distilled water and mixed thoroughly. This solution was standardized using potassium acid phthalate solution.
3.2 Preparing primary standard potassium acid phthalate (KHP) solution for standardizing the o.1 M sodium hydroxide
Apparatus and chemicals
Dried potassium acid phthalate (KHP),three 250mL Erlermeyer flasks (Conical flask), pipette, and Phenolphthalein indicator.
Procedure
About 0.5±0.01g dried KHP was weighed using 4 decimal points analytical balance and all of the weights were recorded down. 75mL of distilled water was pipetted into each of the 250mL Erlermeyer flasks to dissolve the KHP. Lastly, 3 drops of phenolphthalein indicator were added into each of the 250mL Erlermeyer flasks.
3.3 Standardizing the 0.1M sodium hydroxide using potassium acid phthalate solution
Apparatus and chemicals
Buret, 25mL beaker, and double-burette clamp retort stand
Procedure
The buret was rinsed with three 10mL, portions of the sodium hydroxide that has been prepared in section 3.1 before poured the solution into it. The initial volume of the 0.1M NaOH in the burette was recorded. The KHP solution in the 250mL Erlermeyer flasks, which have been prepared in section 3.2 was titrated by using 0.1M NaOH in the burette until the faintest pink persisted for 30 seconds. Final volume of the 0.1M NaOH in the burette was recorded. The titration was repeated for the other two KHP solutions in 250mL Erlermeyer flasks using the same 0.1M NaOH solution.
3.4 Standardizing the 0.1M HCl using standardized 0.1M sodium hydroxide solution
Apparatus and chemicals
Buret, 25mL beaker, double-burette clamp retort stand, and 0.1M HCl solution (prepared by lab technician)
Procedure
25mL of 0.1M HCl was pipetted into three 250mL Erlermeyer flasks. 75mL if distilled water was pipetted into each of the 250mL Erlermeyer flasks. 3 drops of phenolphthalein indicator were added into each of the 250mL Erlermeyer flasks. Initial volume of the 0.1M NaOH in the burette was recorded. The HCl solution in the 250mL Erlermeyer flasks was titrated by using 0.1M NaOH in the burette until the faintest pink persisted for 30 seconds. Final volume of the 0.1M NaOH in the burette was recorded. The titration was repeated for the other two HCl solutions in 250mL Erlermeyer flasks using the same 0.1M NaOH solution.
3.5 Preparing sample solutions
Apparatus and chemicals
Sodium hydroxide, sodium carbonate, 250mL volumetric flask, distilled water, spoon spatula, 25mL beaker, and glass funnel
Procedure
About 1±0.05g of sodium carbonate and 0.5±0.05g of sodium hydroxide were weighed using 4 decimal points analytical balances in a 25mL clean and dry beaker. The exact weight of the sodium hydroxide and sodium carbonate in the beaker were recorded. The purity of the sodium hydroxide and sodium carbonate which stated on the chemical label was recorded. Some distilled water was added to the mixture and swirled the beaker until the mixture has totally dissolved in the water. The mixture solution in the beaker was poured into the 250mL volumetric flask and rinsed the beaker with distilled water for a few times and poured the rinsed water into the volumetric flask. The solution was diluted to 250mL marked with distilled water. The prepared sample was shaking and the sample solution was used for the determination of alkalinity in section 3.7 and 3.8.
3.6 Calibrate pH meter
Apparatus and chemicals
pH meter, buffer solution at pH 4 and 7, squeeze bottle with distilled water, and 500mL beaker used for filling the probe rinsed water.
Procedure
The power button was pressed to switch on the pH meter. The probe was rinsed with distilled water. The probe was dipped into the pH7 buffer solution and pressed "Cal" button. This followed by wait until the decimal point at the reading stop flashing (blinking). The probe was took out and rinsed with distilled water. The probe was dipped into the pH4 buffer solution and pressed "Cal" button. The probe was dipped into the pH7 buffer solution and pressed "Cal" button. This followed by wait until the decimal point at the reading stop flashing (blinking). The probe was took out and rinsed with distilled water. The probed was dipped into the sample solution and pressed "Read" button. The reading was waited to become stable and pressed "Read" button again to static the reading. The reading was recorded.
3.7 Determine the alkalinity of the prepared sample solution using indicators
Apparatus and chemicals
Buret, 25mL pipette, 250mL beaker, 250mL conical flask, calibrated pH meter, squeeze bottle with distilled water, standardized 0.1M HCl solution, phenolphthalein indicator, and methyl orange indicator
Procedure
25mL of prepared sample solution, which has been prepared in section 3.5 was pipetted into a 250mL Erlermeyer flask. 3-5 drops of phenolphthalein indicator were added in the 250mL Erlermeyer flask. The buret was rinsed with three 10mL portions of standardized 0.1M HCl acid solution before the acid solution was filled into the buret. The initial volume of this acid solution in the burette was recorded. The sample solution was titrated with the standardized 0.1M HCl to the endpoint and the final volume of this acid in the burette was recorded. The titration was repeated for two more times. The same procedure from step 1 to step 5 was repeated by using methyl orange as a indicator rather than phenolphthalein.
3.8 Determine the alkalinity of the prepared sample solution using pH meter (potentiometric method)
Apparatus and chemicals
Buret, 25mL pipette, 250mL beaker, calibrated meter, squeeze bottle with distilled water, and standardized 0.1M HCl solution.
Procedure
25mL of prepared sample was titrated into a 250mL Erlermeyer flask. The standardized 0.1M HCl was filled into the buret and the initial volume of this acid solution in the burette was recorded. The initial pH of the sample solution was measured using calibrated pH meter and the pH value was recorded. 0.1mL of the 0.1M HCl solution was added into the sample and swirled the sample before the sample pH was measured using calibrated pH meter. Step 4 was repeated until pH less than 2. The volume of the HCl added and also the pH value after each addition of 1mL of HCl solution was recorded.
