The acid-neutralizing capacity of a water sample is measured by its Alkalinity. It is a cumulative property of the water sample and can only be explained in terms of specific substances. The alkalinity of water is primarily due to the carbonate, bicarbonate, and hydroxide content. Sometimes, borates, phosphates, silicates, or other bases also contribute to alkalinity. These alkaline substances play the role of a buffer in water against sudden changes in pH. Alkalinity is important as an indicator of a water bodys ability to resist pH change with the addition of acid or an accidental spill.
Alkalinity is primarily of two types, Total and Phenolphthalein Alkalinity.
Phenolphthalein alkalinity is present only when free carbon dioxide (CO2) is absent and therefore exists only when the pH exceeds 8.3. Phenolphthalein alkalinity should never be over half the total alkalinityotherwise, a caustic alkalinity is produced.
Total Alkalinity and Phenolphthalein Alkalinity are defined as follows
EMBED Equation.3
Determination
1.) Measure 100 mL of your sample into a 250 mL. Insert a bar magnet and place sample onto a stir plate.
2.) Note initial pH of the sample. If the pH is above 8.3 add several drops of phenolphthalein indicator. Titrate your sample with 0.02 N H2SO4 or HCl until the pH endpoint (color change) is reached. This is the phenolphthalein alkalinity. Record the total volume of acid needed to reach the endpoint.
3.) If pH is below 8.3, add a few drops of bromcresol green indicator. Titrate your sample with 0.02 N H2SO4 or HCl until the pH 4.5 endpoint (color changes from blue to yellow) is reached. Note the total volume of acid needed to reach the endpoint.
4.) The total alkalinity of the sample can be calculated using
Where A is the total volume in mL of the standard acid used N is the normality of the standard acid used 50,000 is a conversion factor to change the normality into units of mg CaCO3L Analysis of Results (assuming all of the alkalinity is due to carbonate, bicarbonate, or hydroxide)
Alkalinity is primarily of two types, Total and Phenolphthalein Alkalinity.
Phenolphthalein alkalinity is present only when free carbon dioxide (CO2) is absent and therefore exists only when the pH exceeds 8.3. Phenolphthalein alkalinity should never be over half the total alkalinityotherwise, a caustic alkalinity is produced.
Total Alkalinity and Phenolphthalein Alkalinity are defined as follows
EMBED Equation.3
Determination
1.) Measure 100 mL of your sample into a 250 mL. Insert a bar magnet and place sample onto a stir plate.
2.) Note initial pH of the sample. If the pH is above 8.3 add several drops of phenolphthalein indicator. Titrate your sample with 0.02 N H2SO4 or HCl until the pH endpoint (color change) is reached. This is the phenolphthalein alkalinity. Record the total volume of acid needed to reach the endpoint.
3.) If pH is below 8.3, add a few drops of bromcresol green indicator. Titrate your sample with 0.02 N H2SO4 or HCl until the pH 4.5 endpoint (color changes from blue to yellow) is reached. Note the total volume of acid needed to reach the endpoint.
4.) The total alkalinity of the sample can be calculated using
Where A is the total volume in mL of the standard acid used N is the normality of the standard acid used 50,000 is a conversion factor to change the normality into units of mg CaCO3L Analysis of Results (assuming all of the alkalinity is due to carbonate, bicarbonate, or hydroxide)
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