How to Determine Mineral Acidity With the Standard Method

Use a buret and stopcock to measure your titrant precisely.

Scientists measure the acidity or alkalinity of waste water from mines, sewage stations and the like by comparing it to water samples containing known levels of various minerals and impurities through titration. That means adding a base to an acid to determine how much it takes until the liquid reaches a balanced pH. This is the standard method of determining both acidity and alkalinity.

Things You'll Need

• Beaker
• Buret
• Stopcock
• Base with known normality value
• Indicator fluid

Instructions

1. Add your acidic solution to a beaker.

2. Add a base to the buret. This is your titrant and has a known base value, or normality. A weak base would be 8.0 or so.

3. Add a few drops of indicator to your solution. This should be one that will turn color when the pH reaches 7.0, or neither acid nor base. Phenol red will turn from yellow to red at about 7.0.

4. Slowly drip precise amounts of the base to your solution until the solution changes color.

5. Determine how much of the base was needed to turn the color to indicate a neutral liquid. This is the mathematical expression (normality of titrant) x (volume of the titrant) = (normality of the unknown amount of acid) x (volume of the unknown).

6. Determine the acidity of your solution. Because you know the quantity of liquid in your beaker, the normality of your titrant and the volume of titrant needed to turn the solution neutral, you can determine the acidity of your solution. For instance, if you add a base with 2.0 normality to 100 mL of your acidic solution, and it takes 29.3 mL of the titrant to reach equilibrium, the pH of your solution is .586, or mildly acidic.

References

• University of Massachusetts: Standard Methods for the Examination of Water and Wastewater
• Elsevier; Net Alkalinity and Net Acidity; Carl S. Kirby and Charles A. Cravotta III
• Virginia Community College System: Titratin, Hardness and Alkalinity