How do you determine the hardness of water?

Answer

Dear Pieter,

I will deal with your question in two parts: a first part deals with the theory of hardness determination by means of the foam number and the second part contains a method of a practical hardness determination.

Theory

The hardness of water is determined by the amount of Ca²⁺ that is present in the water. You can determine the hardness by means of the foam number. This is the number of ml of soap solution you need to keep a solution foaming. A soap solution contains stearate ions (C₁₄huh₃₅COO^–), which together with Ca²⁺ form a precipitate (Ca(C₁₇huh₃₅COO)₂) according to the following response:

ca²⁺ + 2 C₁₇huh₃₅COO^–
-> Ca(C₁₇huh₃₅COO)₂

As soon as a little more soap solution has been added than necessary to remove all Ca²⁺
precipitate, the stearate ions can cause the soap to foam.

First you have to determine how much soap solution is needed to foam water with a certain hardness and then you can determine the hardness of the water to be tested.

Practical

It is not so easy to determine the hardness of water in your home. It takes a fair amount of time and you also need some specific products that may not be that cheap (most likely more expensive than the kit). The success of the test also strongly depends on the glassware you can use. Where possible I have suggested an alternative. If you wish to perform the test, always make sure that an adult assists you!

Necessities

Demineralized water (as used for the iron)

CaCl₂CaCl₂2H₂O OR CaCl₂6H₂O (available from the pharmacist)

Brown soap (liquid)

Measuring cylinder 100 ml (or other container for measuring volumes)

5 Erlenmeyer flasks or jars (glass container and sealable!)

Scale that can weigh small masses (minimum 0.5 g)

Burette (or syringe à Watch out for the needle!)

Bowl / jar to weigh in

1) Prepare standard solutions

The standard solution of 200 mg Ca²⁺ per liter is prepared by adding 0.55 g CaCl₂0.73 g CaCl₂2H₂O OR 1.1 g CaCl₂6H₂O to be dissolved in 1 l of demineralized water.

The standard soap solution is prepared by dissolving 50 g of brown soap in 1 l of demineralized water.

2) Prepare solutions with different concentrations of Ca²⁺

Put 50 ml of the Ca²⁺
standard solution in a graduated cylinder

Pour 25 ml from the graduated cylinder into Erlenmeyer flask 1 (or jar 1) and seal it (200 mg/l)

Add 25 ml of deionized water to the solution in the graduated cylinder and shake well.

Pour 25 ml from the graduated cylinder into Erlenmeyer flask 2 (or jar 2) and seal it (100 mg/l)

Add 25 ml of deionized water to the solution in the graduated cylinder and shake well.

Pour 25 ml from the graduated cylinder into Erlenmeyer flask 3 (or jar 3) and seal it (50 mg/l)

Add 25 ml of deionized water to the solution in the graduated cylinder and shake well.

Pour 25 ml from the graduated cylinder into Erlenmeyer flask 4 (or jar 4) and seal it (25 mg/l)

After thoroughly rinsing the graduated cylinder, place 25 ml of demineralized water in Erlenmeyer flask 5 (or jar 5).

3) Measuring the foam number of all solutions

Fill a burette or syringe with standard soap solution.

Take 5 Erlenmeyer flask (or 5 jar) with distilled water. Add 0.5 ml of soap solution from the burette. Put the stopper on the Erlenmeyer flask and shake vigorously for 5 seconds. Leave the Erlenmeyer flask for a while and see if there is a clear foam head on the water. If no foam head is visible, add another 0.5 ml of green soap solution until the foam head is visible. Note how many times you need to add 0.5 ml! The number of ml of soap solution you have used is the foam number of this solution. Note the foam number.

Do the same for Erlenmeyer flasks 1 to 4 (or jar 1 to 4). In this way you get the foam numbers of all solutions.

4) Establishing the calibration curve

Graph the results. Horizontally you put the concentration of Ca2+ in mg/l (so you have 5 points; 0, 25, 50, 100 and 200 mg/l). Place the foam numbers vertically.

Draw a straight line through these points: the calibration line. You will see that the line does not go through the zero point.

5) Determining the foam number of the solution to be tested and reading the concentration from the calibration curve

You can now determine the foam number of the solution to be tested (25 mL in Erlenmeyer flask/jar). In the graph you can then see the concentration of Ca²⁺
read.

If you like the Ca²⁺ concentration to German degrees of hardness (^ODH), then 1 °DH corresponds to 7.17 mg Ca2+ per liter.

If you have any further questions, I will do my best to answer them as best I can.

Sincerely,

Frederick