Calcium Hardness

A drop-based test that measures the hardness in your water. Once measured, it is used to determine if there is a potential for scaling to the pool surfaces (too hard) or possible damaged to the plaster (too soft).

1. Rinse the sample tube with pool water.
2. Fill the sample tube with pool water to the 10 ml mark. The top of the sample will be curved. This curve is called a meniscus. The bottom of the meniscus should be level with the 10 ml mark.
3. Add 10 drops of R-0010 and swirl to mix.
4. Add 3 drops of R-0011L and swirl to mix. The solution should turn red, pink, or blue. If the sample turns blue, you are done and your CH level is zero.
5. Counting the number of drops as you go, add R-0012 one drop at a time, swirling to mix after each drop.
   • Continue adding drops until the color changes to something more or less blue. If the sample turns purple see the note on “fading endpoint” below.
     
   • Continue adding drops as long as the color continues changing. The final drop, that does not change the color any further, does not count.
     
   • If the sample starts to turn blue and then goes back to red/pink and stays there from several more drops this is also a “fading endpoint”, see the note below.
     
6. Multiply the number of drops by 25 to get your CH level. Remember that the final drop, which didn’t cause any further color change, doesn’t count.
7. Dispose of the sample safely. It is best to pour it down the drain with the water running. Do not add it back to the pool.
8. Rinse the sample tube with tap water and store for next time.

Notes

• Hold the dropper bottles vertically and squeeze gently, so that drops come out slowly and seem to hang on the tip of the dropper bottle for a moment before falling.
• If you expect that your CH level is very low, you can do the test so that each drop is 10, instead of 25, to get some extra precision. Use 25 ml of pool water, 20 drops of R-0010, 5 drops of R-0011L, and multiply the number of drops of R-0012 by 10 to get your CH level.
• Measuring high CH levels is much easier and more precise if you use a magnetic stirrer.
• The sample may turn purple during the test, or go to blue for a moment and then turn back to red/pink. This is called a “fading endpoint” and is caused by interference from metal ions. If this happens, do the test again, but this time add five drops of R-0012 before adding any R-0010 or R-0011L. Remember to count the initial five drops in the total.
  In extreme cases, a fading endpoint may occur even when adding five drops of R-0012 at the start. If that happens, mix pool water with an equal quantity of distilled water, test that, and then multiply the result by two.
• R-0010 is calcium buffer, a strong base which prevents interference from magnesium.
• R-0011L is calcium indicator, an organic dye used to provide the red/blue color. It should be a deep blue color. If the dye stains the plastic bottle it is stored in, it has gone bad.
• R-0012 is hardness reagent, used to titrate until the color changes.
• The precision of the measurement is plus or minus one drop when up to 10 drops of titrant are used, or plus or minus 10% of the final reading, when more than 10 drops of titrant are used.