How to determine calcium level in water

 

This test is based on the determinations of the calcium content of a sample by titration with a sequestering agent in the presence of an organic dye sensitive to calcium ions. The pH is made quite basic so that magnesium is precipitated as the hydroxide, and is therefore removed as an interference in the test. The endpoint occurs when all the calcium ions are sequestered and is observed as a color change from salmon-pink to orchid-purple.

 

Burettes Automatic 25 ml - 1

Casserole Porcelain 210 ml.- 1

Cylinder Graduated 50 ml - 1

Measuring Dipper Brass - 1

Pipet Safety Bulb 2 ml - 1

Stirring Rod Glass       - 1

 

Calcium Indicator

Hardness Titrating Solution, 1 ml = 1 mg. CaCO3

Sodium Hydroxide, 1.0N

 

Measure 50 ml of sample and transfer to casserole. Add 2 ml of 1.0N sodium hydroxide to sample and stir. With the measuring cup provided, add one level measure (approximately 0.2 gram) of calcium indicator and stir. Water solutions of the indicator are not stable for longer than a day. Thus, a dry blend of the dye and an inert ingredient is used. The solution should be titrated immediately after adding the indicator because it is unstable under alkaline conditions. If calcium  is present, the sample will turn salmon-pink. Add the hardness titrating solution (same titrating solution as used in hardness test) from the buret with continued stirring. When approaching the  end point, the sample begins to show a purple tinge. The end point is a final change to orchid- purple. Once the end point is reached further addition of hardness titrating  solution will not produce any further color change. Always check the end point by adding one additional drop of hardness titrating solution and observing if any further colour change occurs.

 

ppm calcium as CaCO3 = ml titrating solution x 1000ml sample

 

Using a 50 ml sample, the calcium in parts per million as CaCO3 is equal to the ml. of titrating solution used multiplied by 20.

 

The magnesium hardness can be determined by subtracting the calcium hardness from total hardness.

 

This method is substantially free from ionic interference in the concentrations normally encountered in industrial water conditioning. By dilution of the sample, ionic interference can be minimized. Orthophosphate will precipitate calcium at the pH of the test. Alkalinity in excess of  30 ppm may cause an indistinct end point with hard waters. Analyze hard waters with high alkalinity by taking a smaller portion and diluting to 50 ml or by neutralizing the alkalinity with acid, boiling one minute and cooling before beginning the titration. Because of the accuracy possible with both the hardness and calcium methods, a reliable magnesium value can be secured by difference.