How to determine calcium level in water
Calcium, Titrimetric Method (0-200 ppm as CaCO3 )
The determination of calcium in a water analysis is closely allied to the determination of hardness, since hardness is caused primarily by the presence of calcium and magnesium.
Under certain circumstances, determination of calcium is made so as to subdivide the total hardness into calcium hardness and magnesium hardness. The total hardness minus the calcium hardness equals the magnesium hardness.
The determination of calcium usually is not made as a control test. However, this determination is necessary for making chemical calculations for treatment-chemicals. For example, where using internal boiler water treatment of the phosphate type, calcium hardness is precipitated by the phosphate where as magnesium hardness is precipitated as magnesium silicate or magnesium hydroxide. The same principle applies to the hot phosphate softener. Therefore, in calculating the quantity of materials required, it is necessary that the hardness be subdivided into calcium hardness and magnesium hardness.
In the treatment of cooling water, the determination of calcium as a control test again plays an important part. An important basis for all cooling water treatment is Langelier’s Saturation Index which predicts the tendency of a water to form calcium carbonate scale - the scale most easily formed in cooling water systems. In using the Langelier Index, the determination of calcium is required. On this basis, a plant control can be established.
Theory of Test
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.
Apparatus Required
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
Chemicals Required
Calcium Indicator
Hardness Titrating Solution, 1 ml = 1 mg. CaCO3
Sodium Hydroxide, 1.0N
Procedure For Test
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.
Calculation of Results
FORMULA:
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.
Limitations of Test
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.