How to determine hardness of water
Hardness, Titration Method (0-200 ppm) Theory of Test
This test is based on the determination of the total calcium and magnesium content of a sample by titration with a sequestering agent in the presence of an organic dye sensitive to calcium and magnesium ions. The endpoint, which occurs when all the calcium and magnesium ions are sequestered, is observed as a red to blue color change.
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Apparatus Required
Buret, automatic, 25 ml
Casserole, porcelain, 210 ml
Cylinder, graduated, 50 ml
Measuring dipper, brass
Stirring rod, glass
Chemicals Required
Hardness Buffer Reagent Hardness Indicator
Hardness Titrating Solution, 1 ml = 1 mg CaCO3
Procedure for Test
Measure 50 ml of sample and transfer to the casserole. Adjust the sample, if necessary to a pH range of 7 to 10 with either dilute ammonium hydroxide or dilute hydrochloric acid. With the brass measuring dipper, add one level measure (0.2 gram) of hardness buffer reagent to the sample and stir. Add one level measure of hardness indicator and stir. If hardness is present, the sample will turn red. Add the hardness titrating solution slowly from the buret, with continues stirring.
When the endpoint is approached, the sample will begin to show some blue coloration, but a definite reddish tinge can still be observed. The endpoint is the final discharge of this reddish tinge further addition of the hardness titrating solution will not produce any further color change.
With the procedure given above, the hardness titrating solution must be added slowly since the endpoint is sharp and rapid. For routine hardness determination, it is suggested that 50 ml of sample be measured, but only add approximately 40 to 45 ml to the casserole at the start of the test.
Add the hardness buffer reagent and hardness indicator as described above and rapidly titrate to the endpoint. Then add the remaining portion of the sample. The hardness present in the remainder of the sample will turn the contents of the casserole red again. Continue titrating slowly until the final endpoint as indicated above is reached. Record the total number of milliliters of hardness titrating solution used:
Calculation of Results
FORMULA:
ppm hardness as CaCO3 = ml of titrating solution x 1000/ml sample
Using 50-ml sample, the hardness in parts per million as CaCO3 is equal to the ml of titrating solution used multiplied by 20.
Limitations of Test
This method of hardness determination can be used to titrate hardness values up to 1200 ppm without dilution with an accuracy of 2 per cent. Low hardness concentrations from 0.1 to 5 ppm can be determined with an accuracy of only 10 per cent.
Interference results with the presence of sodium hydroxide over 300 ppm, copper over 10 ppm, iron over 20 ppm, magnesium over 2 ppm, aluminium over 20 ppm orthophosphate over 100 ppm and polyphosphate over 25 ppm. Strontium titrates as hardness. Other ions in the concentration normally encountered in industrial water conditioning produce no interference.
The effect of interfering ions can be reduced by dilution of the sample. If interference occurs, measure a 25 ml sample, dilute with 25 ml of distilled water and proceed in the normal fashion. Multiply results by 40 instead of 20 as described under “calculation of results”. This procedure will reduce any interfering ions, usually enabling a sharp endpoint to be obtained.
When waters contain free mineral acid, such as the effluent of a hydrogen zeolite, the sample should be neutralized with 1N sodium hydroxide.
In the presence of certain ions, the color change may not be from red to blue. For example, 500 ppm chromate causes no direct test interference, but the endpoint is from red to green due to the color of the chromate. The trace endpoint in each case taken as the disappearance of the reddish tinge of the sample, regardless of the final endpoint color.
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