Hardness is one of the basic parameters of water, which we cannot completely remove in practice. Different hardness of water in the use of certain effects. For example, when the hardness of drinking water is too high, it will affect the health of the body, when the hardness of industrial water is too high, it will affect the quality of the product, and when the hardness of the water used in the laboratory is too high, it will affect the test results.radiation levels geiger counter Therefore, a detailed grasp of the detection of water hardness method is the basic skills of analysts. But we also need to understand the relevant precautions in the test, so as to ensure the accuracy of the final test results.

Precautions for water samples during hardness testing:

1. if the tested water samples are highly acidic or alkaline, you can use 5% sodium hydroxide solution and hydrochloric acid solution (1 + 4) neutralization and then add a buffer solution.

2. If the iron content in the water sample is greater than 2mg, aluminum is greater than 2mg, copper is greater than 0.01mg, manganese is greater than 0.1mg to the determination of interference, you can use 2mL of 1% L-cysteine hydrochloride solution and 2mL of triethanolamine solution (1+4) before adding the indicator to eliminate the interference of the joint masking. If it contains a small amount of zinc can be added β-amino ethanethiol 0.5mL for masking,digital nitrate test but if the content of zinc is high can be measured its specific value, in the total hardness can be subtracted.

Problems to be noted during the test operation:

1. water samples with high carbonate hardness should be diluted or 80%~90% of the required amount of EDTA standard solution should be added before adding the buffer solution (credited into the titration volume), otherwise calcium carbonate precipitates after the buffer solution is added and the end point drags.

2. When titrating water samples with low magnesium content, add a small amount of magnesium salt to the buffer solution, which can make the endpoint change sharply.

3. After the water sample is added to the buffer solution, titration must be done immediately to prevent precipitation of calcium and magnesium salts. If precipitation occurs, the water sample needs to be diluted and re-sampled for determination.

4. Tungsten light gives a purple color to the end point and should not be used. Titration should be done under natural light or fluorescent light.

5. Some metal ions discolor the chromium black T, or make the endpoint not obvious, can be overcome by adding an inhibitor before titration:

a. Add 250 mg of sodium cyanide to the solution being titrated, to which should be added a sufficient amount of buffer solution to give a pH of 10 ± 0.1. b. Dissolve 5.0 g of sodium cyanide in the solution being titrated.

b. Dissolve 5.0 g of sodium sulfide in 100 mL of water and stopper the bottle with a rubber stopper to prevent air from entering. Add 1mL for titration.

Add 1 mL of this solution to the indicator. c. Dissolve 4.5 g of hydroxylamine hydrochloride in 100 mL of 95% ethanol or isopropanol.

6. Titration should be carried out at room temperature, because the temperature is too low color changes more slowly, and the titration process should not exceed 5 min. If the hardness is high, the titration process can produce calcium carbonate precipitation. The water sample can be diluted,best water hardness test kit or 90% or more of the titrant can be added to the water sample before adding the buffer solution, or the water sample can be acidified before adding the buffer solution, and stirred for 2min to exclude carbon dioxide.

7. When titrating with EDTA near the end point, due to the slow complexation reaction, the titration should be slowed down and shaken well until the color changes.

8. If there is a mild color return, it can be titrated until no color return. If the color is serious, should take another water sample drop hydrochloric acid is acidic, heating to boiling, and then add ammonia to neutral and then normal operation.

The above points are in the test of water hardness need to pay attention to the problem.

By:Ariel

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• Different hardness
• hardness testing