Test method of water hardness
① heating mode; (2) Lime soda method: use lime to reduce the hardness of temporary hard water, and use caustic soda to reduce the hardness of non-carbonate hard water; ③ ion exchange method: calcium and magnesium ions are removed by ion exchanger, which is commonly used in household water purifiers at present.
Principle: EDTA-2Na reacts with calcium and magnesium ions in water at PH 10 to form a colorless soluble complex, while chrome black T reacts with calcium and magnesium ions to form a purplish red complex. When calcium and magnesium ions are titrated with EDTA-2na to the end point, all calcium and magnesium ions are complexed with EDTA-2na, which makes chrome black T free and the solution turns from purple to blue.
In water quality monitoring, volumetric method is generally used to measure hardness, and EDTA titrates calcium and magnesium ions in water. You can also measure calcium and magnesium ions by atomic absorption spectrometry, and then add them to calculate the total hardness.
How to use hardness tablets:
1、? Take a clean container, wrap it with aluminum foil, break a piece with hard objects for hardness measurement, then add it all into the container and dissolve it with a small amount of water (V 1). At this time, the solution is blue;
2. Continue to add the water sample to be tested until the solution turns red (v2);
3. Calculate the volume of red solution V 1+V2=V, and check the volume hardness conversion table to get the total hardness value.
Matters needing attention in hardness determination of water quality
1.？ The national standard is based on the determination steps, and the most important thing is to refer to the national standard GB 1576-2008, which is available on this website and covers all kinds of low-pressure boiler water quality indicators, dosing and determination methods. See GBT6909 for the determination of hardness. Both these standards depend on it.
2.？ Detailed determination steps In order to make chemists master the hardness determination steps and make the determination results accurate, Bian Xiao and the members of the unit water quality management team made a determination score table, that is, the standard determination steps were refined and decomposed. This determination step has been used to train chemists for many years, which unified the details of the determination step and played a guiding role in practical application.
3.？ EDTA prepared from EDTA standard solution and ammonia-ammonium chloride buffer solution? The solution must be a calibrated standard solution. If it is simply prepared without calibration, the hardness measurement will be inaccurate. If the deviation of EDTA is 0.00 1mol/l, the corresponding measurement result is 0.01mmol/L. When preparing ammonia-ammonium chloride buffer solution, its hardness should also be compensated to make the determination result accurate.
4.？ Methods of washing bottles and measuring cylinders In order to ensure the accuracy of the determination results, bottle washing must be strictly observed: water bottles should be washed three times, conical bottles should be washed three times with distilled water, and measuring cylinders should be washed three times with water samples.
5.？ When using a measuring cylinder to take water samples, it should be noted that the concave surface of the measuring cylinder liquid is tangent to the scale. Moreover, when the water sample to be measured is poured into the conical bottle, it should be poured slowly along the bottle wall and stopped for a while after pouring, so that there is no residual liquid in the measuring cylinder.
6.？ Before titration, there will be a drop of residual liquid at the head of burette, which is not within the reading range. In order to ensure the accurate determination, this drop must be disposed of, that is, the bottle body of the conical bottle rests on this drop of liquid, disposed of, and then titrated.
7.？ Pay attention to slow titration because the complexation reaction time is slow, and the temperature of water sample affects the complexation reaction speed. Before titration, it is necessary to: heat the water sample to 30~40 degrees Celsius before titration; When titrating, slowly and continuously shake the conical flask in one direction, and pay attention to keep continuous titration until the water sample turns from purple to blue and reaches the titration end point.
8.？ When the end point of colorimetric titration is visually observed, stop titration and compare the color of the solution with white to see if the end point of titration is really reached. If there is still a trace of red in the solution, the titration end point has not been reached.
9.？ Reading and calculation formula If a 25ml acid burette is used, the reading should be read to two decimal places; If a micro burette is used, the reading should be accurate to three decimal places. Pay attention to units when calculating nested formulas.
Bian Xiao will introduce you to the testing method of water hardness. I believe you will know how to test the hardness of water after reading it. In fact, it is very important to master this method, which will help you distinguish the composition of water, so that you will not get sick if you drink high-hardness water. Water hardness testing involves many common sense problems. Therefore, we need to master a lot of common sense questions, which is convenient for us to test the hardness of water quality. Maybe you didn't understand this method before. If you need to meet the problem of water hardness in the future, you can test it according to the method introduced by Bian Xiao.