China's sanitary standard for domestic water stipulates that the total hardness of water shall not exceed 25 degrees. China has a vast territory, and the hardness of water varies from place to place. The suitable hardness of general drinking water is 10 ~ 20 degrees. At present, the hardness of water is determined by EDTA coordination titration [1, 2]. EDTA complexometric titration is a widely used method to determine the hardness of water. Under certain conditions, using chrome black T as indicator and NH3·H2O-NH4Cl as buffer solution, EDTA forms a stable complex with calcium and magnesium ions, thus determining the total amount of calcium and magnesium in water. This method is simple and rapid, and can be used to determine the hardness of different water quality with satisfactory results.
1 material
1. 1 instrument pipette, burette, volumetric flask, AT-26 1 electronic balance and other conventional glass instruments.
EDTA standard solution of 1.2 reagent, ethanol solution of chrome black T, triethanolamine, zinc oxide, dilute hydrochloric acid and methyl red, NH3·H2O-NH4Cl buffer solution with pH of about 10, anhydrous ethanol and high-purity distilled water.
2 methods and results
2. 1 Principle The hardness analysis adopts EDTA method, with chrome black T as indicator, and its determination is called 10mg calcium oxide in 1L water.
Na2[H2Y]+Ca2+→Ca[H2Y]+2Na+
EDTA disodium EDTA calcium complex
(colorless) (colorless)
The disodium salt of EDTA (ethylenediamine tetraacetic acid) can form a stable colorless and soluble complex with calcium salt in water under the condition of controlling the pH value to 10 with ammonia-ammonium chloride buffer. Chrome black T indicator combines with calcium salt in water to form wine red chrome black T calcium complex.
Ca2 ++ H2T→ Cat +2H+
Chrome black t chrome black t calcium
(blue) (Burgundy)
Because the coordination ability of EDTA with calcium is stronger than that of chrome black T, EDTA will coordinate with calcium in chrome black T calcium complex after complexation, making the solution change from wine red to pure blue, that is, showing the end point.
2H ++ 2na 2[H2Y]+CaT+Ca2+→2Ca[H2Y]+H2T+4Na+
(colorless) (burgundy) (colorless) (blue)
2.2 reagent preparation
2.2. 1nh3H2O-NH4Cl buffer solution (pH≈ 10): weigh 13.5g ammonium chloride, dissolve it in water, add 88ml ammonia water, and adjust the volume to 250ml with water.
2.2.2 Chrome Black T indicator Dissolve T0.2g chrome black in 15ml triethanolamine, and then add 5ml anhydrous ethanol.
2.2.3 prepare 0.05 mol L- 1 EDTA standard solution, weigh 9.5g disodium EDTA, add appropriate amount of water to heat and dissolve, prepare 500ml aqueous solution, and shake well.
2.2.4EDTA calibration EDTA standard solution accurately weigh about 0. 1g of constant weight ZnO that has been burned at 800℃, add dilute hydrochloric acid to dissolve it, add distilled water to dilute it, add methyl red, adjust the solution to light yellow with ammonia water test solution, add NH3·H2O-NH4Cl buffer solution and chrome black T indicator, and titrate with the prepared EDTA solution until the solution turns from purple to pure blue. Its concentration can be calculated according to the following formula. See table 1 for the results.
ce DTA = wzno× 1000 vedta×MZnO
Table 1 EDTA standard solution concentration
2.2.50.0102mol l-1preparation of EDTA standard solution accurately suck 20ml EDTA standard solution prepared above into a 100ml volumetric flask and dilute it to scale.
2.3 Take 100ml water sample, put it in a 250ml triangular flask, add 5 5ml NH3·H2O-NH4Cl buffer solution, shake well, add a proper amount of chrome black T indicator, titrate with EDTA standard solution until the solution changes from purple to pure blue, and record the dosage of EDTA standard solution.
2.4 calculation
Total hardness =C×V×56.08V water×10×1000.
In the above formula, the concentration of CEDTA—EDTA standard solution is about (0.01mol L-1); Vedta-volume of EDTA standard solution consumed by titration (ml); V water-water sample volume (100 ml); 56.08—— molar mass of calcium oxide (gram mole); 10-hardness of water. × 1000 —— Convert the water sample volume into1l. The results are shown in Table 2.
Table 2 Hardness of water from different sources
3 discussion
If the indicator is turned off during titration, there may be interfering ions. A suitable masking agent can be added and titrated again.
If the water sample is acidic or alkaline, it should be neutralized and titrated first to make it neutral; If it contains more carbonate, it should be boiled to drive out CO2, and then titrated. If the water sample is turbid or hydroxide precipitation occurs after adding buffer solution, the precipitation should be removed by filtration, and the filter paper used should be fully washed with water sample first, so as to avoid the influence of calcium and magnesium salts brought into the filter paper on the determination results.
The coordination reaction is slow, and the titration speed should not be too fast. When approaching the end point, titrate slowly and shake well.
refer to
Sun Qin. Comparison of two chrome black T indicators in the determination of water hardness [J]. Shenyang Medical, 2003,23 (2): 82. [2] Chen Xiwei. Determination of water hardness in brewing process [J]. Journal of Anhui Education College, 2003,21(5)