——EDTA Titration This method is suitable for the determination of total hardness in circulating cooling water and natural water. 1. Principle At pH= 10, disodium ethylenediamine tetraacetate (EDTA for short) forms a stable complex with calcium and magnesium ions in water, and the indicator chrome black T can also form a wine red complex with calcium and magnesium ions, which is not as stable as the complex formed by EDTA and calcium and magnesium ions. When the titration with EDTA approaches the end point, EDTA takes away calcium and magnesium ions from the wine red complex of chrome black T, which makes the indicator chrome black T free and the solution is wine red. The reaction is as follows: Mg2++hlnd 2- mglnd-+h+mglnd-+h2y2-mgy2-+h+hlnd2-Ca2++hlnd2-calnd-+h+calnd-+h2y2-cay2-+h+hlnd2-where hlnd2-chrome black t indicator. Complex of mg lnd- magnesium and chrome black t (wine red); H2y2-EDTA ion (colorless). 2. Reagent 2. 1.6 mol/L hydrochloric acid solution. 2.2 10% ammonia water: measure 440mL ammonia water and dilute it to 1000mL. 2.3 1+ 1 triethanolamine solution 2.4 chrome black t indicator weigh 0.5g chrome black t and 4.5g hydroxylamine hydrochloride, dissolve them in 100mL95% ethanol and store them in a brown bottle. 2.5 ph = 10 ammonia-ammonium chloride buffer solution. Weigh 54g ammonium chloride, dissolve it in 200mL water, add 350mL ammonia water and dilute it with 1000mL water. 2.6 0.0 1 mol/l EDTA standard solution. 2.6. 1 prepare and weigh disodium EDTA (C 10H 14O8N2Na2? 2H2O)3.72g was dissolved in 1000mL water and shaken well. 2.6.2 Calibrate and weigh 0.2g of reference zinc oxide (accurate to 0.0002g) and burn it to constant weight at 800℃. Moisten with a little water, add 2mL 6mol/L hydrochloric acid solution until the sample is dissolved, transfer to a 250mL volumetric flask, and dilute to scale. Absorb 20mL of this solution, transfer it to a 250mL conical flask, add 30mL of water, neutralize it to pH 7-8 with 10% ammonia water, add 5mL of ammonia-ammonium chloride buffer solution, add 2-4 drops of chrome black T indicator, and titrate with EDTA solution until the solution changes from wine red to sky blue. At the same time, do blank test. 2.6.3 Calculate the molar concentration m (mol/L) of EDTA standard solution according to the following formula: where: the weight of g- zinc oxide, g; V1-dosage of EDTA solution, ml; V 0 —— the dosage of EDTA solution in blank test, ml; 8 1.39—— Molar mass of zinc oxide, g/mol. 3. Instrument 3. 1 burette: 25mL acid type. 4. Analysis Step 4. 1 Suck 50mL water sample, transfer it to a 250mL conical flask, add 5mL ammonia-ammonium chloride buffer solution, 2-4 drops of chrome black T indicator, and titrate with 0.0 1mol/L EDTA standard solution until the solution changes from purple to pure blue. 5. Calculation of analysis results The total hardness content X (mg/L, calculated by CaCO3) in the water sample is calculated as follows:
Where: v-consumption volume of EDTA standard solution during titration, ml; M —— concentration of EDTA standard solution, mole/liter; Vw-water sample volume, ml; 100.08-molar mass of calcium carbonate, g/mol. 6. Note 6. 1 If iron and aluminum in the water sample interfere with the determination, add 1+ 1 triethanolamine 1 ~ 3 ml for masking. 6.2 If there is a small amount of zinc ions in the water sample, 0.5 ml of β-aminoethanethiol can be added for masking after sampling. If the zinc content is high, the zinc content can be measured separately and then subtracted from the total hardness. 6.3 If there is color reversal in the determination, dry filter the water sample with medium-speed filter paper to remove suspended calcium carbonate. 7. When the allowable total hardness in poor water is 300mg/L (calculated by CaCO3), the difference between the two parallel determination results is not more than 3.5 mg/L. 8. The results show that the arithmetic average of the two parallel determination results is taken as the total hardness content of the water sample.