Drinking natural mineral water source water detection

8. Total hardness 8.1 principle: When there is a chromium black T indicator present in the water sample, it forms a purple red chelate with calcium and magnesium ions. The instability constants of these chelates are greater than those of calcium ethylenediaminetetraacetate and magnesium chelates. When pH=10, disodium ethylenediaminetetraacetic acid first forms a chelate with calcium ions and then with magnesium ions. At the titration endpoint, the solution appears blue as a chrome black T indicator. Due to the fact that the reaction between calcium ions and chromium black T indicator cannot show a significant color change when the titration reaches the equivalence point, it is necessary to add a known amount of magnesium salt when the magnesium content in the water sample is low to make the equivalence point color change clear. When calculating the results, subtract the amount of magnesium salt added, or add a small amount of complex ethylenediaminetetraacetic acid magnesium salt to the buffer solution to ensure a clear endpoint. 8.2 Reagents and Materials Unless otherwise specified, all reagents used in this method are analytical pure, and the water is Grade III water as specified in GB/T 6682. 8.2.1 Buffer solution (pH=10); Dissolve 67.5 g of ammonium chloride (NH, CI) in 300 ml. of distilled water, add 570 mL of ammonium hydroxide (zo=-0.90 g/mL), and dilute with water to 1000 ml. 8.2.2: Chromium black T indicator (5 g/L): Weigh 0.5 g of chromium black T (CH: N; NaO, S) and dissolve it in 100 ml. of triethanolamine (C. H, NO,). 8.2.3 Sodium sulfide solution (50 g/L): Weigh 5.0g of sodium sulfide (Na, S · 9H; O), dissolve in water, and dilute to 100 ml. 8.2.4 Hydroxylamine hydrochloride solution (10 g/L): Weigh 1.0g of hydroxylamine hydrochloride (NH, OH · HCl), dissolve in water, and dilute to 100 ml. 8.2.5- Potassium cyanide solution (100 g/1): Weigh 10.0g of potassium cyanide (KCN), dissolve in water, and dilute to 100 ml. Warning - This solution is highly toxic. 8.2.6. EDTA-2Na standard solution [c (C,. H, N-O, Na · 2H, O)=0.01 mol/1L]: Weigh 3.72 g of EDTA-2Na, dissolve it in 1000 mL of distilled water, and calibrate its accurate concentration according to 8.2.6.1 and 8.2.6.2. 8.2.6.1 Zinc standard solution: Accurately weigh 0.6 g to 0.7 g of pure metal zinc particles, dissolve them in hydrochloric acid solution (1+1), place them on a water bath and heat until completely dissolved. Transfer them to a volumetric flask and bring to a constant volume of 10 mL

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