Experiment for determination of total hardness of water
Experiment for determination of total hardness of water
Total hardness (Total hardness) is the total concentration of calcium and magnesium ions in water and converted to calcium oxide for calculation. Hardness has a great deal to do with industrial water, especially boiler water. Various industries have certain requirements for water hardness. Excessive hardness in drinking water will affect the digestive function of the stomach and intestines, China's domestic drinking water health standards stipulate that the total hardness (in terms of CaCO3) should not exceed 450 mg/L.
Operation method
EDTA complex titration
Principle
Complexation titration of calcium and magnesium ions with EDTA solution at pH 10, using Chrome Black T as indicator. Chrome Black T can form purple or violet complexes with calcium and magnesium ions in water. Free calcium and magnesium ions in the titration firstly react with EDTA, and the calcium and magnesium ions complexed with the indicator then react with EDTA, and when reaching the end point, all the calcium and magnesium ions are complexed with EDTA to make the chromium black T free, and the solution changes from violet-red to azure.
Materials and Instruments
watery Move I. Instrumentation Caveat 1. EDTA complex titration is much slower than the acid-base reaction, so the titration speed should not be too fast, near the end point, each drop of EDTA solution should be fully shocked, otherwise the end point will appear too early, the results are low. 2. buffer solution in water samples, in order to prevent Ca2+, MgMg2+, MgTo prevent Ca2+ and Mg2+ from precipitating, the titration must be completed within 5 min. 3. If the titration to the blue endpoint, a little time and then reappear purple-red, this may be due to the presence of small particles of calcium and magnesium salts. In this case, we should take another water sample, add hydrochloric acid to make it acidic, heat it to boiling, and then add ammonia to neutral, and then proceed according to the steps of determination. 4. In order to minimize the interference of Zn, Cu, Co, add 250 mg of sodium cyanide (highly toxic) or a few milliliters of triethanolamine masking agent before titration. The solution must be alkaline before adding sodium cyanide. 5. If the specimen has been preserved acidified, it may be neutralized with a measured amount of sodium hydroxide, and the dilution of the sample or specimen by the addition of acid or alkali should be taken into account when calculating the results. Common Problems I. Introduction Total hardness (Total hardness) is the total concentration of calcium and magnesium ions in water and converted to calcium oxide for calculation. Hardness has a great relationship with industrial water, especially boiler water. Various industries have certain requirements for water hardness. Drinking water in the high hardness will affect the digestive function of the stomach and intestines, China's drinking water health standards for total hardness ( CaCO3 ) shall not exceed 450 mg / L. Total hardness determination methods are: ① EDTA complex titration method, this method is simple to use the equipment, fast, is generally often used. ② atomic absorption method, the method is simple, fast, sensitive, accurate, easy to eliminate interference. When the EDTA method has interference, it is best to change to atomic absorption method. ③ plasma emission spectrometry, this method is fast, sensitive, less interference, and can be measured at the same time a variety of elements. For more product details, please visit Aladdin Scientific website.
50 ml acid buret, 100 mL measuring cylinder, 250 mL triangular flask.
II. Pharmaceuticals
Disodium ethylenediaminetetraacetic acid (referred to as EDTA-2Na or Na2H2Y-2H2O), ammonium chloride, magnesium sulfate, chromium black T, sodium sulfide, hydroxylamine hydrochloride, concentrated ammonia, calcium carbonate or zinc granules, sodium chloride.
III. Reagents
1. Ammonium chloride-ammonia solution (Reagent A): Weigh 16.9 g of ammonium chloride and dissolve in 143 mL of concentrated ammonia.
2. EDTA-Mg solution (reagent B): weigh 0.78 g of magnesium sulfate ( MgSO4-7H2O ) and 1.179 g of EDTA-2Na dihydrate, dissolve in 50 mL of distilled water, add 2 mL of reagent A, 5 drops of chromium black T indicator or 0.2 g of chromium black T dry powder, titrate with 0.01 mol/L EDTA solution to the solution from violet-red to azure.
3. Ammonia buffer solution (pH 10): Mix reagent A and reagent B, dilute to 250 mL with distilled water and store in a plastic bottle, at this time the solution is still azure, if the solution turns purple, the reagent blank should be subtracted from the calculated results.
4. 0.01mol/L calcium standard solution: weigh about 0.2500 g CaCO3 accurately, put it in 100 mL beaker, add a few drops of water to moisten it, cover the tableware, add HCl slowly until CaCO3 is completely dissolved, add 20 mL of water, boil it over low heat for 2 min, cool it down and then quantitatively transfer it to a 250 mL volumetric flask, dilute it to the mark by adding water and shake it well.
5. 0.01 mol/L EDTA-2Na standard solution: weigh 3.725 g of EDTA-2Na dried at 80 ℃ for 2 h and cooled in a beaker, add 500 mL of water, slightly heat and stir to make it completely dissolved, cooled and transferred to a volumetric flask to 1L, stored in polyethylene bottles, and calibrate its concentration periodically.
Calibration of EDTA-2Na standard solution: Aspirate 25.00 mL of calcium standard solution in a conical flask, add 50 mL of water and 3 mL of Mg-EDTA solution, add 15.00 mL of EDTA standard solution, then add 5mL of ammonia buffer solution and the appropriate amount of chromium black T indicator, and then titrate with EDTA standard solution immediately, and the solution will change from violet red to pure blue as the end point. Parallel titrationParallel titration three times, the positive difference should be less than 0.05 mL, the average volume to calculate the concentration of EDTA standard solution. CEDTA (mol/l) = CCa × VCa/VEDTA
6. Chrome Black T powder indicator: 1.0 g of Chrome Black T mixed with 100 g of NaCl, finely ground, stored in a brown bottle, tightly corked spare, this indicator can be stored for a long time. Chromium black T indicator solution: 0.5 g of chromium black T, dissolved in 10 mL of buffer solution, diluted to 100 mL with 95% ethanol, stored in the refrigerator, can be stable for one month.
IV. Experimental steps
1. Aspirate 50 mL of water sample (if hardness is too high, more than 3.6 mmol/L can take less water sample, dilute with distilled water to 50 mL), put in 150 mL triangular bottle.
2. If there are other metal ions in the water sample to interfere with the titration, the end point drags or the color is dark, add 1 mL of 5% sodium sulfide solution and 5 drops of 1% hydroxylamine hydrochloride solution.
3. Add 2 mL of buffer solution and 5 drops of chromium black T indicator (or a small spoon of about 0.2 g solid indicator), immediately titrate with EDTA standard solution, the initial titration speed should be slightly faster and should be shaken vigorously, near the end of the drop should be slow and more shaking, the color of the violet-red or violet to azure for the end point. Parallel titration of three. Volume of EDTA standard solution consumedThe volume of EDTA standard solution consumed should not be greater than 0.1 mL.
V. Calculation of results
Total hardness (CaO, mg/l) = (VEDTA × CEDTA × MCaO × 1 000)/V Water sampleFormula:MCaO- refers to the molecular weight of calcium oxide, 56.