(2) In the filtrate, add an appropriate amount of NaOH and Na2CO3 solution, and filter to remove Mg(OH)2, CaCO3, and an excess of BaCO3. Mg2+ + 2OH- =Mg(OH)2? =Mg(OH)2? Ca2+ + CO32- = CaCO3? Ba2+ + CO32- = BaCO3?
(3) Neutralize the excess NaOH and Na2Co3 with hydrochloric acid. test with pH paper to make the solution nearly neutral. h+ + OH- = H2O
2H+ + CO32- = CO2?+ H2O
(4) The solution is evaporated, concentrated and structured, NaCl is filtered out and the mother liquor is separated by containing K+.
B:
I. Purpose of the experiment
According to what has been learned, on the basis of summarizing the experiments on alkali metals and alkaline-earth metals, purify the crude table salt according to the scheme designed by yourself.
Second, the experimental principle
1. Crude table salt contains mud, sand and other insoluble impurities as well as calcium, magnesium, potassium, sulfate ions and other soluble impurities.
2. When considering the removal of impurities such as calcium, magnesium, sulfate ions, etc., the solubility data of their insoluble salts should be consulted first, and the precipitant for the removal of the above ions should be selected under the principle that no new impurities will be introduced or that the introduced impurities can be removed in the next operation.
Third, the contents of the experiment
1. Design an experimental program (including detailed steps and specific conditions for each operation), and start the experiment after approval by the instructor.
2. Weigh 20 grams of crude table salt, dissolve it in as little water as possible and purify it.
IV. Product testing
1. Determination of NaCl content
Weigh about 0.2 g of the dried sample weighed to 0.0002 g, dissolved in 70 ml of water in a conical flask with 3-5 drops of 0.5% fluorescent red alcoholic solution and 10 ml of 0.5% starch solution without oxidation, and then under constant shaking , titrated with 0.1 mol?dm-3 AgNO3 standard solution, until the turbid solution color changes abruptly (showing pink color).
The percentage of NaCl content (x) was calculated according to the following formula:
x = M ?V×0.05845×100/G = 5.845MV/G
Wherein, V - amount of AgNO3 standard solution (cm3), M - amount concentration of AgNO3 standard solution, G - grams of sample. amount concentration, G - grams of sample, 0.05845 - grams per millimole of NaCl, NaCl content should not be less than 99.8%.
2. Test of purity of crude salt and product
Take a small amount of crude salt and purified sodium chloride, respectively, dissolved in a small amount of distilled water, and compare their purity with the following method test.
(1) test of sulfate ions
To the two test tubes containing crude table salt and pure NaCl solution (two kinds of test liquids), respectively, add a few drops of 0.2 mol?dm-3 BaCl2 solution, and observe whether there is any BaSO4 precipitation.
(2) Calcium ion test
Add a few drops of saturated sodium oxalate solution to the two test solutions, and observe whether calcium oxalate precipitate is produced.
(3) Test for magnesium ions
Add 6 mol?dm-3 NaOH solution dropwise to each of the two test solutions to make them alkaline. Add a few drops of magnesium reagent solution and when the solution is blue in color, it indicates the presence of magnesium ions.