Page images
PDF
EPUB

herewith, should be used. In each of two or three small weighingtubes with tightly-fitting glass stoppers are placed 2 to 2.5 grams of pure potassium iodide free from iodate and 0.5 c.c. of water (not more); the tubes are stoppered and accurately weighed by the method of swings. The tubes are then opened, 0.3 to 0.4 gram of pure iodine is added to each, the tubes are quickly stoppered and again weighed; the difference shows the amount of iodine. The iodine dissolves almost instantly in the concentrated potassium iodide solution. One of the tubes is then placed in the neck of a 500 c.c. Erlenmeyer flask which is held in an inclined position and contains 200 c.c. of water and about 1 gram of potassium iodide. The tube is dropped to the bottom of the flask, but just as it begins to fall the stopper is removed and allowed to follow it. In this way there is no iodine lost, which will be the case if the contents of a tube are washed into the water. A solution is thus prepared containing a known amount of iodine and to it the sodium thiosulphate solution to be standardized is added from a burette until the liquid is pale-yellow in color. Now, 2 or 3 c.c. of starch solution are added and the solution carefully titrated until it becomes colorless. From the mean of three determinations, the strength of the thiosulphate solution is calculated.

307. Standardization by Metallic Copper. In acetic acid solution cupric ion and iodide ion react to form iodine and cuprous ion, the latter being precipitated out of solution as the insoluble cuprous iodide, the entire mechanism of the reaction being represented by the scheme:

2 Cu+++ 2 I− → 2 Cu++ I2

2 I


2 Cu I

The reaction is quantitative and can be used as the basis of standardizing sodium thiosulphate solutions. It is important in using this reaction to observe that the anion of any strong acid, i.e., sulphate ion, chloride ion, nitrate ion, etc., must be excluded (except for very small quantities of same), as the presence of such ions prevents more or less completely the quantitative pre

cipitation of the cuprous ion, which quantitative precipitation is absolutely essential in order that the reduction of the cupric ion be quantitative. Furthermore, there should not be present any other oxidizing agent than the cupric ion, or any other reducing agent than the iodide ion, for the first would liberate iodine and cause too high results, while the second would reduce some of the liberated iodine and cause low results.

Procedure. Thoroughly clean a strip of pure copper foil and cut from it three pieces, each weighing about 150 milligrams. Dissolve the three pieces in separate small beakers by treating each with about 5 c.c. of 8 molar nitric acid, place on a waterbath or steam hot-plate, and evaporate to the point at which the solution will just solidify upon cooling. Care must be taken, however, not to evaporate too far as such over evaporation will produce basic salts of copper which are very difficult to dissolve. Dissolve the copper nitrate in about 15 c.c. of cold water, and add dropwise a 1 molar solution of sodium hydroxide until a slight permanent precipitate of cupric hydroxide forms. Add about 5 c.c. of 8 molar acetic acid and dilute with cold water to about 50 c.c. Add 10 c.c. of 1.8 molar potassium iodide solution to the contents of the first beaker, allow to stand for about thirty seconds, and then titrate immediately with the sodium thiosulphate solution until the brown color has changed to a light yellow, then add 3 c.c. of starch so ution, and continue the titration with constant stirring until the blue color disappears. This blue color will return more or less quickly upon standing, 12 but the end point is to be taken as its first complete disappearance. Proceed in turn the same way with the contents of the second and the third beakers, and from the mean of the three determinations calculate the value of the thiosulphate solution.

308. Exercise No. 32. Iodimetric Determination of Copper in an Ore. Weigh out from 0.5 to 1 g. of the finely ground ore 13 and transfer to a covered casserole. Add about 20 c.c. of 16 molar

12 Large amounts of sodium acetate cause a quick return of the blue color. Therefore, care should be taken to add not more than a few drops excess of sodium hydroxide solution in neutralizing the nitric acid which remains after evaporation.

13 This weight of sample presupposes that the copper content is between 15 and 30%; if the copper content happens to be under or above these percentages, a proportionately larger or smaller weight of sample must be taken accordingly.

nitric acid, 5 c.c. of 12 molar hydrochloric acid, and 5 c.c. of 18 molar sulphuric acid. Heat until the decomposition of the ore is complete, remove the cover and evaporate to dense fumes of sulphuric trioxide. Cool, add carefully about 40 c.c. of water, and heat to dissolve soluble salts. Filter through a small filter and wash with hot water.

Most ores are completely decomposed by the above treatment. Certain silicates, slags, etc., require a sodium carbonate fusion of the residue from the acid treatment. In case fusion is necessary, lead sulphate, if present in the residue, must first be removed by repeated treatment with hot 3 molar sodium acetate solution. Silver chloride, if present, must be removed by repeated treatment of the residue with hot 6 molar ammonium hydroxide solution. The residue free from lead and silver salts is then fused with sodium carbonate and the fused mass decomposed with hydrochloric acid in the usual manner. Then 5 c.c. of 18 molar sulphuric acid are added to the solution, and the latter evaporated to fumes of sulphur trioxide as above. The soluble salts are dissolved in water, the solution filtered, and the filtrate and washings added to the main solution. If silver is present in the main solution, it must be removed by precipitation as ch oride, the solution filtered and evaporated to fumes of sulphur trioxide, after the addition of sulphuric acid as above.

The volume should now be about 100 c.c., and the concentration of sulphuric acid about 1 molar. If the acid is more concentrated than this, the excess must be neutralized by ammonium hydroxide; if less concentrated, the proper volume of 18 molar sulphuric acid must be added. Place two pieces of sheet aluminum, about 1.5 inches square and with the corners turned over so that the pieces are held away from the bottom of the beaker, in the solution, cover with a watch-glass and boil the solution for about 15 minutes. Copper and other metals of lower solution tension than iron will be deposited (among such elements the only ones likely to be encountered here are lead, cadmium, bismuth, arsenic, and antimony). Decant at once through a 9 cm. filter paper and wash thoroughly with boiling water, keeping the aluminum and as much of the copper as possible in the beaker. Add about 10 c.c. of 8 molar nitric acid to the beaker

containing the aluminum and copper, and warm until all the copper has been dissolved. Pour this warm solution through the filter in such a manner as to dissolve any particles of copper it may contain and receive the filtrate in a small beaker. Wash the original beaker, the aluminum and the filter paper two or three times with a very small amount of water. When the solution has drained from the funnel, remove the beaker and place a small watch-glass under the funnel to receive any further drops of solution that may run through the funnel. To the copper solution add about 30 c.c. of bromine water and boil to oxidize any arsenic and antimony that may be present. Continue the boiling until all excess bromine is expelled. Again place the beaker under the funnel, wash any solution from the watch-glass into the beaker and complete the washing of the filter paper with hot water. Evaporate the solution on the water-bath or hotplate until it just solidifies upon being removed and allow to cool somewhat, then complete the determination exactly as described for the standardization of the thiosulphate solution. From the data obtained, and an auxiliary determination of the moisture content of the sample, calculate the percentage of copper in the ore on the dry basis. Bismuth and lead, if present, are without effect upon the titration except that the color change in this case is from a greenish color to a pale yellow instead of from blue to colorless.

309. Example.

In standardizing a solution of sodium thiosulphate by means of the iodine liberated by the cupric ion from a known weight of copper (§ 307), the following results were obtained:

[blocks in formation]

What was the value of the sodium thiosulphate solution in terms of copper? What was the average deviation?

Ans. 1 c.c. Na2S2O3 = 0.006421 g. Cu;

1.2 parts per 1000

CHAPTER XX

ELECTROLYTIC DETERMINATIONS. THEORY

Current
Enters

Current

Leaves

310. Electrolytic determinations are based upon the fact that when a direct current of electricity is caused to pass through a solution of a salt there always results an oxidation, to the elemental state, of some of the negatively charged ions present and a reduction, to the elemental state, of some of the positively charged ions present. This phenomenon is known as. electrolysis; and the solution which is thus subjected to the action of the current is called the electrolyte. The conductor by which the current enters the solution is known as the anode, while that by which it leaves the solution is known as the cathode; the anode and cathode are often spoken of simply as the electrodes.

Electrolyte

(Oxidation)

Cathode (Reduction)

FIG. 40

Anode

The respective oxidations and reductions never take place in the mass of the solution itself but only at the electrodes, oxidation taking place at the anode, reduction at the cathode. Moreover, for each amount of substance oxidized at the anode there is always an exactly equivalent amount of substance simultaneously reduced at the cathode in accordance with Faraday's Law that for each 96,500 coulombs of electricity passed through an electrolyte one gram equivalent of substance is oxidized at the anode and one gram equivalent of substance is reduced at the cathode.

It is to be further noted that the various kinds of anions are not all oxidized with the same ease, their relative order being (the easiest coming first) sulphide, hydroxyl, iodide, bromide,

« PreviousContinue »