A Text-book of Assaying: For the Use of Those Connected with Mines Part 47

"Hypo" required 20.4 " 20.4 " 20.4 " 20.5 "

But if the starch is added before the t.i.tration, the results are liable to error.

Starch added 1.0 c.c. 50.0 c.c.

"Hypo" required 20.4 " 24.0 "

The starch should be used fresh, and is best made on the day it is used; after four days the finishing point is not so good.

~Effect of Varying Pota.s.sium Iodide.~--An excess of iodide is always required to keep the iodine in solution; a larger excess has little effect.

Iodide added -- 1 gram 20 grams "Hypo" required 20.4 c.c. 20.5 c.c. 20.6 c.c.

The 20 c.c. of iodine used, itself contained 0.5 gram of pota.s.sium iodide.

~Effect of Foreign Salts.~--

Bicarbonate of soda added -- 0.5 gram 1.5 gram 5.0 grams "Hypo" required 20.4 c.c. 18.2 c.c. 17.1 c.c. 16.0 c.c.

The solution obviously must be free from bicarbonate of soda. This should be remembered, since when t.i.trating a.r.s.enic a.s.says with iodine it must be present; and students must avoid confounding the two t.i.trations.

In some other experiments, in which 10 grams each of the salts were taken, the following results were obtained:--

Salt added -- AmCl AmNO_{3} Am_{2}SO_{4} "Hypo" required 20.4 c.c. 20.5 c.c. 20.3 c.c. 20.2 c.c.

Salt added NaCl NaNO_{3} Na_{2}SO_{4} "Hypo" required 20.3 c.c. 20.4 c.c. 20.4 c.c.

~Effect of Varying Iodine.~--

Iodine added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

"Hypo" required 1.3 " 10.2 " 20.4 " 51.0 " 102.0 "

~Determination of Dioxide in a Manganese Ore.~--Weigh up 0.25 to 0.3 gram of the powdered ore; place in a flask, cover with 10 c.c. of hydrochloric acid, and close the flask with a paraffined cork, and bulbs (as shown in fig. 60), having previously charged the bulb with 5 grams of pota.s.sium iodide in strong solution. Heat the flask, and boil cautiously for about fifteen minutes. Wash the contents of the bulbs into a large beaker, nearly (but not quite) neutralise with dilute ammonia, and t.i.trate with the standard "hypo."

As an example, 0.2675 gram of pyrolusite was taken, and required 60.3 c.c. of standard "hypo" (100 c.c. equal 1.185 gram iodine, or 0.4042 gram MnO_{2}), which equals 0.2437 gram of the dioxide or 91.1 per cent.

COLORIMETRIC METHOD.

When compounds of manganese free from chlorides are boiled with nitric acid and dioxide of lead,[87] the manganese is converted into permanganic acid, which is soluble and tints the solution violet. The depth of colour depends on the amount of manganese present, and this should not much exceed 10 milligrams. A quant.i.ty of substance containing not more than this amount of manganese should be boiled for a few minutes with 25 c.c. of a solution containing 5 c.c. of nitric acid, and 10 or 20 c.c. of dilute sulphuric acid, with 2 or 3 grams of lead dioxide. Filter through asbestos, wash by decantation with dilute sulphuric acid, make up with distilled water[88] to a definite bulk, and take a measured portion for the colorimetric determination.

The standard solution of manganese is made by dissolving 0.1435 gram of permanganate of potash (KMnO_{4}) in a little water acidulated with nitric acid, and diluting to 1 litre. One c.c. will contain 0.05 milligram of manganese.

PRACTICAL EXERCISES.

1. What percentage of manganese (Mn) is contained in permanganate of potash (KMnO_{4})?

2. Ten c.c. of a solution of permanganate of potash is found to oxidise 10 c.c. of an acid solution of ferrous sulphate. The manganese is determined in the t.i.trated solution by precipitation as dioxide and t.i.trating. How much of the ferrous solution will be oxidised in the second t.i.tration?

3. What weight of pota.s.sium iodide would be just sufficient to absorb the chlorine evolved by 0.5 gram of pure dioxide of manganese?

4. What weight of iron must be dissolved up so as to have an excess of 0.25 gram after oxidation by 1 gram of pure dioxide?

5. What weight of the brown oxide, Mn_{2}O_{4} will be left on igniting 1 gram of the pure dioxide?

CHROMIUM.

Chromium occurs in nature chiefly as chromite or chrome iron ore (FeO_{2}Cr_{2}O_{3}, with more or less MgO and Al_{2}O_{3}), which is the chief ore. It is a const.i.tuent of some silicates, and is frequently met with in very small quant.i.ties in iron ores. It occurs as chromate in crocoisite (PbCrO_{4}), and some other rare minerals.

The metal is used in steel-making. Steel containing about 0.5 per cent.

of it is rendered very hard; but its chief value is in its salts, the chromates. These are highly-coloured compounds, generally red or yellow.

Some of the insoluble chromates are used as pigments; chromate of lead or chrome-yellow is the most important. The soluble chromates, those of soda and potash, are valuable chemicals, and are largely used in the preparation of pigments, dyeing and tanning, and as oxidising agents.

Chromium forms two important cla.s.ses of compounds--chromic salts, corresponding to the oxide Cr_{2}O_{3}, and chromates, which contain the trioxide CrO_{3}. Solutions of chromic salts are green, whilst those of the chromates are yellow. Chromates are reduced to chromic salts by the action of most reducing agents in the presence of an acid; and this property is used in a.s.saying for the volumetric determination of ferrous iron, &c. The chromates in solution are more stable than other similar oxidising agents, and consequently are generally used in the laboratory as one of the standard oxidising agents for volumetric a.n.a.lysis. They have the disadvantage of requiring an outside indicator. Bichromate of potash (K_{2}Cr_{2}O_{7}) is the salt generally used for this purpose.

Chromic salts are oxidised to chromate by fusion with "fusion mixture"

and nitre, or by treating with chlorine in an alkaline solution.

Chromic salts closely resemble those of ferric iron, and in the ordinary course of a.n.a.lysis chromic hydrate (green) is precipitated together with ferric hydrate, alumina, &c., on the addition of ammonic chloride and ammonia. The ignited oxide, Cr_{2}O_{3}, however, is not reduced on heating to redness in a current of hydrogen.

~Detection.~--Chromium is detected by fusing the powdered substance with "fusion mixture" and nitre. The melt is extracted with water and filtered. The filtrate is acidified with acetic acid, and treated with a few drops of a solution of lead acetate. A yellow precipitate indicates chromium. Substances containing chromium impart a green colour to the borax bead in both flames. Small quant.i.ties of chromate in neutral solution can be found by the dark or violet-red colouration imparted thereto on boiling with a dilute decoction of logwood.

~Solution and Separation.~--Chromates and chromic salts are generally soluble in water or dilute acids. Chrome iron ore, however, and ignited chromic oxide are insoluble; and the former presents considerable difficulty on attempting to open up by the usual methods. A large number of mixtures have been tried in order to get all the chromium in a soluble form. Among these are the following. One part of the very finely-powdered ore is fused with any of these mixtures.

(1) 10 parts of bisulphate of potash.

(2) 5 parts of bisulphate of potash and 5 parts of pota.s.sium fluoride.

(3) 5 parts of hydric pota.s.sic fluoride.

(4) 12 parts of bisulphate of potash; and, afterwards, with 6 parts of carbonate of soda and 6 parts of nitre.

(5) 8 parts of borax; afterwards, with carbonate of soda till it ceases to effervesce; then, with 3 parts of carbonate of soda and 3 of nitre.

(6) 4 parts of borax and 6 parts of fusion mixture.

(7) 12 parts of caustic potash.

(8) 10 parts of caustic soda and 30 of magnesia.

(9) 5 parts of caustic soda and 3 of magnesia.

(10) 2 parts of carbonate of soda and 1 of lime.

(11) 6 parts of soda-lime and 2 of chlorate of potash.

(12) Sodium peroxide.

Of these, numbers 1, 2, and 3 yield the chromium in a form soluble in dilute acids, as chromic salt. The rest in a form soluble in water, as pota.s.sium or sodium chromate.

On boiling an insoluble chromium compound with chlorate of potash and nitric acid, the chromium pa.s.ses into solution as chromate. This method, however, does not answer for chrome iron ore. In the fusion methods the ore must be very finely powdered, well mixed with the fluxes, and subjected to a prolonged fusion in a platinum vessel at a high temperature. Undecomposed particles require re-fusion.

The aqueous extract containing the chromate is ready for volumetric work, except in those cases where nitre has been used. For gravimetric work the solution is acidified with hydrochloric acid, then mixed with ammonia in slight excess, boiled, and filtered. The filtrate is acidified with hydrochloric acid, and treated with sulphuretted hydrogen, warmed, rendered slightly alkaline with ammonia, and the gas again pa.s.sed. The chromium is precipitated as chromic hydrate mixed with sulphur from the reduction with sulphuretted hydrogen. It is filtered off, washed with hot water, and ignited. It is weighed as chromic oxide.

GRAVIMETRIC DETERMINATION.

The solution containing the chromium, freed from other metals and earths and in the form of (green) chromic salt, is heated to boiling. If any chromate is present reduce it with sodium sulphite or sulphuretted hydrogen. Add ammonia in slight excess, boil till the liquid is free from a red tint, and allow to settle for a few minutes. Filter, wash with hot water, dry, and ignite strongly in a loosely-covered crucible.

Cool, and weigh. The substance is chromic oxide, Cr_{2}O_{3}, and contains 68.62 per cent. of chromium. It is a dark-green powder insoluble in acids.