[88] The water for dilution and the dilute sulphuric acid used for washing should be previously tested, to see they have no reducing action, with dilute permanganate of pota.s.sium solution.
[89] Arnold and Hardy, _Chemical News_, vol. lvii. p. 153.
CHAPTER XIV.
EARTHS, ALKALINE EARTHS, ALKALIES.
ALUMINA.
Alumina, the oxide of aluminium (Al_{2}O_{3}), is found in nature fairly pure in the mineral corundum; transparent and coloured varieties of which form the gems sapphire and ruby. A coa.r.s.er compact variety contaminated with oxide of iron const.i.tutes emery. Compounded with silica, alumina forms the base of clays and many rock-forming minerals.
China clay (or kaolin) is used as a source of alumina. Bauxite, hydrated alumina, is also used for the same purpose--that is, for the preparation of sulphate of alumina. The mineral cryolite is a fluoride of aluminium and sodium.
Corundum is characterised by a high specific gravity (4.0) and extreme hardness. By these it is distinguished from felspar and similar minerals, which it somewhat resembles in general appearance.
Aluminium is used for a variety of small purposes: it is white, light, and very tenacious; but owing to the difficulty of its reduction it is expensive.
Aluminium forms one series of salts which closely resemble those of ferric iron. It forms an interesting series of double sulphates, known as the alums. Common potash alum is Al_{2}(SO_{4})_{3},K_{2}SO_{4},24H_{2}O.
~Detection.~--Alumina is not precipitated from its acid solution by sulphuretted hydrogen, but it is thrown down by ammonia (with the other earths) as a white hydrate, soluble in soda and insoluble in ammonic carbonate. Filtered off and ignited, it a.s.sumes, after treatment with nitrate of cobalt before the blowpipe, a blue colour which is characteristic. With natural compounds containing metallic oxides this colour is masked. It is more satisfactory to make a separation in the wet way and to test the ignited oxide.
~Separation and Solution.~--If the substance is insoluble in hydrochloric acid it is finely powdered and fused with "fusion mixture"
with the help, in the case of corundum (which is very refractory) of a little caustic soda or potash. The method of working is the same as that described for opening up silicates. See under _Silica_. Corundum cannot be powdered in Wedgwood, or even agate, mortars; since it rapidly wears these away and becomes contaminated with their powder. It is best to use a hard steel mortar and to extract the metallic particles from the bruised sample with a magnet or dilute acid.
When the substance has been completely attacked and dissolved, it is evaporated to dryness with an excess of hydrochloric acid on the water-bath to render any silica present insoluble. The residue is extracted with hydrochloric acid and freed from the second group of metals by means of sulphuretted hydrogen. The filtrate from this (after removing the sulphuretted hydrogen by boiling) is nearly neutralised, and treated with 8 or 10 grams of hyposulphite of soda[90] in solution.
It is then boiled till the sulphurous oxide is driven off. The precipitate is filtered off, ignited, and weighed as alumina.
It is sometimes more convenient to proceed as follows:--After boiling off the sulphuretted hydrogen peroxidise the iron with a little nitric acid, add a solution of ammonic chloride, and then ammonia in very slight excess; boil, filter, wash, ignite, and weigh the oxides. These generally consist of ferric oxide and alumina. It is a common practice to determine the iron, calculate it to ferric oxide, and so to estimate the alumina indirectly. This may be done either by igniting in a current of hydrogen and estimating the iron by the weight of oxygen lost; or, by dissolving with sulphuric and hydrochloric acids, and determining the iron volumetrically. It should be borne in mind that these oxides will also contain any phosphoric oxide that happened to be in the mineral.
In general a.n.a.lyses of samples containing alumina, it may be contained in both the soluble and insoluble portions. In these cases it is better to fuse the sample with "fusion mixture" before treatment with acids.
The alumina in the fused ma.s.s will exist in a state soluble in acids.
GRAVIMETRIC DETERMINATION.
Solutions containing alumina free from the other metals are diluted to a convenient bulk and heated nearly to boiling. Add chloride of ammonium, and then ammonia in slight excess; boil, allow to settle, filter, and wash with hot water. Dry the precipitate, and ignite in a platinum or porcelain crucible at the strongest heat. Cool, and weigh. The substance is alumina, Al_{2}O_{3}, which contains 52.94 per cent. of aluminium.
It is only in special cases, such as the a.n.a.lysis of metals and alloys, that it is reported as aluminium. The percentage of alumina is generally given.
Ignited alumina is difficultly soluble in acids; it is not reduced by hydrogen at a red heat. Ignited with ammonium chloride portions are volatilised.
~Direct Determination of Alumina in the Presence of Iron.~--The iron and alumina are precipitated as hydrates by ammonia. The precipitate is dissolved in hydrochloric acid and the iron reduced to the ferrous state. It is then added to a hot solution of potash or soda. The solution is boiled till the precipitate settles readily, filtered, and washed with hot water. The alumina is contained in the filtrate, which is acidified with hydrochloric acid and the alumina precipitated therefrom as hydrate with ammonia, as just described.
~Determination of Alumina in the Presence of Phosphates and Iron.~--For details, see a paper by R.T. Thomson in the "Journal of the Society of Chemical Industry," v. p. 152. The principles of the method are as follows:--If the substance does not already contain sufficient phosphoric oxide to saturate the alumina, some phosphate is added. The iron is reduced to the ferrous state and phosphate of alumina precipitated in an acetic acid solution. It is purified by reprecipitation, ignited, and weighed as phosphate (Al_{2}O_{3},P_{2}O_{5}), which contains 41.8 per cent. of alumina, Al_{2}O_{3}.
EXAMINATION OF CLAYS.
~Moisture.~--Take 5 grams of the carefully-prepared sample and dry in the water-oven till the weight is constant.
~Loss on Ignition.~--Weigh up 2 grams of the sample used for the moisture determination, and ignite in a platinum-crucible to redness, cool, and weigh.
~Silica and Insoluble Silicates.~--Weigh up another 2 grams of the dried sample, and place them in a platinum dish; moisten with water, and cover with 20 c.c. of sulphuric acid. Evaporate and heat gently to drive off the greater portion of the free acid. Allow to cool; and repeat the operation. Extract by boiling with dilute hydrochloric acid, filter, wash, dry, ignite, and weigh. The quant.i.ty of insoluble silicates is determined by dissolving out the separated silica with a strong boiling solution of sodium carbonate. The residue (washed, dried, and ignited) is weighed, and reported as "sand."
~Alumina and Ferrous Oxide.~--To the filtrate from the silica add "soda"
solution till nearly neutral, and then sodium acetate. Boil and filter off the precipitate. Reserve the filtrate. Dissolve the precipitate in hydrochloric acid, and dilute to exactly 200 c.c. Divide into two parts of 100 c.c. each. In one determine the iron by reducing and t.i.trating in the way described under volumetric iron. Calculate the percentage as ferrous oxide, unless there are reasons to the contrary, also calculate its weight as ferric oxide. To the other portion add ammonia in slight excess, and boil. Filter, wash with hot water, dry, ignite, and weigh as mixed alumina and ferric oxide. The weight of the ferric oxide has already been determined in the first portion: deduct it, and the difference is the weight of alumina.
~Lime.~--To the reserved filtrate, concentrated by evaporation, add ammonium oxalate and ammonia; boil, filter, ignite strongly, and weigh as lime.
~Magnesia~ is separated from the filtrate by adding sodium phosphate. It is weighed as magnesium pyrophosphate.
~Potash and Soda.~--These are determined in a fresh portion of the sample by Lawrence Smith"s method, as described on page 333.
THORIA.
This is an oxide of thorium, ThO_{2}. It is only found in a few rare minerals. It is a heavy oxide, having, when strongly ignited, a specific gravity of 9.2. In the ordinary course of a.n.a.lysis it will be separated and weighed as alumina. It is separated from this and other earths by the following method. The solution in hydrochloric acid is nearly neutralised and then boiled with sodium hyposulphite. The thoria will be in the precipitate. It is dissolved, and the solution heated with ammonium oxalate in excess. The precipitate is thorium oxalate, which is washed with hot water, dried, and ignited. It is then weighed as thoria, ThO_{2}. Thoria which has been ignited is not readily soluble in acids.
ZIRCONIA.
The oxide of zirconium, ZrO_{2}, is found in the mineral zircon, a silicate of zirconia, ZrSiO_{4}. When heated intensely it becomes very luminous, and is used on this account for incandescent lights.
In the ordinary course it is thrown down by ammonia with the other earths, from which it is thus separated:--The hydrates precipitated in the cold, and washed with cold water, are dissolved in hydrochloric acid, nearly neutralised with soda, and precipitated by boiling with hyposulphite of soda. Dissolve; and from the hydrochloric acid solution precipitate the thoria (if any) with ammonium oxalate. To the filtrate add carbonate of ammonia, which will precipitate any t.i.tanium present.
The zirconia will be in solution, and is recovered by precipitating with pota.s.sium sulphate, or by evaporating the solution and igniting. It is separated from alumina by taking advantage of its insolubility in pota.s.sic hydrate.
It is estimated in zircons in the following way:--The powdered substance is fused with bisulphate of potash, and extracted with dilute sulphuric acid. The residue is fused with caustic soda and extracted with water.
The portion not dissolved, consisting of zirconate of soda, is dissolved in hydrochloric acid. The solution is diluted, filtered if necessary, and treated with ammonia in excess. The precipitate is filtered off, washed with hot water, dried, ignited, and weighed as zirconia, ZrO_{2}.
This is a white powder, which is insoluble in acids; even in hydrofluoric acid it is only slightly attacked.
CERIUM.
Cerium occurs as silicate (together with the oxides of lanthanum, didymium, iron and calcium) in the mineral cerite, which is its chief source. It also occurs as phosphate in mon.a.z.ite, and as fluoride in fluocerite. The oxalate is used in medicine. Cerium forms two cla.s.ses of salts corresponding to the oxides, cerous oxide (Ce_{2}O_{3}) and ceric oxide (CeO_{2}). Compounds of cerium with volatile acids yield dioxide on ignition; and this, on solution in hydrochloric acid, yields cerous chloride and chlorine.
In the ordinary course cerium is thrown down along with alumina and the other earths by ammonia. It is separated by dissolving the hydrates in hydrochloric acid, and oxidizing with chlorine water. On treating with oxalic acid, cerium, lanthanum, and didymium are precipitated as oxalates, which on ignition are converted into oxides. These are soluble in acids. Their solution in hydrochloric acid is nearly neutralised; acetate of soda is then added, and an excess of sodium hypochlorite. On boiling, the cerium is precipitated as dioxide, which is filtered off, ignited, and weighed.
Cerium is detected by giving with borax a bead which is yellow in the oxidising, and colourless in the reducing flame. Traces of cerium compounds boiled with dioxide of lead and nitric acid will give a yellow solution.
LANTHANUM AND DIDYMIUM
occur together with cerium in cerite, and are separated with that metal as oxalates, as described under _Cerium_.
Didymium salts have a rose or violet colour, and impart (when in sufficient quant.i.ty) the same colour to the borax bead. Solutions have a characteristic absorption-spectrum.
The separation of lanthanum and didymium in the solution from which the cerium has been precipitated is effected by precipitating them together as oxalates, igniting, and dissolving in dilute nitric acid. This solution is then evaporated to dryness and ignited, for a few minutes, just below redness. A subnitrate of didymium is formed, and remains as an insoluble residue on extracting with hot water. The separated salts are treated with ammonia and ignited, and weighed as oxides (La_{2}O_{3} and Di_{2}O_{3}).