CHAPTER III.
WET GRAVIMETRIC METHODS.
In _dry a.s.says_ the metal is almost always separated and weighed as metal; in _wet_ gravimetric a.s.says the metal is more usually weighed in the form of a definite compound of known composition. The general methods of working resemble those of ordinary chemical a.n.a.lysis, and their successful working is greatly helped by a knowledge of, at any rate, those compounds of the metal which enable it to be separated, and of those which are the most convenient forms in which it can be weighed.
But the work of the a.s.sayer differs from that of the a.n.a.lyst, inasmuch as the bulk of his estimations are made upon material of practically the same kind, varying only in richness; consequently in a.s.saying, it is possible (and necessary) to work on such a definite plan as will involve the least amount of labour in weighing and calculating.
The a.s.sayer connected with mining has generally two cla.s.ses of material to deal with: those comparatively rich and those comparatively poor. For example, silver in bullion and in ores; copper precipitates or regulus, and copper ores and slags; and "black tin" and tin ores. He is only occasionally called on to a.s.say the intermediate products. It is indispensable that he should have an approximate knowledge of the substance to be determined. With new ores this information is best got by a qualitative testing. Knowing that only certain bodies are present, it is evident that the number of separations can be reduced, and that simple methods can be devised for arriving at the results sought for.
The best method is that which involves the least number of separations.
The reactions must be sharp and complete, and yet not be liable to error under varying conditions.
To bring the richer and poorer materials under the same conditions for the a.s.say, a small weight, say 1 gram of the richer, and a larger weight (5 or 10 grams) of the poorer, substance is weighed up. A method is then adopted which will concentrate the whole of the metal (either during or after solution) in a product which need not necessarily be pure. The work on this product is comparatively easy. In separating small quant.i.ties of a substance from a large bulk of impurities, the group separations must not as a rule be too much relied on. Very large precipitates carry down small quant.i.ties of bodies not belonging to the group, more especially when there is a tendency to form weak double compounds. The re-dissolving and re-precipitating of bulky precipitates should be avoided.
When a large number of a.s.says of the same kind have to be carried out, a plan something like the following is adopted:--The samples, after having been dried, are placed in order on a table at the left hand of the a.s.sayer. He takes the first, marks it with a number, samples and weighs up the quant.i.ty required, and transfers it to a flask, which is similarly marked. As the weighings are finished, the samples are placed in the same order on his right hand. The a.s.sistant takes the flasks in batches of four or five at a time to the fume cupboard, where he adds a measured quant.i.ty of acid. When solution has been effected, dilution with a measured volume is generally necessary. The a.s.sayer sees to this and (whilst the funnels and filters are being prepared) makes any separation that is necessary. The filters are arranged in order on a rack (fig. 11), and need not be marked unless the precipitates or residues have subsequently to be dried. The filters are washed with hot water, and if the filtrates are wanted flasks are placed beneath, if not, the solution is drained off down the sink. Precipitation or reduction (or whatever it may be) is now made; the a.s.sistant filters the prepared samples, one at a time, whilst the a.s.sayer is engaged with the others. The same style of work is continued until the a.s.says are completed. If one should be spoiled, it is better to allow it to stand over for a.s.saying along with the next batch. If one filters slowly or is in any way less forward than the rest, it may lessen the accuracy of the other a.s.says, owing to oxidation, &c., it should, therefore, be put on one side. The a.s.says are dealt with in batches of ten or twenty, so that a large quant.i.ty of work can be quickly finished.
[Ill.u.s.tration: FIG. 11.]
When the a.s.says are finished, it is the duty of the a.s.sistant to clean the apparatus (with reagents, if necessary), and to put the vessels in the place set apart for them. Flasks are best kept inverted on a rack, so that they may be dry and clean by the next morning. Berlin crucibles must be cleaned and ignited.
The amount of apparatus employed should be as little as is feasible. The a.s.say should be carried out as much as possible in the same flask. The bench must be clean, and altogether free from apparatus not in actual use. Crucibles and dishes in which weighings are made should be marked with numbers or letters; and their weights recorded, together with the date of weighing, in a small ledger, which is kept in the drawer of the balance. By this means a record of the "wear" of each piece of apparatus is obtained, and, what is more important, much weighing is saved, and increased confidence is gained. The weight of each piece of apparatus need not be taken daily. It will be seen from the record in the book and a knowledge of the use it has been put to how often a checking of the weight is necessary. The entries are made in black lead as follows:--
Dish, A. Feb. 3 9.4210 grams.
5 9.4225 6 9.4230 7 9.4200
Platinum vessels and apparatus lose, and porcelain ones slightly gain, weight with continued use.
The special details of the work is given under each a.s.say; certain general instructions will be given here.
~Solution.~--It is not always necessary to get the whole of the mineral in solution, provided the body sought for is either completely dissolved or altogether left in the residue. It is often only by a qualitative examination of the solution (or residue, as the case may be) that the a.s.sayer can satisfy himself that it is free from the substance sought.
But previous experience with the same kind of ore will show to what extent this testing is necessary.
Solution is generally best effected in flasks; but where the resulting liquid has afterwards to be evaporated to dryness and ignited, evaporating dishes (fig. 12) are used. With them clock gla.s.ses are used as covers during solution to avoid loss through effervescence.
Evaporating dishes are also best when an insoluble residue has to be collected, since it is difficult to wash out most residues from a flask.
b.u.mping occurs less frequently in dishes than in flasks.
[Ill.u.s.tration: FIG. 12.]
After the addition of the acid, and mixing by agitation, the vessel containing the substance is heated. This is best done on the "hot plate" (fig. 13). This consists of a slab of cast iron about half or three-quarters of an inch thick, supported on loose fire bricks, and heated by two or three ring burners (figs. 14 and 15). The burners are connected to the gas supply by means of _lead_ tubing, to which they are soldered. Flasks and dishes after being put on the plate are not further handled until solution is complete or the evaporation is carried to dryness. The hot plate is contained in a cupboard so as to be out of the reach of cold draughts.
[Ill.u.s.tration: FIG. 13.]
[Ill.u.s.tration: FIG. 14.]
[Ill.u.s.tration: FIG. 15.]
The action of the acids and other solvents is described in the chapter on Reagents.
~Precipitation.~--In precipitating add sufficient of the reagent to complete the reaction. The student must be on his guard against adding a very large excess, which is the commoner error. In some reactions the finishing point is obvious enough; either no more precipitate is formed, or a precipitate is completely dissolved, or some well-marked colour or odour is developed or removed.
In those cases in which there is no such indication, theoretical considerations should keep the use of reagents within reasonable limits.
The solutions of the reagents (_see_ REAGENTS) are generally of five or ten per cent. strength. A small excess over that demanded by theory should be sufficient.
[Ill.u.s.tration: FIG. 16.]
[Ill.u.s.tration: FIG. 17.]
[Ill.u.s.tration: FIG. 18.]
[Ill.u.s.tration: FIG. 19.]
~Filtration.~--Solutions are best filtered hot whenever the a.s.say allows of this being so done. The precipitate should be allowed to settle, and the clear liquid decanted on the filter with the aid of a gla.s.s rod if necessary. The filter-paper must not be too large, but at the same time it must not be overloaded with the precipitate. There should be ample room for washing. For general use three sizes of filter-paper are sufficient. Common quick filtering-paper (English) is best for most work in a.s.saying. The specially prepared paper (Swedish or Rhenish) is used for collecting those precipitates which have to be weighed. The papers are folded as shown in fig. 16, and should not project above the funnel.
The filter-paper works better if damped with hot water. In special cases filtering is hastened by means of an air-pump. The apparatus used consists of a water-jet (fig. 17), which is connected with the tap, as also with a bottle fitted as shown in fig. 18. The pump draws the air out from the bottle, and atmospheric pressure forces the liquid through the filter-paper. The bottom of the funnel is provided with a platinum cone, which supports the filter-paper, and prevents its breaking. The pump is only used in exceptional cases; nearly all the filtrations required by the a.s.sayer can be made without it. The usual methods of supporting the funnel during filtration are shown in fig. 19. Where the filtrate is not wanted, pickle bottles make convenient supports. After the precipitate has been thrown on the filter, it is washed. In washing, several washings with a small quant.i.ty of water are more effective than a few with a larger quant.i.ty of that fluid. The upper edge of the filter-paper is specially liable to escape complete washing. Excessive washing must be avoided; the point at which the washing is complete is found by collecting a little of the filtrate and testing it. The precipitate is removed from the filter-paper for further treatment by opening out the paper and by washing the precipitate with a jet of water from a wash-bottle into a beaker, or back through the funnel into the flask. In some cases, when the precipitate has to be dissolved in anything in which it is readily soluble, solution is effected in the filter itself allowing the liquid to run through as it is formed.
~Drying and Igniting.~--Precipitates, as a rule, require drying before being ignited. With small precipitates the filter-paper may be opened out, and placed on a warm asbestos slab till dry; or the funnel and the filter with the precipitate is placed in a warm place, and supported by any convenient means. The heat must never be sufficient to char the paper. Some precipitates must be dried at a temperature not higher than 100 C. These are placed in the water-oven (fig. 20), and, when apparently dry, they are taken from the funnel, placed between gla.s.ses, and then left in the oven till they cease to lose weight. Such precipitates are collected on tared filters. Those precipitates which will stand a higher temperature are dried in the hot-air oven at a temperature of from 120 to 150. The drying is continued until they appear to be free from moisture, and until the precipitate ceases to adhere to the filter. In drying sulphides the heat must not be raised to the melting point of sulphur, since, if there is any free sulphur present, it fuses and filters through.
[Ill.u.s.tration: FIG. 20.]
The precipitate, having been dried, is transferred to a watch-gla.s.s. The filter-paper is opened out over a sheet of note-paper, and, with a camel-hair brush, the precipitate is gently brought into the gla.s.s. Most precipitates come away easily, and the transfer can be made without apparent loss. The watch-gla.s.s is covered by the funnel, and the filter-paper (folded into a quadrant) held by the tweezers and set fire to with the flame of a Bunsen burner. It is allowed to burn over the crucible, into which the black bulky ash is allowed to drop, and two or three drops of nitric acid are then added. The crucible is placed on a pipe-stem triangle (fig. 21), supported on a tripod. It is at first heated gently with a Bunsen burner, and afterwards more strongly, until the residue is free from carbon. It is cooled, and treated with any acid necessary to convert the small amount of precipitate into the state in which it is to be weighed; heated again, and cooled. The main precipitate is transferred to the crucible, and the heating repeated very gently at first, but more strongly towards the end of the operation. It is next placed in the m.u.f.fle, and, after two or three minutes at a red heat, it is removed and allowed to cool in the desiccator before weighing. This is for bodies that will bear a red heat; for those compounds that require a lower temperature the heating in the m.u.f.fle is omitted. The m.u.f.fle used for this purpose must not be used at the same time for cupelling; a gas m.u.f.fle (fig. 22), such as one of Fletcher"s, is best. A desiccator (fig. 23) is an air-tight vessel which prevents access of moisture, &c., to the substance. Usually the air in it is kept dry by means of a basin containing sulphuric acid.
[Ill.u.s.tration: FIG. 21.]
[Ill.u.s.tration: FIG. 22.]
[Ill.u.s.tration: FIG. 23.]
The crucible is removed from the m.u.f.fle with the tongs and carried to the desiccator. It is best, in an office, to have a large desiccator permanently fixed alongside the balance, into which all substances may be put before being weighed. The substance is removed from the bench or m.u.f.fle in the small hand apparatus generally sold, and carried to the balance room to be transferred to the large desiccator, where it is allowed to become thoroughly cold before being weighed. Twenty minutes is generally the time allowed after ignition before it is advisable to weigh. Bodies allowed to cool in the air after they have been ignited will absorb moisture, and hot bodies placed in the balance-pan will disturb the equilibrium and show false results. Compounds that absorb moisture must be weighed quickly; they should, therefore, be weighed in covered vessels. Such compounds are detected by their continually-increasing weight. They should be ignited and weighed again in a well-covered dish.
Substances that have been washed with alcohol, ether, or any readily volatile liquid are dried in the water oven. They quickly dry if there is no water present, and are generally fit for weighing in less than one hour. Sometimes drying for a few minutes only will be sufficient.
The weight of the crucible and precipitate having been obtained, the weight of the crucible and ash is deducted; for example--
Crucible and precipitate 10.183 grams.
Crucible and ash 9.527 "
------ 0.656 "
The weight of the ash is best added to that of the crucible. The amount of ash in filter-papers must not be neglected, although papers are now made almost free from ash, and the amount to be deducted is found by taking eight or ten papers and burning them until they become white, and then weighing the ash. The amount varies from 0.004 to 0.0005 gram for different papers. Having determined the ash, place in the balance-drawer three of the filter-papers pinned together, with the weights marked on them in the way shown in fig. 24, so as to be readily seen when there is occasion to refer to them.
[Ill.u.s.tration: FIG. 24.]
It must be remembered that the determination of small quant.i.ties of substances generally involves the use of reagents which are often contaminated, as an impurity, with the body sought for. Thus, in a.s.saying silver, the oxide of lead or metallic lead used is rarely free from silver; and in the case of a.r.s.enic, the acids, zinc or ferric chloride are sure to contain a.r.s.enic. The same observation applies to the precipitation of lead by zinc, &c. The errors caused by these impurities are more marked in the determination of material having small quant.i.ties of metal than in that of ores which contain larger quant.i.ties. Errors of this kind are counteracted or neutralised by "blank" or "blind" determinations. These consist in carrying out by the side of and during the a.s.say a duplicate experiment with the reagents only, which are thereby subjected to the same processes of solution, evaporation, filtration, &c. The final result thus obtained is deducted from that given by the a.s.say, the difference gives the corrected result.
In some cases, where it is desired or necessary to have a tangible residue or precipitate, some _pure_ inert material is added.