After solution, certain minor operations may or may not be necessary, but as a rule the next essential operation is that of precipitation. In his qualitative work the student has already come across many cases of precipitation, and he will find that many of the methods there used are again applied for quantitative purposes. Silver, for instance, is precipitated as the chloride AgCl, copper as the sulphide by H2S, iron as the hydroxide by NH4HO, and so on. He will also find that some methods are introduced which are either not used or are of minor importance in qualitative work. Copper, nickel and other metals are quantitatively estimated by electrolysis.
As in this work the equivalent system of reagents is employed unless otherwise specified, the student may in every case calculate very closely the amount of reagent required to effect complete precipitation in a given case if he knows the quantity present of the substance to be precipitated. Even where this is not known, he may approximately estimate, for instance, that in half a gram of iron pyrites there will be somewhere between 40% and 50% of iron, that is, about .2 to .25 gm. of iron, and he may then proportion his precipitant accordingly, and the result will not be far from that desired. The habit of drowning the analysis with large doses of precipitant must be carefully avoided, not because, as the student sometimes thinks, the authorities grudge him materials, but because such a practice leads to very uncertain results. There are sufficient difficulties already in existence without increasing them by adding variable quantities of precipitant. In every precipitation there is a certain point, more or less definite, at which the best precipitation is attained. It is not claimed that this point is exactly known in all cases, or even in the majority of cases, but the instructions given in the following pages will in all cases at least give an approximation to the desired end—an approximation giving sufficiently accurate results for the purposes of the chemist employing gravimetric methods.
As the methods of gravimetric analysis are more and more investigated, the conditions securing accuracy become more defined; points of doubt are cleared up and the science becomes more exact. To benefit by the results of such research the student must have access to the current chemical literature; and whenever a criticism or research connected with his work appears, he should note its bearing and claims, and if possible try any variations or innovations suggested, bearing in mind that a new method must be viewed with caution, and must not be adopted till its claims are proved (personally) to be true.
In precipitation the following two main conditions should be aimed at:
(a) The precipitation should be as complete as possible.
(b) The precipitate should be obtained in the best possible condition for easy filtration and thorough washing.
Regarding completeness of precipitation, something has already been started in the Qualitative Section. Here it is even more important than there. The point at which precipitation is complete is rather difficult to determine in some cases.
Generally the calculated quantity may be added, and one or two c.cs. allowed in addition. A test may be made of the supernatant liquid, or by filtering a small portion and testing the filtrate. The student must bring all his chemical knowledge to bear in ascertaining the completeness of precipitation, and must always apply tests to check, the operation, that is, to ascertain whether too little or too much of the precipitant has been added.
Regarding the physical condition of the precipitate it is impossible to generalise, and whenever necessary, special instructions will be given. Frequently, though not always, precipitates thrown down in hot to boiling solutions, with the gradual addition of the precipitating solution, aided by continual stirring, produce the best results.
The precipitating reagent should always be added, unless otherwise specified, in the liquid form, or where a gas is the precipitating agent, it should be passed through the liquid, and in this case again heat frequently aids precipitation. Care must be taken that the gas is purified in a suitable wash bottle before passing through the liquid. The gas delivery tube should be drawn to a point about 2 mm. in diameter, and in certain cases slight pressure is beneficial, and may be obtained by precipitating in a flask, through the cork of which the gas delivery is led and a small exit tube is fitted, drawn to a fine point. Care should be taken that all the precipitate attached to the gas tube is removed. Further instructions will be given where needed in the details of methods given in the following pages. No special notes need be given regarding the vessels used for precipitation. For the generation of H2S, CO2, etc., Kipp’s Generator will be found convenient.