QUANTITATIVE ANALYSIS.] 



CHEMISTRY. 



403 



By means of the indications thus afforded by hydro- 

 snlphuric acid and hydro-sulphate of ammonia, the cha- 

 racteristics of the metals, in respect to their sulphides, 

 are readily grouped; and hence a great assistance is 

 afforded in analytical investigations. The results obtained 

 by the addition of the alkalies and their carbonates are 

 equally valuable ; and the previous tables will thus be 

 found of essential service to the student. As they are the 

 result of direct experiment, he may place every reliance 

 on them, although a repetition of the experiments will 

 prove of great use, as tending to impress the facts on 

 the mind. 



We have thus afforded the student ample means for 

 commencing: a course of qualitative analysis. Com- 

 mencing with bodies of which he knows the constitution, 

 or can find it by referring to our previous pages, he may 

 gradually pass on to examine others of a more complex 

 character. From such he may proceed to analyse minerals 

 of various kinds ; and having acquired tact, skill, extended 

 information, keenness of observation, and last, but by no 

 means least, patience, he will be qualified to detect the 

 presence of poisons, under circumstances in wliich, from 

 the minuteness of their presence, all his chemical know- 

 ledge and qualifications will be called into exercise. 

 We shall now give general directions for the prosecution 

 of quantitative analysis, presuming that our readers have 

 already acquired suflicient information in the previous 

 department of analytical research to enable them to make 

 f iirther progress. 



INORGANIC QUANTITATIVE ANALYSIS. 



WHILST in the qualitative analysis of any substance we 

 are content to simply ascertain the presence of the various 

 constituents, the object of quantitative analysis is that 

 of finding the quantity of each substance composing the 

 body under examination. We need hardly state that 

 great skill and care in manipulating is required. In 

 former days 100 grains were generally taken for an 

 analysis ; a course wliich, whilst in appearance would 

 afford a chance of obtaining reliable results, actually 

 defeated that object, by the diffusion, of both the sub- 

 stance tested and the reagent, in large quantities of 

 liquids. The operations, of course, were on the large 

 scale. Furnaces, vessels, and every apparatus were of 

 the largest size ; and a laboratory for such purposes 

 could only be commanded by persons possessed of great 

 pecuniary resources. At the present time, a table, a 

 gas-furnace, test-tubes* and other small apparatus, are 

 considered quite sufficient for all purposes. Only a few 

 grains of any substance are needed ; and in some cases 

 less than a grain satisfies the requirements of an ex- 

 perienced chemist. Former sources of error are entirely 

 avoided ; and an accuracy, which in the earlier days of 

 chemical science was not dreamt of, is expected from 

 even the pupil ; much more so from the professor. It 

 will be impossible for us to give detailed instructions for 

 the quantitative analysis of every class of bodies which 

 come under examination in the laboratory. For such 

 information we must refer our readers to works ex- 

 clusively devoted to the subject. We shall bo content 

 to give general outlines for the direction of the student, 

 and must leave to his industry and skill the chance of 

 his becoming proficient in this interesting branch of ex- 

 perimental science. 



Of course, before attempting to ascertain the relative 

 quantities of the constituents in any substance, their 

 existence must be ascertained by means of a qualitative 

 analysis. And this must not be cursorily perform* ! ; 

 for if anyone, however minute, of the bodies c" 

 tnting the whole quantity be neglected in a preliminary 

 examination, then an accurate quantitative analysis 

 cannot be effected. Hence, as we have already remarked, 

 considerable chemical experience is necessary to qualify 

 any one for an analyst. A vast variety of minor circum- 

 stances must also be kept in mind. Tims, some sub- 

 stances almost entirely insoluble in distilled water, are 

 readily dissolved if others are present ; and this often 

 occurs through the introduction of salts, (fee., in the 



course of analysis. For instance, many oxides, <fea, 

 precipitated by an exact equivalent of potass and am- 

 monia, are insoluble in water. If, however, an excess 

 of the test be present, a portion of the precipitate will 

 be redissolved, and a source of serious error introduced. 

 Many cases of this kind may be noticed in the tables of 

 tests, <fec. , in our previous pages. Occasionally, the pre- 

 sence of certain salts, or their absence, will interfere with 

 precipitation completely. Thus, the sulphate of lead is 

 readily precipitated from the acetate by means of sul- 

 phuric acid, or a sulphate ; but if the sulphate of am- 

 monia, or other salts of that alkali be present, precipi- 

 tation may be entirely prevented. 



In quantitative analysis, it is of the utmost importance 

 that the whole of the precipitate be thrown down. This 

 will not occur where a sufficiency of liquid is not used ; 

 but caution in this respect is requisite, for an excess 

 may interfere injuriously e.g., the sulphate of lime, 

 which is barely soluble in water, and may be almost 

 entirely precipitated in strong solutions ; whilst a portion 

 will remain in solution in those which are soluble. Tem- 

 perature, also, materially influences precipitation ; for 

 occasionally a precipitate will remain suspended in the 

 fluid at ordinary temperatures ; wlu'Ist, on boiling, it will 

 be speedily thrown down. Such is the case' with the 

 ferro-cyanide of iron, and many other salts. At times 

 the physical condition of the precipitating vessel will 

 influence the result e.g., the bitartrate of potass will 

 only be thrown down on a roughened surface. 



Great care is required in collecting precipitates during 

 filtration. In transferring them from the precipitating 

 jar to the filter, a lipped vessel should always be used, 

 wliich should be repeatedly washed out, and the washing 

 added to the filtrate. The choice of filtering-paper is 

 also of great importance. Two sorts are sold for che- 

 mical purposes; one being of a porous, open nature, 

 suitable for common operations ; and the other of a line 

 texture, which alone should be employed for analytical 

 purposes. All paper contains a portion of earthy mat- 

 ter, the quantity of which varies according to the 

 material employed, and the water used at the mill. In 

 some parts of England we have detected both lead and 

 iron in paper manufactured with water derived from 

 proximity to mines of those metals. It is therefore a 

 good plan to purchase a quantity of filtering-paper of 

 one manufacture ; and having ascertained the amount 

 of ash it affords per 100 grains, by burning it the next 

 step is to make a qualitative analysis of the ash, so as to 

 learn the possible nature of the impurity it may com- 

 municate. Some recommend to soak the paper in 

 dilute nitric or hydrochloric acids ; but this, whilst not 

 affording any great advantage, is a trtiublesome process; 

 and even, if not carefully employed, may introduce fresh 

 sources of error. Thus, if the paper were soaked in 

 hydrochloric acid, and every trace of the latter had not 

 been removed, an analysis of any substance containing 

 salts of silver or lead, would afford a deficiency of the 

 base, by the conversion of a portion into a chloride of 

 the metal, which would most likely remain in the pores 

 of the paper itself. 



In very delicate investigations account must bo taken 

 of the moisture which the paper contains. Persons 

 unaccustomed to the delicate nature of some analyses, 

 would scarcely credit how necessary it is to ensure accu- 

 racy in all respects. It is best to use the filtering- paper 

 after completely drying it. Pieces are then to be cut off, 

 as near as possible the same size, one of which is em- 

 ployed to weigh the substance before analysis, and the 

 there in its course ; for nothing should be weighed in 

 the scale-pan itself. The exact weight of the dried 

 filter must, of course, be ascertained, and carefully noted 

 down. It is by no means a useless plan to make a nick 

 on all the papers likely to be used for one analysis, as 

 by that means a chance of error may be avoided ; and no 

 inconvenience can arise, because they can thereby be all 

 identified as belonging to the same kind of paper, and to 

 be used for the same purpose. 



For quantitative analysis a most delicate balance is 

 required. In the previous experiments, any pair of 



