MISTRY. 



[QUANTITATIVE ANALYSIS. 



be cautious how they draw oat their proportion!. Some 

 yean ago, we entrusted to a pupil the :i some 



r cm, by way of trying his abilities. Wo gave 

 him fifty grain* on which to try hia skill. In the count 

 of a day or to, he stated that he had obtained 200 

 grain* of pure copper from it The result w:i certainly 

 astonishing: but it> won<ior immediately lessened on 

 the arithmetical process being examined. The analysis 

 was near enough ; but in casting the proportion, ho hat 

 misstated it, and brought out 300 grain* instead of 

 two per cent , by some mysterious process of calculation. 

 It will be seen, from the preceding remarks, that an 

 extended knowledge of the equivalents and constitution 

 of chemical compounds is essential to progress in quanti- 

 tative analysis, and a free use of the symbols is equally 

 requisite, as much time is saved thereby. It is an 

 excellent plan for the student to copy out the equiva- 

 lents of all the simple and oonpoud bodies he can gain 

 information of, and writing them, in a tabular f<>rm, 

 on a piece of cardboard, he should hang it up in some 

 conspicuous place in his laboratory. liy this much time 

 is saved, and reference to books is rendered unnecessary. 

 The latter soon get spoilt in a laboratory. It is also 

 advisable to have extended tables of specific gravities, 

 >er particulars, for the purpose of constant re- 

 ference, which should always be ready to hand. 



The foregoing general hints will be sufficient to remind 

 the student of many important matters requiring his 

 attention whilst pursuing quantitative analysis. We 

 shall now pass on to give an instance or two of the 

 practical method, and to mention various particulars 

 which have to bo attended to in special cases. We must 

 mention, however, the following rules as never in any 

 case to be neglected. 



1. A qualitative analysis should always first be made. 



2. Care must be taken in quantitative analysis never 

 to lose a drop of solution or a particle of a precipitate. 



3. All washings must be added to the original solu- 

 tion from which the precipitate has been thrown down, 

 and the precipitate itself must be washed (if insoluble in 



; water) until every trace of soluble matter has been 

 r m i. 



4. The precipitate must be thoroughly and carefully 

 dried. It is not to be weighed till cool ; and if s 



; metric, must, for that purpose, be placed in a thin glass 

 tube, or between two watch-glasses. 



6. And, lastly, every step of the analysis must be 



carefully noted down as it occurs, and each vessel should 



' be labelled with letters, Arc., corresponding to those at- 



.1 to entries in the note-book. 



PRACTICAL 1 LUSTRATIONS. For the sake of morn 

 fully explaining the methods adopted in quantitative 

 analysis, we shall select the following examples, which 

 may be taken a* types of the general plans to be 

 followed in most cases. There will be sufficient variety 

 to enable the student to comprehend numerous other 

 | instances with which he may meet. 



Experiment 90. Dissolve a three- penny piece in nitric 

 acid. Now, as silver is always alloyed with copper to 

 f"nn the material from which our coin is manufactured. 

 nitrates of silver and copper will be left in solution. It 

 is required to make an analysis affording the actual 

 iit of each metal composing the coin. The solution 

 is to be diluted with distilled water, and to this a solu- 

 tion of common salt is to be added so long as a white 

 precipitate is formed. The silver is thus removed as 

 the chloride, whilst the copper remains in solution. 



i I'our solution ami precipitate cautiously and 

 gently on to a weighed filter,* supported in a funnel, 

 and receive the liquid, which will run through, in an- 

 other vessel Wet the filter with distilled water before 

 pouring in the solution, A-e.t When all the liquid has 

 passed through, wash the precipitate by means of the 

 washing- bottle, j so as to remove ev.ry trace of the 

 r solution, .Vc. As the Utter will spread by capil- 

 lary attraction through every part of the paper, wash 

 the filter as carefully as the precipitate iteelf. The 

 washings, of course, must run through to the solution in 



. 4M. 



the lower vessel, as they will contain a minute p..rti..n of 

 copper. When the washing is completed, remove the 

 filter carefully from the funnel, and place it on the 

 water-bath, leaving it there till completely dry. Whilst 

 the chloride of silver is being dried, the copper left iu 

 solution may be obtained. 



(6.) If the solution left after removing the chloride 

 have been largely increased in bulk by using too much 

 water for washing, evaporate until not more than an 

 ounce in measure is left. To this add a little hydro- 

 chloric acid, and then a few zinc shavings or some clean 

 iron scraps. Heat the solution gently for some time, 

 and then evaporate nearly to dry ness. Add a little 

 dilute sulphuric acid, to remove all excess of iron or 

 zinc. The precipitate produced will be metallic copper, 

 which is to be collected on a weighed filter, and well 

 washed. It is then to be quickly dried by means of the 

 water-bath. 



Tho next step is to weigh each product of the 

 analysis. We will take the chloride of silver first. If 

 the operation have been properly conducted, and sup- 

 posing the filter to weigh 20 grains, it and the precipitate 

 together should weigh 4<>J grains nearly. Hence the 

 precipitate will afford 26J grains of chloride of silver. 

 The amount of silver is easily found from this ; || for the 

 proportion will be 



As an equivalent of chloride of silver is to that of the 

 product of the analysis, so is the amount of silver con- 

 tained in an equivalent to that contained in the pre- 

 cipitate ; or, expressed in figures 



As 144 : 2CJ : : 108 : 20^ grains. 

 2CJ 



048 

 216 

 64 

 27 



144) 2889 (20. ? T = 

 988 



9 



grains. 



We thus find that the coin contained 20^ grains of 

 pure silver. 



If the copper be then weighed after drying, it will 

 afford about Ij- grains. Thus the composition of the 

 three-penny piece will be 



Of silver 20,^ grains 



Copper 1J 



Total 21}J grains. 



Which is a little less than that of the new coin, as issued 



Erom the mint. 

 It is usual to express all figures in an analysis in 



whole numbers and decimals ; the latter being used in 



place of fractions : the results of the analysis will then 



stand as under : 



Silver 20-0ti'_'."> 



Copper 1-7500 



Total 21-8125 grains. 



The above examples may be easily repeated with 

 successful results, because both the products of the 

 operations are insoluble in water ; and therefore no error 

 can arise except that proceeding from carelessness. In 

 nan}' instances, the slight solubility of a precipitate 

 nterferes with the accuracy of the result, and in such 

 cases the total of the original weight is made up after 

 inalysis. by adding in to the amounts of the found bodies, 

 * representing the actual loss, and marking them 

 as such. 



In the next example we shall confine our attention to 

 weighing only one of the substances present in the body 

 analysed namely, the sulphuric acid. 



Kmrimenl 91. Di.s>nl\e t \\eiity grains of the sulphate 

 of potash in an ounce of distilled water, and add thereto 

 so much solution of the chloride of barium as is required 



i See aalr, p. 401. | Ante, p. 40}. 



