(3) CERTAIN PHYSICAL DETERMINATIONS. It is generally expected of the chemist that, if required, he is able to make certain physical determinations which are of value in analytical processes. Of these, the most important are determinations of Specific Gravity, Melting and Boiling Points, and Vapour Density. It is not intended here to do more than briefly touch on these matters. For further information the student is referred to text-books on Practical Physics, Ostwald's Physico-chemical Measurements, or the larger works on Analytical Chemistry. Specific Gravity. The following tabulation gives some of the most im portant methods used in the determination of the Specific Gravity of Solids and Liquids : Solids.-1. Not acted on by water.-A lump is weighed in air and then in water, and the S.G. calculated from the weighings. 2. Acted on by water.-A lump is weighed in air and then in some liquid such as naphtha, benzene, or alcohol of known S.G., and the S.G. calculated as before. 3. In a powdered state.-A small S.G. bottle (Pyknometer) is weighed full of water. Then a weighed quantity of the powder is introduced, and from the quantity of water displaced and the weight of powder taken the S.G. is calculated. Liquids.-1. Approximately by the Hydrometer. 2. Accurate to 001 by a 1 c.c. pipette. 3. Accurate to +00002 by the Sprengel Tube. In all of these methods the temperature is important; and where glass vessels are weighed, care must be taken that a uniform method of wiping is adopted, or a varying film of moisture will exist on the surface. Before weighing, the glass should be carefully dried with a clean, soft, linen cloth, and this treatment should be followed in all cases. As in his mineralogical and physical laboratory work the student has already had practice in determining the S.G. of solids, these will not be considered here. The Specific Gravity of liquids may generally be determined by the Hydrometer, the S.G. being read directly from the scale on the stem, either in terms of water as a standard of 1·000 (or 1000), or in degrees Baumé. The following formulæ serve to convert degrees Baumé to the usual standard : 135+n where n = the number of degrees Baumé. Thus 24° B. for a liquid lighter than water corresponds to = 145 145 912. These results are only approximate, and for accurate 135+24 159 conversions the student may consult the tables given in more advanced works. Rapid and accurate determinations may be made by the method described by Ostwald. A 1 c.c. pipette with almost capillary tubes is marked to hold 1 c.c. of water. Its weight when empty and dry is carefully ascertained. It is then filled (at the same temperature as before) to the mark with the liquid and the point carefully dried. It is then reweighed, and the weight gives the S.G. directly. For convenience in weighing, the pipette is laid flat on a small wire stand, the weight of which is accurately determined. For more accurate determinations the student may consult the works referred to. Determinations of Melting and Boiling Points and Vapour Density belong more to the sphere of General than Metallurgical Chemistry, and are simply mentioned to call the student's attention to certain auxiliary Physical methods which may in some cases be of service when determining the constitution of a liquid or solid. (4) POINTS ON LABORATORY WORK. (a) Economy of Time.-Economy of time is best secured by working from first to last as carefully as possible. "The more haste the less speed " holds here as in other things. "Quality" must be attained before attempting "Quantity." This must not be construed, as is sometimes done, into wasting time. A slow filtration goes no faster by staring at it, and in the time so lost a crucible could be ignited and weighed, or some other operation commenced. As many operations should proceed concurrently as can be attended to without confusion. At the outset one or two operations may demand the student's whole attention, but as time goes on his capacity for work will increase; and if, preparatory to his day's practical work, he carefully read up his instructions and map out his scheme of work, further time will be saved. He should so arrange his work that at no time is he idle, and at no time is he 'rushed.' If in doubt, it is preferable to err on the side of wasting time than to 'rush' a number of operations, as the repetition of work likely to ensue from undue haste will in the end consume much more time than would be required to do the work thoroughly in the first place. The student, then, in this matter must exercise to the full his common-sense, and carefully consider how far he may run several operations concurrently without affecting the thoroughness of either. He will also find that time is saved by strictly following all instructions laid down. If inclined to criticise any instruction as unnecessary or as an undue refinement, he must remember that it is inserted for some definite purpose, which may be revealed to him on more mature consideration. There is no 'short-cut to the end desired except that of patient and intelligent work. (b) Bench Work. As the experienced employer can at a glance estimate a skilled workman's value by the clean, free style in which he handles his tools, so the experienced chemist will at a glance discern from his work-bench the skill and value of the student. While not denying that accurate and quick results may be obtained on a bench littered with apparatus, clean and unclean, and in an order known (if to anyone) only to the student, such a student must remember that he is heavily handicapped if he seeks a living in the Chemical world, and at the very outset has earned a reputation for carelessness. All apparatus should be cleansed immediately after use, and unless required at once should be replaced in the locker on a clean sheet of white paper. All apparatus on the bench should be set in order. Beakers, flasks, wash bottle, and funnels should not be indiscriminately mixed, but each beaker should be placed beside its respective filter funnel, and all other apparatus except the wash bottle placed in a row behind the filtration. Similar methods should be followed with other operations. Care should be taken to exclude all foreign matter from an analysis. Cigarette ash cannot add to the accuracy of an analysis. The same remark applies to reagent bottles covered with dust. Again, when conducting more than one operation at a time, or putting away an unfinished analysis, the various vessels should be labelled with neat paper labels, or better, with a special blue pencil. These remarks could be greatly multiplied, but perhaps the best advice that can be given to the student is that he look round him and criticise the work of any careless fellow-students. He may then notice that in some points he himself is not quite perfect; and to avoid giving openings to would-be critics, he must immediately set to work to remedy these deficiencies. (c) The Equivalent System of Reagents.-This has been noticed previously, but a few additional remarks are necessary here. The student should form the habit of calculating the quantity of reagent required in every Thus 1 gm. Zn requires 308 c.cs. E. acid for solution. The equivalent of Zn is =32·5, and 1 c.c. E. acid will dissolve 32.5 mgms. Zn, therefore 2 case. 65 Again, 1 gm. Cu requires 315 c.cs. E. KHO for precipitation from solution as Cu(OH)2. The equivalent of Cu is 63.5 = 31.75, and 1 c.c. E. Therefore 1 gm. Cu KHO precipitates from solution 3175 mgms. Cu. =3 •03175 = 31.5 c.cs. E. KHO. Again, on neutralising say 5 c.cs. 36 E. H2SO with E. KHO there are produced 5 × 36 = 180 equivalents of Na2SO4; and as the equivalent of Na2SO is 142 = 2 12780 mgms. or 12.780 gms. Na2SO, in solution. 71 we have 180 × 71 = These calculations might be indefinitely extended, but the few given will be sufficient to show the simplicity of the equivalent system, and its suitability to the needs of the chemist who desires to conduct all operations under known and accurately defined conditions. CHAPTER IV. SIMPLE GRAVIMETRIC DETERMINATIONS. In this chapter certain simple gravimetrie determinations will be considered in detail. În each case the following order is adopted :: Apparatus and Chemicals required. Calculation and Accuracy of Results. Where no further apparatus than that already described is used, no details under this head will be given unless some special arrangement is adopted. It is always assumed that before commencing an analysis the student has all apparatus thoroughly clean and in readiness. Regarding the methods employed, brief notes will be given explaining the reactions on which the method is based, and any comments thereon which may aid the student. For reference on such points the student may consult Quantitative Analysis by Fresenius, or Qualitative Analysis by Prescott and Johnson. The student who wishes to excel must take advantage of these reference works, and should understand the reason for every step in the analysis. A merely mechanical attention to the instructions, however perfect, although it may yield good results, can never produce more than a rule-ofthumb' analyst. In the detailed instructions for each analysis the student is to understand that he is never to weigh exactly quantities such as 5 gm. or 1 gm. unless specially instructed to do so. When performing filtrations he may use the Gooch or the ordinary method, unless specially instructed otherwise. The Gooch method is generally preferable; but as the paper method is more difficult and is still largely used, it is described in the instructions. When instructed to incinerate a precipitate in a crucible, one of platinum may be used with advantage, provided it is not attacked by the material. If a platinum crucible is not available, porcelain may be used, and with care gives good results. Calculations of results are not worked out in detail, an outline merely of the necessary steps being given. The student who has studied the foregoing pages will have little difficulty in completing the required calculations. The authors would have much liked in every case to definitely state the degree of accuracy attainable by good work, but regret that, owing to the absence of data, they can do little more than state in many cases whether a method is regarded as good or otherwise. In simple estimations there is, as a rule, little difficulty in obtaining accurate results; but in complex substances, where an element has to be separated by precipitation from a number of others, complete precipitation can rarely be obtained in one step, as traces of other elements are also precipitated. Such a precipitate must be redissolved and reprecipitated until the desired end is attained. In these cases the precipitate must be examined qualitatively to detect the presence of impurities. Too little stress is laid by many text-books on the necessity of re-solution and precipitation when dealing with complex substances. The student will find a good concrete example of the necessity of this procedure when he comes to the analysis of an insoluble silicate; and if he compare the older methods with those laid down in the paper of Dr Hillebrand, he will find that considerable discrepancies are accounted for by incomplete precipitation. THE ESTIMATION OF COPPER IN COPPER SULPHATE, CuSO4,5H,O. The student may Apparatus, Chemicals, etc.-The usual equipment. take for analysis the salt which he has previously purified, or he may be given a certain volume of solution prepared and checked by the demonstrator, in which case the preliminary solution being already effected, the analysis is to be proceeded with from that point. Method, Reactions, etc.-The method here given is selected with the object of giving the student practice in the various manipulations of gravimetric analysis. Being rather tedious, it is not used by the chemist in routine work, and is replaced by the Cyanide, Iodide, or Electrolytic methods, which are given further on. In the method here employed the copper is precipitated from solution as the hydroxide Cu(OH), by means of a solution of KHO, and on boiling, CuSO4 + 2KHO= Cu(OH)2 + K2SO4 3Cu(OH), Cu(OH), 2CuO+ 2H,O 2 = a black hydrate of copper being formed. On gently heating the dried precipitate, Cu(OH)2=CuO+ H2O Fusion of the precipitate must be avoided or 5CuO = CuO,2Cu2O+O2 which will introduce an error of between 8% and 9% in the cupric oxide. On incineration of the paper with traces of precipitate, 2CuO+C=2Cu + CO2 The ash is then treated with a few drops of HNO3 3Cu+8HNO2 = 3Cu(NO3)2 + 4H2O + 2NO And on evaporation and re-ignition 2 Cu(NO3)2, 3H,O=CuO+2HNO3 + 2H2O Cupric oxide being again formed, and from the total cupric oxide obtained the copper is calculated. Details of the Analysis.-Transfer a few grams of the purified salt to a weighing tube. Insert the stopper and wipe the tube with a soft linen cloth. Weigh the tube and contents. Remove it from the balance and shake about |