ANALYTICAL CHEMISTRY.1. INTRODUCTION.IN continuation of our policy of last year we have preferred thechoice of a few themes t o an attempt a t a Report covering all phasesof Analytical Chemistry. By this preference, most of the subjectsdealt with have been given fairer and more detailed treatment thanwould have been possible had our range been extended over a widerfield.The microscope is a familiar instrument in the analytical labor-atory. A review of its diverse uses for identification and estimationand of recent advances and extensions of its applicability is given.A note on a related method of identification dependent uponcrystalline form is included under the heading of crystallochemicalanalysis.The marked scientific attention which industrial hygiene hasdemanded during the past ten or fifteen years has prompted anaccount of the analysis of dusts and smokes.It is primarily fromthe industrial aspect that the progress in gas analysis is also reported,and the account which follows may be regarded as being supple-mentary to the article on gas analysis which appeared in the AnnualReports, 1933.To the analyst the occurrence of pronounced adsorption is usuallyan unwelcome phenomenon in analytical procedure, resulting as itdoes in the loss of a fraction of the material to be estimated inpreliminary processes of separation, or in contamination of theultimate precipitate, and this is one aspect of sulphide precipitationwhich is discussed below. Adsorption has, however, been put tocertain analytical uses.These are usually specific, as for examplein the collection of minute amounts of bismuth from solution bycoprecipitation with ferric hydroxide, but a widely applicableanalytical technique, namely, chromatographic analysis, has beenfounded on adsorption phenomena.2. MICROSCOPY.It has been mentioned that any property of a substance, physicalor chemical, which is characteristic of that substance may be utilisedto identify that substance and determine its proportions inmixtures,l and it has been said that the chance of two differentAnn. Reports, 1938, 35, 380GRIFFITHS, GULL, AND WHALLEY. 389substances having the same 10 or 12 characters directly observablemicroscopically is about 1 in lo6, so it is not surprising that incertain fields of analytical chemistry, the microscope can play avery important part.The fact that without undue difficulty it is possible to observemicroscopically the shape or crystal habit of particles of the orderof 10 p in diameter means that in principle it is possible to identifya particle weighing about g., i.e., 1 pg., in so far as the shape,colour, refractive index, action on polarised light, fluorescence andother features, and in addition changes brought about in these byexternal agency, are observable microscopically and are indicativeof a particular substance.Perhaps the simplest case is that of adding a drop of reagent to adrop of the solution of the unknown on a microscope slide or in acapillary tube and observing microscopically the properties of anysolid, amorphous or crystalline, which may separate out.Thisparticular method has been developed during many decades, andwhen compared with ordinary qualitative analysis has advantagesof speed, since it is evident that a drop of fluid can be decanted,filtered, or evaporated much more rapidly than 10 ml. There arealso advantages of economy and certainty, the latter being due tothe fact that some physical attributes of the solid separating are notgenerally noted in ordinary “ macro ” qualitative analysis, but areeasily observed under the microscope and have considerablediagnostic value. An authoritative description of microscopicalqualitative inorganic analysis was published in 1931 and progresshas continued steadily in the inorganic and the organic fields in thedirection of discovering more sensitive and more highly selectivereactions and in the analysis of mixtures, but owing to the volumeof work, only a small portion can be referred to directly in thisReport.In order to identify a substance by micro-crystal methods, itis frequently necessary to remove certain other material ; e.g.,0.5% aqueous anthranilic acid gives characteristic crystallineprecipitates serving for the detection of not less than 0.013 pg.ofcopper, 0.06 pg. of mercurous, 0.015 pg. of palladous, 0.05 pg. ofzinc, and 0.24 pg. of silver ions in the presence of each other andother metals, but cobaltous, ferric, and ceric ions must be a b ~ e n t . ~In view of the interference of one element with the tests for another,methods of separation into groups have been devised.In the caseE. M. Chamot and C. W. Mason, “ Handbook of Chemical Microscopy,Vol. 11, Chemical Methods and Inorganic Qualitative Analysis,” Chapmanand Hall.0. G. Scheintzis, J. Gen. Uhern. Russia, 1938, 8, 596390 ANALYTICAL CHEMISTRY.of non-ferrous alloys, separation may be effected by dissolution innitric acid followed by the use of a series of reagents not all normallyregarded as (( group reagents " in ordinary qualitative analy~is.~Progress has also been made in the direction of finding veryselective and sensitive reagents for ions, as in the case of lead, with0-02 pg. of which, at a concentration of 1 part in 40,000 of pure dilutenitric acid, a crystal of thiourea gives characteristic crystals.Onlythallium and some platinum metals give similar crystals, but excessof silver or copper may alter the cryst'al form, and together withbismuth should be removed by a preliminary electrolysis, lead beingdeposited on a platinum anode.5 Zinc in concentration as low as1 in 100,000 can be detected by the characteristic crystals of zinchydrogen p-naphthaquinoline thiocyanate in the presence of calcium,magnesium, beryllium, aluminium, tervalent chromium and cerium,ter- and quinque-valent arsenic, manganese, and nickel, as well asalkali metals.6 Silver, mercury, and lead may be detected in thepresence of each other without separation, by dividing the test dropinto three portions, one each of which serves for the identificationof silver as chromate, mercury as cobalt mercury thiocyanate, andlead as its triple nitrite with copper and potassium.'As illustrative of the close connection between organic and in-organic microscopical analysis, it may be mentioned that previousmethods of detecting yohimbine, a potent alkaloidal stimulant, havebeen neither highly selective nor very sensitive, but if a solution of notless than 2 pg.of the hydrochloride at a concentration of not less than1 in 5000 is heated with a particle of potassium cyanide, character-istic crystals are obtained, as is also the case when borax, sodiumselenite, sodium tellurite, and potassium oxalate are used'as reagents.Conversely, yohimbine can be used to detect borate, selenite, tellur-ite, and oxalate ions.8Microcrystalline tests are of particular value in the recognition ofalkaloids, drugs, and other organic substances, and only a limitedamount of work done in this field can be referred to.A saturatedsolution of lead iodide in potassium acetate is a sensitive reagentgiving characteristic crystals with 15 alkaloids and synthetic drugs,and a number of drugs give amorphous precipitate^.^ A newsystematic classification of some 100 reagents in use for identifyingalkaloids is suggested; and a table is given showing them in the4 W. F. Whitmore and F. Schneider, Ind. Eng. Chem. (Anal.), 1930, 2, 173.C . Mahr, Mikrochem., 1939, 26, 67.E. B. Sandell, D. M. Wishnick, and E. L. Wishnick, Milcrochim. Acta,B. Berisso, Mikrochem., 1939, 26, 221.A.Martini, ibid., p. 227.G. H. Wagenaar, Pharm. Weekblad, 1939, 76, 276.1938, 3, 204GRIFFITHS, m L , AND WHALLEY. 39 1order of their precipitating power.1° Work on the recognition ofthe ingredients of commonly occurring mixtures of drugs is proceed-ing. For example, Reinecke's reagent, [Cr(NH,),,(SCN),]NH,, givescharacteristic crystals with 1 pa'rt of brucine in 500 parts ofstrychnine, and morphine may be detected in the presence of otheropium alkaloids.ll Brucine and strychnine give widely differentcrystals with rhodium chloride, and both can be recognised therebyin certain mixtures.12 Recently, the use of mixtures of hydroxy-benzoic acids and their esters came into prominence, as preservativesof foodstuffs, and it was necessary to distinguish between smallquantities of salicylic and p-hydroxybenzoic acids.It was foundthat the latter, alone or in presence of the former, gives characteristiccrystals with copper ~u1phate.l~In addition to the general nature of the crystal habit, otherproperties assist discrimination. The angles between crystal faceshave long been used as a means of identifying substances, and arecent development of this method is referred to later (p. 398).The use of measurements of the angular constants of microcrystallineprofiles and silhouettes in the conclusive identification of substancesis discussed.14 Such measurements may be made microscopicallyby means of a rotating stage or a goniometer eyepiece. In thisconnection, the effect on crystal angles of depositing thin filmsof crystallisable substances on glass has been investigated.As anexample, it is found that sodium thiogulphate so deposited can beidentified from the crystal data.15A knowledge of the refractive index is a valuable adjunct to therecognition of a pure substance, and in the case of a solid particle itmay be determined by microscopic observations when the particleis immersed in liquids of known refractive index, but in which theparticle is insoluble. This requires series of suitable liquids whichdiffer in refractive index by small and regular intervals over theappropriate range. In view of difficulties in collecting such a setof pure liquid compounds, mixtures having definite refractive indicesdetermined refractometrically have been used.In order to ensureconstancy of composition during use, the components should havevery similar vapour pressures. In the case of inorganic substances,a-bromonaphthalene, butyl phthalate, and heptoic acid mixturescover the range 1.658-1.423, and mesitylene-ethyl propionatelo C. C. Fulton, Amer. J. P h m . , 1939, 3, 184.l1 P. Duquenois and Mlle. Faller, Bull. SOC. chim., 1939, 6, 998.l2 A. Martini, Mikrochem., 1937-1938, 23, 164.l3 F. W. Edwards, H. R. Nanji, andM. K. Hassan, Analyst, 1937,62, 178.l4 A. C. Shead, I n d . Eng. Chem. ( A w l . ) , 1938,10, 662.l5 G. Cesko and J. MBlon, Bull. Acad. roy. Belg., 1938, [v], 24, 558392 ANALYnCAL CHEMISTRY.mixtures cover the range 1.498-1.384.16 The refractive indexof small quantities of organic liquids is determined by observationson a scratch at the bottom of a hole 5 mm.deep and 1 mm. indiameter in which the fluid in question is placed.17 Characteristiccrystal habits and optical data are described for the salts of picrol-onic acid with 27 amino-acids, and in nearly all cases the refractiveindex of the crystals can serve for identification.lsThe polarising microscope is of assistance in identifying crystallinematerial, particularly as many organic compounds exhibit bire-fringence, and the magnitudes of the properties are of diagnosticvalue. A new technique depends on allowing the substance tofreeze under a cover glass supported a t one side so as to form wedge-shaped crystals, the polarisation colours of which are examinedbetween crossed Nicol prisms with a grating microspectrograph.The patterns obtained assist comparison and identification ofsubstances. The utility of a grating microspectrograph in recognis-ing small quantities of material by means of their absorption spectra(cf.Ann. Reports, 1938, 35, 394) is self-evident, and an instrumentin which these and polarisation phenomena can be photographed isdescribed.lg In another case, optic axial angles of binary mixturesof acet- and propion-p-bromoanilide were determined at five wave-lengths, and two of the three types of crystal dispersion exhibitedare found to be functions of composition, thereby providing a basisfor identifying small amounts of acetic and propionic acids anddetermining approximately the composition of Thepolarising microscope can be used in detecting and identifyingotherwise unworkably small quantities of material, in identifyingproducts of reaction, often in a crude state, and in detecting andidentifying unsuspected products of reaction,21 and it is consideredthat any crystalline substance is adequately defined by the followingdata taken together : solubility, elementary composition, m.p., andthe magnitudes of the refractive indices, all of which are derivableby micro-met hods .22Fluorescence microscopy has found considerable application inthe examination of biological material, and i s used analytically as asorting test, as, e.g., in a mixture of novocaine and cocaine, theparticles of the first ingredient fluoresce whilst those of the secondare not luminescent in ultra-violet light, thereby showing that the16 A.H. Kunz and J. Spulnik, I n d . Eng. Chem. (Anal.), 1936, 8, 485.l7 P. L. Kirk and C. S. Gibson, ibid., 1939, 11, 403.R. Dunn, K. Inouye, andP. L. Kirk, Mikrochem., 1939, 2'7, 154.l9 E. E. Jelley, J . Roy. Microscop. SOC., 1936, [iii], 56, 101.2o W. M. D. Bryant, J . Amer. Chem. SOC., 1938, 60, 1934.21 H. C. Benedict, I n d . Eng. Chem. (Anal.), 1930, 2, 91.22 P. L. Kirk and C. S. Gibson, Zoc. cit., ref. (17)GRIFFITRS, GULL, AND WHALLEY. 393powder is not homogeneo~s.~~ The method, incorporating a micro-spectroscope, has been used quantitatively, as in the detection anddetermination of approximately g.of samarium and dysprosiumin a borax bead.24Within recent years, the accuracy of the determination of themelting point of substances by means of a microscope heating blockhas been increased, and it is advantageous, particularly in the caseof dark-coloured crystals, especially porphyrins, to illuminate thecrystals with polarised light and observe the material through ananalysing eyepiece. Birefringent solids shine brightly when thesurrounding field is at maximum extinction. The colour fades atthe m. p., but reappears with s~lidification.~~ The identification oforganic substances by the m. p. and refractive index of the melt,determined microscopically, has been described.26 The separation,isolation and identification of traces of substances and discriminationbetween chemically similar organic compounds is frequently facilit-ated by forming derivatives having well-defined melting points.Thefield in which the microscopic method is available is merely indicatedby reference to some of the papers describing the characterisation ofsubstances by means of the m. p. of derivatives. Certain aromaticpolynitro-compounds are identified thus by means of their additioncompounds with na~hthalene,~' and a number of naphthyl ethersafford suitable picrates.28 Dinitro-derivatives of certain sulphonesare highly chara~teristic.~9 Isomeric hexanols are differentiated bythe m. p. of their dinitrobenzoates and the additive compoundsof these esters with a-naphthylamine.30 Many organic acidsform suitable salts with benzylisothiourea,3l monobasic saturatedaliphatic acids are characterised by the m.p. of their derivativeswith pp'-diaminodiphenylmethane 32 and by ' the m. p. andoptical crystallographic properties of their p-bromoanilides.33Examples of the use of 3 : 5-dinitrobenzoyl chloride for identifyingamino-acids and peptides are given.34 P. P. T. Sah and his co-workers describe the use of semicarbazides for the identification of23 M. Servione, Ann. Chim. analyt., 1937, [iii], 19, 313.24 M. Haitinger, Mikrochem., 1934-1935, 16, 321.25 C. Rimington and P. Symons, Mikrochim. Acta, 1938, 3, 4.2 6 L. Kofler, Angew. Chem., 1938, 51, 703.27 0. C. Dermer and R. B. Smith, J . Amer. Chem. SOC., 1939, 61, 748.28 V. H. Dermer and 0. C. Dermer, J .Org. Chem., 1938, 3, 289.29 C. A. Buehler and J. E. Masters, ibid., 1939, 4, 262.30 P. Sulter, Helv. Chim. Acta, 1938, 21, 1266.31 S. Veibel and K. Ottung, Bull. SOC. chim., 1939, 6, 1434.32 A. W. Ralston and M. R. McCorckle, J . Amer. Chem. SOC., 1939, 61, 1604.33 W. M. D. Bryant and J. Mitchell, jun., ibid., 1938,80, 2748.34 B. C. Saunders, J., 1938, 1397394 ANALYTICAL CHEMISTRY.aldehydes and ketones, and azides in the case of amines, alcohols,and phen0ls.3~ Ethers may be identified by heating a small sampleto 500" and converting the products into a tolylsemicarbazide or abenzhydrazide by which the aldehyde or ketone is identified and theether deduced.36 Small quantities of alkyl halides easily affordS-alkylisothiourea picrates with convenient m.p.37Many organic compounds can be identified by means of thecharacteristic forms of their microsublimates, and, incidentally,separated from admixed non-volatile substances. Fresh cases ofthe utility of this method are still forthcoming, as for example in theidentification of a large number of organic pigments by means oftheir microsublimates .38The microscope serves for the identification of the ingredientsof mixtures frequently without any form of mechanical separation,for, in a thin layer, particles of different ingredients may be observedisolated from other constituents which may be tested in situ optically,chemically, and by their m. p. A relatively recent feature of suchanalysis has been the use of micro-manipulators operated by micro-meters whereby tests may be carried out on specks, invisible to thenaked eye, a t magnifications as great as 200-fold.For instance,the action of hydrochloric and nitric acids on a speck supported ona hook can be investigated by holding under the particle an electric-ally heated micro-crucible containing acid, whereby the liquid isdistilled on to the particle. After evaporation of the acid, chloridesand nitrates have been found with protruding crystal faces suffi-ciently well developed to permit recognition of the crystals bydeterminations of crystal angles and polarisation effects.39The value of observations of shape, markings and linear dimensionsof particles in microscopical analysis cannot be over-estimated ,particularly when it is by such means as these that naturallyoccurring substances of different origin, but chemically almostidentical, are recognised in mixtures, as, e.g., the various starches,and pollen as found in honey.Measurements with micrometereyepieces are time-consuming, since each particle has to bebrought under the scale, and two alternative methods have beendeveloped.Photomicrography affords a permanent record of the particles inthe microscope field, and these images can be counted and measuredat leisure. As an example of the utility of this method, reference35 Rec. Trav. chim., 1939, 58, 8, 12, 453, 459, 582.36 Ibid., p. 758.37 (Miss) W. J. Levy and N. Campbell, J., 1939, 1442.3* A. Kutzelnigg and E. Eraake, Mikrochirn. Acta, 1938, 3, 33.39 R.N. TitusandH. L. Gray, Ind. Eng. Chem. (Anal.), 1930, 2, 368GRIFFITHS, GULL, AND WHALLEY. 395may be made to the photomicrography of lithopone in ultra-violetlight. Cumar gum is used as mountant, and zinc sulphide particles,being opaque to ultra-violet light, give black images on the positive,whilst the barium sulphate particles, transparent to ultra-violetlight, appear white against a grey background due t o the gum whichis intermediate in tran~parency.~~The projection of real images on to a white opaque screen or atransparent screen is also finding increasing favour. The dimensionsof the images in the field can be determined by means of transparentscales or graduations on the screen, etc., and slight adjustments offocus for the detailed examination of particles in different parts ofthe field and of different thickness can be made with e a ~ e .~ 1 Mostmicroscopes can be used as microscope projectors provided asufficiently powerful light source of small dimensions, e.g., a smallPoint-o-lite lamp, is available.42For quantitative work, it is very desirable that the particles ofthe different substances in the mixture be easily distinguishable,and it may be necessary to have recourse t o preliminary treatment.For example, the quartz content of ground felspars used in ceramicsis important and is not accurately or conveniently determined bypurely chemical means. A preliminary treatment with hydrogenfluoride etches the felspars, leaving the quartz clear, and subsequenttreatment with sodium cobaltinitrite stains the potassium felspargrains yellow, thereby permitting microscopical differentiationbetween quartz, orthoclase, and plagioclase felspar fragments.43The merits and limitations of staining methods and observations ofthe effect of reagents are well.illustrated in a recent report onmicroscopic methods used in identifying commercial fibres.44Quantitative Microscopical Methods.There are certain problems in analysis which involve the determin-ation in mixtures of entities which are very similar or even identicalchemically but can be distinguished microscopically. I n such cases,microscopical methods are the only ones available. In addition,there are problems which are capable of solution by ordinarychemical means, but are more rapidly solved microscopically, andspeed of operation is frequently decisive when a routine process is40 G.S. Haslam and C. H. Hall, J. Opt. SOC. Amer., 1934, 24, 14.41 C. E. Brown and W. P. Yant, U.S. Bur. Mines Re@. Invest., 3289, Oct.,42 J. G. A. Griffiths, Analyst, 1937, 62, 519.43 A. Gabriel and E. P. Cox, Amer. Min., 1929, 14, 290.44 T. M. Platt, U.S. Department of Commerce National Bureau of Standards,1935.Circular C. 423396 ANALYTICAL CHEMISTRY.being selected. Microscopical methods are, however, subject tocertain limitations, and recent progress has been concerned withthe examination and extension of the range of validity of thesemethods.Inasmuch as microscopical observation is limited to objects in theplane on which the instrument is focused, only material in thatplane can be observed with any accuracy.Observation is thereforelimited to surfaces with but small irregularities in a direction parallelto the optical axis of the instrument or to particles of which thedepth is small or such as not to interfere with observations of the“ silhouettes.” A random plane through a compacted mass ofparticles does not cut all grains at their maximum diameters, andthe effect of this in determining grain size has been considered in thecase of alloys.45If a plane or a line is passed through an aggregate of hetero-geneous material orientated a t random, the total intercepts of eachconstituent with that plane or line are proportional to the volumes,and the weights of the respective constituents can be deduced ifthe densities are known.The microscopic method is suited, there-fore, to the analysis of compact mixtures which are easily surfaced,such as certain alloys, rocks, and fragmented materials which areunited into a mass by means of a suitable cement. The accuracy ofsuch methods has been investigated with particular reference t o theanalysis of rocks.46 The frequencies of the different ingredientsfound by noting the substances present at regularly spaced pointsalong the line are proportional to the volumes, and an integratingdevice for making rapidly the 1000 or so observations necessary forthe analysis of an alloy or an ore, etc., has been de~eloped.~’The principles on which these methods are based do not applyaccurately to loose grains of material which have not been sectioned,but satisfactory results have been obtained, without sectioning, inthe case of ground quartz-felspar mixtures.48 A comprehensiveexposition of these and other aspects of microscopical analysis isgiven by E.M. Chamot and C. W. Mason.49The complexity of the problem involved in the analysis of apowder consisting of several ingredients, A, B, etc., is evidentfrom the equation by which the proportion by weight of ingredient45 J. J. B. Rutherford, R. H. Aborn, and E. C. Bain, Metals and AZZoys,1937, 8, 345.4 8 H. L. Alling and W. G. Valentine, Arner. J. Sci., 1927, 14, 50.47 A. A. Glagolev, Eng. Min. J., 1934, 135, 399.49 “Handbook of Chemical Miscoscopy, Vol.I, Principles and Use ofPhysical Methods for the Study of ChemicalC. L. Thompson, Bull. Amer. Ceram. SOC., 1934, 17, 257.Microscopes and Accessories.Problems,” 1938, Chapman and Hall, LondonGRIBFITHS, GULL, AND WHALLEY. 397A can be deduced from the relative numbers nA, nB, etc., of theparticles :Simplification may, however, be possible, since the specificgravities, p,, etc., may be neglected if they are nearly identical, andthe diameters, d,, etc., may be neglected if they all lie within afairly narrow range, and in these circumstances, only the numbersof particles need be counted. The nature of the material may notpermit such simplifications, or it may be impracticable to count allthe particles. Sampling is not a serious source of error whenadequate precautions are taken to prevent ~egregation,~~ and whenthe significant ingredient of a powder is of fairly uniform grain size,two different methods may be adopted for the analysis.One procedure consists in suspending uniformly in a viscousmedium a known small weight of the powder, spreading a smallaliquot of the suspension uniformly over a prescribed area, andcounting the particles in question in a number of microscope fields.Statistical analysis shows that in some cases, the distribution of theparticular particles over the prescribed area is not random, owingto surface tension or other effects, and serious error may be causedthereby.51 This error, and others due to the difficulty of measuringsmall aliquots accurately and ensuring the uniform spreading of thesuspension, etc., are obviated in the second method.This consistsin mixing with the powder a definite proportion of a referencesubstance of uniform particle size approximately equal to that ofthe particular particles to be determined, but easily distinguishedtherefrom visually, preparing a uniform suspension of the mixture,and determining the numbers of reference particles and particularparticles in several microscope fields. If the number of referenceparticles per unit weight is known, the number of particular particlesper unit weight of the original powder can be calculated from theobserved ratio of reference particles to particular particles.For the analysis of mixtures containing particles ranging fromabout 10 p to 100 p in diameter, lycopodium powder (spores of theclub-moss) is frequently used as reference substance;52 in the caseof finer powders, the standard particles must be smaller, and of manysubstances tested for use in the determination of bacteria in soil aspecially prepared suspension of indigotin proved most suitable.51The reference substance method has been extended recently to50 J. D. Wildman, J. Assoc. Off. Agric. Chem., 1931, 14, 563.61 H. G. Thornton and P. H. H. Gray, Proc. Roy. Soc., 1934, B, 115, 522.52 T. E. Wallis, Analyst, 1916, 41, 357398 ANALYTICAL CHEMISTRY.complex mixtures in which the particles of two of the ingredients tobe determined are identical chemically and as regards shape, andonly differ in that the ranges within which the diameters of thegrains fall do not wholly c0incide.~33.CRYSTALLO-CHEMICAL ANALYSIS.The possibility of using the crystal form of a material as the basisof an analytical method has long been recognised. Since ampledata are available, by the measurement of the angles between thefaces of a crystal-which angles are characteristic of a substance-identification is in theory possible. In practice, however, the modeof recording and classifying crystallographic data has made identifi-cation by means of crystal form a difficult and tedious process.For example, in Groth’s “ Chemische Crystallographie ” the 7,000substances listed are classified according to chemicaZ composition.In contrast, the classification of the late Dr. T.V. Barker makescrystallo-chemical analysis feasible, and M. W. Porter and R. C.Spiller have described the progress made in the continuation of thissystem.Barker’s classification was based, not on chemical composition oron theories of crystal structure (as Federov’s system was), but ongeometrical form. Within each crystal system, for every crystal amain classification angle was chosen according to a simple set ofrules.3 Though subsequent work has required minor modificationsand additions to be made to the system, its simplicity has not beenlost. Its practical value has been demonstrated by a particular teston substances belonging to the orthorhombic and monoclinic systems.From 1,230 substances, 16 were chosen by an independent selectorand submitted for identification.These included : 01-2 : 4-dinitro-phenylethylaniline, ammonium sulphate, l-phenyl- 3-methyl-4-benzylidenepyrazalone, potassium dihydrogen orthophosphate. Allthe 16 were identified save one, of which the crystal faces were sopoor that not even fair reflexions could be obtained, and consequentlymeasurement on the reflecting goniometer was impracticable. Thetime taken per sample was Since crystals as small as1 cu. mm. can be measured ‘accurately, the classification whencomplete will make crystallo-chemical analysis a reality.hours.63 J. G. A. Griffiths, AnaZyst, 1937, 62, 510.See, e.g., Ann. Reports, 1923, 20, 289.Nature, 1939,144,298.“ Systematic Crystallography,” Thomas Murby, London, 1930, p.2GRIFFITHS, GULL, AND WHALLEY. 3994. DUSTS AND SMOKES.The investigation of the properties of dusts and smokes 1 duringthe past decade has been stimulated by a number of social problemssuch as atmospheric pollution, and the dangers to health of certainindustrial and mine dusts. Not many years ago the law of com-pensation in cases of silicosis was based upon the analysis of the rockbelieved to be the source of the dangerous dust, but since that timeit has been recognised that the percentages of the different mineralsin the dust of the rock are not necessarily the same as those in therock itself, and the problem of dust analysis has gained added socialimportance and increased scientific attention.Changes of timeand temperature alter certain of its characteristics : e.g., by aggrega-tion and partial and preferential precipitation, the number ofparticles per unit volume, the particle size distribution, and theaverage chemical composition of the cloud may change.The takingof a sample, therefore, presents greater difficulties than those offeredby a stable system, heterogeneous though it may be. Frequently itis necessary to maintain a constant temperature throughout theprocess of sampling to prevent any change in concentration of thedisperse phase. The fineness of the disperse phase frequently makescomplete collection of the dispersed medium difficult even whenmethods relying upon aggregation are used, and the danger that theminor fraction of the material which escapes the collecting instru-ment may differ both physically and chemically from the majorfraction which is trapped, cannot be ignored.The true sample, i.e.,100% separation of the disperse from the continuous phase, and itscollection without any change in its chemical and physical character-istics, is virtually unobtainable. The seriousness of this defect isminimised by the fact that a satisfactory analysis can often be madeby a series of separate determinations, none of which requires a truesample as defined above but for each of which a sample is obtained insuch a way that the particular property which the determinationmeasures is unchanged. Further, since comparative data betweentwo or more dusts are often adequate, complete sampling is rarely anecessity.Sampling.-A sample of the disperse phase may be collected bydeposition, filtration, precipitation, or impact methods. Theexposure of a plate or vessel of known surface area to the aerosol fora known time is the basis of the deposition method.It is commonlyemployed for the determination of the amount of material thatsettles from the air in, say, one year per square metre. Though theSee, e.g., '' Disperse Systems in Gases; Dust, Smoke, and Fog," Trans.Faraday SOC., 1936, 32, 1041.A dust or smoke is usually an unstable system400 ANALYTICAL CHEMISTRY.deposit so obtained may be ample for chemical and microscopicexamination, results obtained can only be approximate, andevidently only apply to that fraction of the suspended matter whichhas settled.has used this principle in determining iron,lead, and tar in dust samples.When a stream of particles impinges on a surface, depositiondepends upon the velocity and temperature of the particles, thetemperature and the physical and chemical character of the surface.The stream may impinge upon a surface or pass between areascoated with glycerol or vaselin, and the plates so obtained may beexamined microscopically, or the adhesive film dissolved off and thesuspension so obtained examined directly, or filtered. The koni-meter is an instrument based on this principle, and J. B. Littlefield,C. E. Brown,’and H. H. Schrenk have described a typical one. Thedensity of koaimeter dust spots has been measured photoelectricallyby W. H. Walton? who found that with dusts from two types ofshale, anthracite, and coal, the method allowed the determination ofthree times the maximum number of particles which could beconveniently counted.For dusts of similar particle size, the ratio ofthe light absorbed to the number of particles was found to beconstant and almost independent of the nature of the material.Owens’s method4 has been the subject of much development.The incoming dust cloud is saturated with water vapour, and as itpasses out of a jet to impinge on a microscope slide, almost adiabaticexpansion occurs and water vapour condenses on the particles sothat on striking they stick the more easily. By using a high velocityof air in certain cases 90% efficiency is claimed. S. W. Gurney, C.R.Williams, and R. R. Meigs 5 describe a typical instrument based onthese principles, in which the essentials are an air pump, moisteningchamber, slit 6 x 0.4 mm., circular glass slide, and microscope. Thefactors influencing dust determinations by the impinger method, e.g.,the importance of using dust-free water and of removing air byboiling, are discussed by M. H. Kronenberg, A. N. Setterlind, andC. H. McClure.6The risk of changing the character of finely ground felspar andquartz and dried spores of Penicillium oxalicum by the act of collec-tion has been examined.’ It was found that on dry surfaces thefineness to which particles shatter appeared to be limited to approxi-A. HellerGesundh.-Ing., 1934, 57,322.U.S. Bur. Mines, 1938, Inf.Circ. 6993.J . I n d . Hyg., 1936, 18, 689.Proc. R o y . SOC., 1922, A , 101, 18.IbicE., 1937, 19, 198.J. B. Ficklen and L. L. Goolden, Science, 1937, 85, 587.ti J . I n d . Hyg., 1938, 20, 24GRIFFTTHS, GULL, AND WHALLEY. 401mately lp, and on a wetted surface to 0 . 5 ~ . It was deduced thatwith felspar and quartz any estimate of particle size distribution inair from the resultant particles was erroneous, and with each of thethree dusts the determined number of particles in air samples wasuntrustworthy. E. L. Anderson 8 has reached a similar conclusion.The impinger method has also been used by P. Drinker and W. G .Hazard to measure, record, and control dilute dust concentrations,the dusty air being drawn through a slit-shaped jet to impinge on amoving sensitised film.An alternative apparatus based on the impinger principle butemploying a liquid collecting medium is described by H.H. Schrenkand his co-workers.1° The dust impinges on a smooth surface undera bubbling column of liquid, and is picked up from this surface bythe liquid, or removed from the air as it passes through the liquid.In the earlier stages of the work water was used as the collectingmedium, but owing to the solubility of even siliceous dusts if keptfor 24 hours before testing, ethyl alcohol or ethyl alcohol-watermixture (1 : 3) was preferred. The latter has the additionaladvantages of lower freezing point, bactericidal action, wetting andretaining dusts difficultly wetted by water, and preventing " clump-ing " of asbestos dust.Ropy1 and isopropyl alcohol were alsofound to be suitable. A washing technique suitable for zinc chlorideclouds in hydrochloric acid vapour, and for phosphoric oxide andsulphur trioxide has been devised by A. Czernotzky.llFiltration of the disperse phase is a special form of impinging anddeposition, since the fibres of the filter act primarily not as a sievebut as a number of surfaces to which the particles adhere. Collec-tion is assisted by the relatively high velocity of the aerosol throughthe channels of the filter and by the eddy currents set up therein.As a method of collection it is suited to coarse particles, preferably ofhigh concentration. The character of the aerosol, of course, deter-mines the choice of filter. Further reference to this method is madelater.A fourth method of sampling depends upon precipitation, whichmay be effected electrostatically or thermally. Electrostaticprecipitation is suited to coarse and to fine suspensions, and a recentapparatus employing this principle is described by E.C. Barnes andG. W. Penney.12 A ground, cylindrical, aluminium tube serves asthe collecting electrode, a central electrode being the ionising an(J . In&. Hyg., 1939, 21, 39.U.S.P. 2,076,554; 2,076,553, 13.4.37.lo C. E. Brown and H. H. Schrenk, U.S. Bur. Min., 1938, Inf. Circ. 7026l1 Chem. Pabr., 1937, 10, 218.lZ J. I n d . Hyg., 1938, 20, 259.F. L. Feicht and H. H. Schrenk, U.S. Bur. Min., 1937, Rept. Invost. 3360402 ANALYTICAL CHEMISTRY.precipitating electrode.The rate at which the air is drawn throughis measured by a manometer, and precautions are taken to avoid anychange in weight of the apparatus during use. For greater accuracya glass tube bearing a conducting glaze is used, with a platinum-rhodium wire as the central electrode. Under certain conditions100% efficiency of all particulate matter is claimed. S. Blacktin l3has described a collecting instrument , the essential component ofwhich is a rotating ebonite or celluloid disc which is electrified byfriction, and over which the aerosol is. pulled.The basis of thermal precipitation is that when an aerosol passesbetween two coaxial tubes maintained at different temperatures,precipitation on the colder tube occurs.H. H Watson l4 describesthe use of such a precipitator, claimed to be 100% efficient, whichmay be regarded as a standard method. Coagulation and precipita-tion by sonic and supersonic waves are known to occur in certaincases.l?Andy&.- The identification and analysis of the sample collectedis necessarily specific in character and dependent upon the particularproblem in hand. Optical methods may be used. Schrenk and hiscollaborators l6 describe a microprojection method for countingimpinged dust particles which differs from the normal microscopicmethod in that the images of the dust particles are magnified to 1000diameters and projected on a ruled translucent screen. This pro-cedure permits more concentrated samples to be counted withoutthe need for secondary dilution. Identification based upon themeasurement of the optical properties, e.g., the refractive index,birefringence, and extinction angle, is described by C.R. Wi1liams.l'Petrographic work with particles as small as 5p is, however, ingeneral difficult, whilst identification of particles less than 2p isalmost impossible, and as many industrial dusts fall within thiscategory, the value of this technique-so useful in other fields-isminimised. As a result of their observations on the optical examina-tion of quartz dusts, W. D. Foster and H. H. Schrenk l8 deducedthat most particles were so small that ordinary petrographic methodswere of little use.A general scheme for the chemical examination of aerosols,l3 J .Ind. Hyg., 1936, 18, 613.Bull. Inst. Min. Met., Nov. 1036 ; H. L. Green and H. H. Watson, MedicalResearch Council, Spec. Report Series, No. 199, H.M.S.O., 1935.l5 0. Brandt and E. Hiedemann, Z'runs. Furuduy SOC., 1936, 32, 1101;E. N. da C. Andrade, ibid., p. 1111 ; R. C. Parker, ibid., p. 1115.l6 C. E. Brown, L. A. H. Baum, W. P. Tant, and H. H. Schrenk, U.S. Bur.Min., 1938, Rept. 3373.17 J . Ind. Hyg., 1937, 19, 44.l8 U.S. Bur. Min., 1938, Rept. Invest. 3368GRIFF'ITHS, GULL, AND WHALLEY. 403dependent as it must be upon the amount of sample available, itsnature, and the degree of accuracy required, cannot be laid down.Once a satisfactory sample has been obtained, any difficultiespresented by its analysis will be similar to those encountered ingeneral macro- or, more usually, micro-analysis. Colorimetric andnephelometric methods are frequently suitable.A. Heller l9 detectedthe presence of iron particles in dust by collecting a sample on a slidecoated with gelatin containing 30 yo of potassium ferricyanide andadding hydrochloric acid; the appearance of blue specks ofTurnbull's blue was a positive test. Similarly, for the detection oflead, potassium iodide was used, and after exposure to the air,contact with acetic acid vapour gave a yellow precipitate of leadiodide. The exposure of pure gelatin followed by treatment withchloroform vapour revealed the presence of tar particles by thedevelopment of a brown halo round each particle. D. N.Finkelstein 2o determined copper in aerosols by filtration of theair through a 20-cm.cotton-wool layer, ashing the filter, dissolvingthe residue in hydrochloric acid, precipitating iron by excess ofammonia, and estimating the copper nephelometrically with salicyl-aldoxime. Inorganic cations and anions were found not to interfere.The same worker 21 absorbed fumes of selenium dioxide in a mixtureof hydrochloric acid, potassium bromide, and bromine, and reducedthe solution with sodium sulphite t o yield selenium hydrosol, whichwas then determined nephelornetrically or colorimetrically. Iron,copper, and arsenic were found not to interfere, but tellurium shouldbe absent.Fog nuclei have been condensed on the surfaces of flasks containingcooling mixtures of ether, ice and salt, liquid air, or solid carbondioxide.22 The precipitated water waa analysed in 0.2-ml.portionsfor chloride, sulphate, sulphite, nitrate, nitrite, carbonate,ammonia, and hydrogen peroxide, and its p , determined by knowncolorimetric and turbidimetric methods.Since most methods used to isolate mineral dust in the lung fail todifferentiate between the dust and the mineral content of the lungtissue, or to avoid the risk of chemical and mineralogical changesoccurring, N. Sundius and A. Bygd6n 23 recommend dissolving thelung tissue in hydrogen peroxide which has only a negligible actionon ordinary dust minerals save a moderate oxidation of ferrous toferric iron. Since the action of the peroxide is to give dilute organicGesundh.-Ing., 1934, 57, 322.2o J.Appl. Chem. Russia, 1937,10, 2123.21 Ibid., 1938,11, 1033.22 E. Quitman and H. Cauer, 2. anal. Chem., 1939, 116, 81.2s J . I n d . Hyg., 1938, 20, 351404 ANALYTICAL CHEmSTRT.acids (pH 5-6) in the resulting liquid, the method cannot be usedwhen carbonates are present.The valuable work of H. V. A. Briscoe and his collaborators on theanalysis of siliceous dusts has been summarised by J. W. Matthews.24Two types of collectors for the dusts-of which the physiologicallydangerous concentration was of the order of 1 mg. /cam.-wereemployed. In the first type the air was filtered through a pad ofsolid (e.g., benzoic acid, naphthalene, anthracene) removable bysublimation, or a filter pad of porous solid removable by solution innon-aqueous media from which the dust particles were separated bycentrifuging : the latter type being the most suitable for low con-centrations of dust.“ A.R.” Salicylic acid, sieved through a 40-mesh sieve, and of mean crystal length lop, was packed on stainlesssteel gauzes 7 cm. in diameter to a thickness of 4 mm. At thebeginning of a run, penetration was appreciable, but the leakagerapidly fell to a negligible amount as the collected dust itself beganto act as a filter: for this reason the filter was unsuited to thecollection of more than a few grams of dust. The salicylic acid wasremoved by absolute alcohol.The second type, the “ labyrinth ”, was used for prolonged runsand collected 1 4 0 0 g. of sample in 1-8 weeks. It consisted of atube holding a number of copper baffle-plates on which the dust wasdeposited as the air stream was drawn through the tube.Itsefficiency was found to vary considerably with the rate of air flowand the type of dust, though this factor is unimportant as long asa representative sample is obtained. The labyrinth has theadvantages of giving a large sample which requires no furthertreatment, such as alcohol extraction, and of automaticallyfractionating the dust, i.e., the sample is graded in particle size alongthe baffled tube. A microtechnique with an accuracy of 5 0.2-0.5% was used for the chemical examination of the sample obtained,water, silica, iron, aluminium, calcium, magnesium, sodium, andpotassium being determined. In the determination of silica,platinum crucibles weighing about 2 g.and of 2-3 ml. capacity wereused; for alkali metals, platinum microbeakers of 5 ml. capacity;for other elements, glass beakers of about 5 m1. capacity. Forfiltration, porcelain filter-sticks and the Emich filtering techniquewere employed.X-Ray diffraction methods of analysis have been used, forexample, by W. 3’. Bale and W. W. Fray,25 and by H. C. Sweaney,R. Klaas, and G. L. Clark 26 for the detection of quartz in lungtissue, and the size of crystallites in metal and metal oxide smokes has24 Analyst, 1938, 63, 467.26 Radiology, 1938, 31, 299.25 J . I n d . Hyg., 1935, 17, 30GRIFFITHS, GULL, AND WHALLEY. 405been determined with X-ray and electron-diffraction diagrams andwith electron microscope ph0tographs.~75.GAS ANALYSIS.Since the last comprehensive reports on this subject were made,ldevelopments have proceeded according to analytical requirements,in three main directions.(a) Problems involving the analysis of small quantities ofmaterial have led to the perfection of micro-methods for the precisehandling of very small quantities of gas. An excellent illustrationof such a problem and its analytical solution is provided by thedetermination of minute quantities of free alkaline-earth metals inthe oxide coatings of thermionic valve filaments to which reference ismade later.( b ) The development of methods for the detection and determin-ation of small concentrations of gaseous impurities (e.g., in the atmo-sphere) has been stimulated partly by the exigencies of modern war-fare but mainly by the increasing industrial use of solvents andvolatile materials known hitherto only in the laboratory.Manysuch vapours have unexpected and dangerous effects on humanbeings at concentrations as low as 1 in lo4. Analytical methods forthis type of work often involve automatic apparatus enablingcontinuous records to be made; many are simple and specific incharacter, enabling tests to be made, as required, by unskilled hands.( c ) Several new reagents are noted, together with improvements inthe technique of using old ones. Certain physical methods havebeen developed for some constituents, and factors limiting generalaccuracy have been examined.Methods of micro-gas analysis which have undergone recentdevelopment are of two types : those in which the necessary volumemeasurements are made by drawing the sample into a calibratedcapillary tube and measuring its length in that tube, and those whichemploy the principle of the .McLeod gauge.T. Carlton Sutton2describes apparatus and technique of the former type and gives anexcellent bibliography of the subject. The sample (0-1-0.3 c.c.) isconfined by mercury and is measured dry (P,O,) in a horizontalcapillary tube. This is closed at one end by a piece of rubber tubingand screw clips, which are manipulated so as to move the sample upand down the capillary as required. The other end is bent down-wards at right angles and sealed to a short length of wider tubingwhich dips into a mercury trough and serves as a reaction vessel,Reagents are introduced through the mercury seal on loops of27 F.Krause and D. Beischer, 2. Elektrochem., 1939, 45, 117.1 Ann. Reports, 1933, 1934, J . Sci. Instr., 1938, 15, 133406 ANALYTICAL CHEMISTRY.platinum wire or on beads of porous earthenware fused to platinumwire. An ingenious steel guide prevents the reagent holders fromtouching or fouling the sides of the reaction vessel so that theapparatus keeps perfectly clean. Since minimum quantities ofreagents are used, errors due to the solubility of non-reacting com-ponents of the gas sample are reduced to vanishing point. I n thismanner, every reaction employed in macro-analysis can be utilisedwith a similar degree of accuracy. The design of measuring tubesfor vertical apparatus of this type is discussed by D.Grahame? anda similar apparatus is described by D. Gilm~ur.~ It is essential thatapparatus of this type should be easy to clean, since a little dirt onthe walls of the capillary completely upsets any attempt at accuratemeasurement.The McLeod type of micro-apparatus has been neglected for someyears, but C. H. Prescott and J. Morrison describe an apparatus ofthis kind for the rapid analysis (1 hour) of 5-25 cu. mm. with errorsnot greater than 2%. Samples of 1 cu. mm. can be analysed with anaccuracy of 5y0, and under special conditions it is claimed that thesmallest quantity of a component that can be detected is0-025 cu. mm., equivalent to the carbon monoxide in 1 sq. cm.of aunimolecular film. The observed errors appear to be due largely toadsorption and desorption of gas on the apparatus, particularly onthe powdered reagents employed. Gas samples are handled at lowpressure with Toepler pumps over mercury, and the scheme describedprovides for the determination of water, hydrogen, carbon monoxideand dioxide, oxygen, or methane, the residue being taken as nitrogen.Water and carbon dioxide are removed by conventional reagents, butthe use of a heated platinum or platinum-rhodium wire is limited tothe combustion of oxygen in excess of carbon monoxide or hydrogen,since it is attacked by excess of oxygen at temperatures above 700".The apparatus and technique were used for the oxidation withcarbon dioxide of the free metal in the oxide coatings of thermionicvalve filaments referred to previously.Further observations on theuse of platinum for combustions are made by G. Thanheiser and H.Ploum They findthat combustion of hydrogen in excess of oxygen and nitrogen leadsto slight production of oxides of nitrogen with a consequent highresult for hydrogen, but even with this disadvantage, the method isbetter than explosion or combustion over copper oxide.The detection and determination of small quantities of gaseousimpurities in the air likely to have undesirable effects on humanInd. Eng. Chem. (Anal.), 1939, 11, 351.Austral. J . Exp. Biol., 1938, 16, 208.Ind. Eng. Chem. (Anal.), 1939, 11, 230.6 ATcTL. Eisenhiittenw., 1937-1938, 11, 81.in a paper on analysis of the gases from steelGRIFFITHS, G U U , AND W€€ALLEY.407beings have received much attention recently. S. H. Wilkes andD. Matheson direct attention to some of the more commonexamples, such as oxides of nitrogen (produced in closed orpoorly ventilated places by the use of oxy-acetylene torches uponmasses of cold steel), hydrogen sulphide, chlorine, carbon monoxide,etc.I n some cases, automatic apparatus for the continuous or periodicdetermination of expected impurities in the surrounding atmospherehas been devised. Such apparatus is often connected to some formof warning device which operates when the concentration of gaseousimpurity reaches a predetermined limit. Hydrogen sulphide, said tohave ill effects when present in concentrations exceeding 1 in 30,000,8is continuously recorded by J.Bell and W. K. Hall,9 who use areversible indicator consisting of sodium nitroprusside in sodiumcarbonate + bicarbonate solution. This turns red in the presenceof hydrogen sulphide but is bleached by pure air. Another record-ing lo apparatus employs lead acetate paper combined with aphotoelectric cell. This gas has also been determined by absorptionin a solution of zinc acetate and acetic acid, the zinc sulphide beingdetermined by titration with iodine and thiosulphate.llL. B. Berger and H. H. SchrenkI2 discuss methods for thedetection and determination of carbon monoxide in air, including thepyrotannic acid method (0.01-0.2 yo), the activated iodine pcnt-oxide method (0.1-1 - O ~ o ) , methods utilising palladous chloride(2-10 parts in lo4), together with gas volumetric and thermalconductivity methods, and methods involving measurement of theheat liberated by combustion of the carbon monoxide.An accountof the U.S. Bureau of Mines’ continuous carbon monoxide recorderfor use in vehicular tunnels is also given ; this records concentrationsexceeding 1 part in a million of air. I n a colorimetric method 13 thegas is passed through a suspension of platinum on silica gel in adilute solution of ferric sulphate containing potassium ferrocyanide.A blue coloration, approximately proportional to the concentrationof carbon monoxide, is obtained. Apparatus suitable for applicationof the “ wet ” iodine pentoxide (solution in sulphuric acid containing10% of sulphur trioxide) method to concentrations exceeding 0.1 YoChem.and Ind., 1939, 58, 316.“ Methods for the Detection of Toxic Gases in Industry,” Leaflet No. 1,H.M. Stationery Office, 1937.9 Chem. and Ind., 1936, 55, 89.lo S. Roberts and G. Minors, ibid., 1934, 53, 526.l1 M. Strada and A. Macri, Ann. Chim. appl., 1939, 29, 64.l2 U.S. Bur. Mines, Tech. Paper No. 582, 1938.l3 S. M. Tschumanov and M. B. Axelrod, J . Appl. Chem. Russia, 1938, 11,1236408 ANALYTICAL CHEMISTRY.and for the haemoglobin method (0~003-0~1~0) has also beendescribed .14Carbon dioxide has been measured by changes in the light trans-mission of a solution of methyl-red l5 or by changes in the con-ductivity of a solution of barium hydroxide l 6 when gas containingsmall amounts of the dioxide is passed through it.Methods for the detection of mercury vapour in air have beenreviewed by L.R. Briggs,l' including those involving the blackeningof selenium sulphide paper and the absorption of ultra-violet light bytraces of the vapour.F. A. Paneth and J. L. Edgar l8 deal with errors in previousmethods for the determination of ozone in the atmosphere. In theirown method the ozone from not less than 500 1. is condensed on silicagel at liquid-air temperature. It is then redistilled below - 120°,when all the nitrogen dioxide in the original sample is held back (andcan be finally determined by the m-4-xylenol method); the ozonemay then be determined with potassium iodide and starch, or it maybe collected in a glass tube with quartz windows and determined bymeans of its ultra-violet absorption spectrum.The determination of iodine and bromine together in air hasreceived attention.lg Since sea air was examined, it is probable thatthese substances were present as salts or sprays.The importance of ethylene in the ripening and storage of fruit hasled to methods for its determination. R.C. Nelson 2o purifies thegas extracted from the fruit and oxidises the ethylene with standardpotassium permanganate, whereas B. E. Christensen and his co-workers 21 prefer bromination with standard bromate solution, andin this way 0.0014-06 C.C. of ethylene can be determined in a totalvolume of 3 5 4 0 C.C.Present knowledge of the toxicity of volatile solvents used inindustry is reviewed by T.McClurkin,22 and the detection andapproximate determination, together with what are regarded as safeand dangerous concentrations of most of them, are given in a paperby R. B. Vallender.23 A form of hand pump for general use insampling is described. Each stroke of the pump serves as a unitmeasure of volume, and the air being sampled is drawn through14 H. A. J. Pieters and K. Penners, Het Gas, 1938, 58, 252.15 R. J. Winzler and J. P. Baumberger, Ind. Eng. Chem. (Anal.), 1939, 11,16 A. Lassieur, Compt. rend., 1938,206,606 ; T . Krasso, Tech. Kurir, 1938,9,63.l7 J . Ind. Hyg., 1938, 20, 161.l9 E. S. Burkser and V. V. Burkser, J . Appl. Chem. Russia, 1937,10, 2153.2o Plant Physiol., 1937, 12, 1004.21 Ind.Eng. Chem. (Anal.), 1939, 11, 114.22 Chem. and Ind., 1939, 58,339.371.l8 Nature, 1938, 142, 112.23 Ibid., p. 330GRIFFITHS, GULL, AND WHALLEY. 409standard test papers or solutions. Approximate determinations arethen made by comparing stains on the papers or colorations of thesolutions with sets of standards. In this way hydrogen sulphide(1 in 150,000) is determined with lead acetate paper, hydrogencyanide (1 in 100,000) by the production of a blue stain on paperimpregnated with benzidine and copper acetates or by the change incolour of paper containing silver nitrate and Congo-red (productionof nitric acid), arsine (1 in 250,000) with mercuric chloride paper,sulphur dioxide (1 in 250,000) with paper impregnated with starch,potassium iodide and iodate, and glycerol, carbonyl chloride (1 inlo6) by the production of a yellow stain on paper containing di-phenylamine-p-dimethylaminobenzaldehyde.Oxides of nitrogen(1 in 100,000) are determined by the Griess-Ilosvay test understandard conditions, chlorine (1 in lo6) by the production of a yellowcolour in an acid solution of o-tolidine, carbon disulphide by theorange-brown colour produced in a solution of diethylamine andcopper acetate, aniline (1 in 100,000) by absorption in acid solutionfollowed by the addition of a few drops of bleaching powder solutionand then an ammoniacal solution of phenol-this produces a bluecolour suitable for colorimetric comparison. Benzene is determinedby absorption in concentrated sulphuric acid containing a littleformalin, and the reddish-brown colour produced is compared with astandard solution of sodium nitroprusside.The determination ofcarbon monoxide in this scheme is not so satisfactory, but someresults were obtained with palladium chloride paper. Chlorinatedhydrocarbons in general use are detected by a ‘‘ halide lamp.” Thetraces of vapour are drawn into the flame of a small, alcohol-fedblast lamp which impinges on a small copper screw where they aredecomposed with the formation of small quantities of copper halides.These impart green colorations to the flame, and it is understood 24that the range of coloration from a faint greenish tinge to brightgreen covers concentrations of the chlorinated hydrocarbons ingeneral use ranging from 1 in 25,000 to 1 in 10,000.It is apparentthat, although these tests are of great use in indicating dangerousconcentrations of the various vapours, they are not necessarilyspecific ; for instance, the test for benzene vapour is given even morereadily by toluene and also to a certain extent by coal-tar naphthaand other hydrocarbon vapours.Industrial developments have led to a search for new reagents.One for the absorption of hydrogen from a mixture containingsaturated hydrocarbons and nitrogen 25 consists of an aqueoussuspension of dinitroresorcinol and kieselguhr covered with nickel24 Private communication.25 H. N. Bannerjea, L. H. Bhatt, and R. B. Forster, Analyst, 1939,64,77410 ANALYTICAL CHEMISTRY.catalyst freshly prepared by reduction of nickel carbonate ; thecatalyst is not poisoned by traces of carbon monoxide.The absorption of gaseous olefins and hydrocarbons by sulphuricacid has been examined fully,26 and apparatus and techniquedevised to make the determination accurate for low and high con-centrations. For the production of hydrogen from commercialhydrocarbon mixtures containing hydrogen sulphide, nitrogen, andthe oxides of carbon by heating with steam over a catalyst, it isnecessary to know the general formula C,H, of the material involved ;a scheme for doing this, involving a combination of fractional com-bustion and absorption reagents, has been devised.27 For theanalysis of gases consisting mainly of hydrogen but containinghydrocarbons and some carbon monoxide, A.G. Fleiger 28 removesthe hydrogen by diffusion through a palladium tube at 300".Higher hydrocarbons are previously frozen out at - 180", and afterremoval of the hydrogen the residual gas is examined for methaneand carbon monoxide by combustion over heated copper oxide. Afurther scheme for the analysis of light hydrocarbon mixtures hasbeen prepared by E. C. Ward ; 29 150 C.C. of the sample are liquefied,and fractionally distilled at a pressure of 1 mm. Hg. Fractionationis controlled with liquid air, and an accuracy of 0-02-0-1~, isclaimed. The use of heated calcium as an absorbent has beenexamined; 30 absorption of nitrogen begins at 370" and is quantita-tive a t 385". Hydrogen is absorbed best at 360" but carbon dioxideis only absorbed weakly a t 730-930".Methane is stronglyabsorbed at 530-650", and at 700" a 1 : 1 mixture of nitrogen andmethane gets richer in nitrogen. The absorption of oxygen andcarbon dioxide by calcium nitride is also described.The use of alkaline pyrogallol solutions for the absorption ofoxygen has been condemned by several workers, but A. V. Mazov 31finds that it is quite satisfactory until it has absorbed 20 C.C. ofoxygen per g.-mol., after which it should be replaced. The use ofthis reagent is made more rapid by spraying the absorbent throughthe gas, and the same technique can be applied with advantage toother reagents hitherto regarded as too slow in their action.32 Thepreparation and use of neutral or acid solutions of chromous chloridehas been examined and recommended 33 as an absorbent for oxygen,26 M.P . Matuszak, Ind. Eng. Chem. (Anal.), 1938, 10, 354.27 G. Pastonesi, Chim. e E'Ind., 1939, 21, 4.28 lnd. Eng. Chem. (AnaZ.), 1938, 10, 544.30 P. de Cori, Congr. int. Quim. pura appl., 1934, 9, vi, 225.31 Zavod. Lab., 1938, 7 , 359.32 C. M. Blair and J. H. Purse, Ind. Eng. Chem. (Anal.), 1939, 11, 166.33 W. J. Gooderham, J . SOC. Chem. Ind., 1938, 57, 388; J. R. Branham,29 Ibid., p . 169.J . Res. Nat. Bur. Stand., 1938, 21, 45GRIFFITRS, GULL, AND WHALLEY. 41 1and a further reagent for this gas is indicated by J. Boeseken’sobservation 34 that a solution of thiocarbamide dioxide in aqueousammonia is an excellent absorbent for it.Carbon dioxide at very low concentrations is absorbed by anaqueous solution of dipiperidyl~.~~ The absorbent can be used attemperatures up to 80°, and when spent it may be prepared for useafresh by distilling it at 140°, all the carbon dioxide then beingdriven off.The absorption of carbon monoxide by ammoniacal solutions ofcopper carbonate 36 and of copper chromate has been examined indetail by K.Leschewski 37 and his co-workers, and G. Venturoli 38has devised a method based on the oxidation to carbon dioxide byN/lO-potassium permanganate solution.Further work on the absorption of oxides of nitrogen by alkalinesolutions is reported.39 It was already well known that the degreeof absorption varied with the rate of gas flow, and the generalkinetics of reactions involved have been further investigated.Ithas also been shown that traces of sulphur dioxide can be measuredin the presence of oxides of nitrogen by absorption in neutralhydrogen peroxide solution, and the sulphuric acid formed deter-mined by a precipitation method with ben~idine.~~Of the developments in physical methods of gas analysis, two maybe mentioned. The difficulties encountered in the quantitativespectroscopic analysis of mixtures of gases and the methods adoptedto overcome them are discussed by R. A. Wolfe and 0. S.Duffendack,4l including the “ cleaning-up ” effect of the discharge ingases which varies with the type of discharge and the electrodevoltage. The electrodeless or glow discharge appears to be best foranalytical work, although the intensities of the lines of some elementsvary with the amounts of other gases present. This difficulty isparticularly marked where an inert gas is present.Interferences ofthis type are overcome by using helium in excess as a carrier of thedischarge and a small addition of argon as an internal control. Themethod has been used to determine hydrogen, oxygen, nitrogen, andcarbon monoxide and dioxide.The general aspects of thermal separation processes in gases have34 Proc. K. Akad. Wetensch. Amsterdam, 1938, 41, 70.35 R. B. Evans and D. W. Parkes, J . SOC. Chem. Ind., 1938, 57, 302.36 2. anorg. Chem., 1938, 235, 369.37 Ibid., 1939, 240, 322.38 Boll. Chim. farm., 1939, 78, 1.39 V. I. Atroschtschenko, Ukrain. Chem. J., 1937, 12, 442 ; J .Appl. Chem.4Q G. V. Rakovski, Zavod. Lab., 1938, 7 , 174.41 Proc. 6th Cod. Spectros., 1938, 66.Russsia, 1939,12, 167412 ANALYTICAL CHEMISTRY.been discussed by A. Eucken42 and W. van der Grinten,43 and themethod has been applied by K. Clusius and G. Dickel 4 with a viewto separating gaseous isotopes. The apparatus consists of a verticalhot surface opposite to a cold one. Thermal diffusion and convec-tion result in the heavier component of the mixture becomingrelatively more concentrated at the bottom of the apparatus. A1 : 3 mixture of bromine and helium was completely separated bythis process, and good results were also obtained with a 2 : 3 mixtureof carbon dioxide and hydrogen and with air. Normal neon andalso hydrogen chloride were partly separated into their respectiveisotopes.The thermal conductivity method has been further examined inrelation to the analysis of binary mixtures, and errors of less than0.1% are claimed in the cases of N, + H, and CO + H,.45 Thetechnique consists in using a conductivity wire as one arm of aWheatstone bridge and measuring the voltage necessary to keep thetemperature of the wire constant.The major developments in technique and apparatus have alreadybeen dealt with, but improvements in the familiar constant-volumeapparatus and in the technique of coal-gas analysis effected by W.J.Gooderham 46 should be noted ; further, attention is directed to thedisplacement of nitrogen and other inert gases from, and theirdissolution in, certain reagents during analy~es.~’ Errors arisingfrom this source have been shown to depend on the solubility of theinert gas in the reagent, the rate of absorption of other componentsof the gas phase, and the form of the apparatus.A 1” layer of oildoes not appreciably affect the passage of oxygen or nitrogenthrough the open surface of the liquid phase.6. CHROMATOGRAPHIC ANALYSIS.whenworking on the separation of plant pigments, and it has undergoneconsiderable development recently as an analytical tool.A solution of tlie material in some suitable solvent is allowed tofilter slowly through a very evenly packed column of a suitableadsorbing material such as alumina, whereupon the various solutesare preferentially adsorbed at different levels in the column.Afterpassage of the solution, the so-called chromatogram is developed byThis interesting method was introduced by Tswett in 190642 Osterr. Chem.-Ztg., 1938, 41, 137.43 Naturwiss., 1939, 27, 317.45 F. Ishikawa and K. Hijikata, Bull. Inst. Phys. Chem. Res. Tokyo, 1939,P6 LOC. cit. (ref. 33).47 J. R. Branham and M. Sucher, J. Res. Nat. Bur. Stand., 1938,21, 63.1 Ber. deut. bot. Ges., 1906, 24, 316.44 Ibid., 1938, 26, 546.18, 401GRIFFITHS, GULL, AND WHALLEY. 413washing the column many times with clean solvent. Successiveelutriations and readsorptions occur resulting in the separation of theadsorbed materials into sharply defined zones down the column.The various adsorbates may then be separated mechanically or theymay be elutriated separately with different solvents.The choice ofsolvents and general operation of the method have been discussed byH. G. Cassidy.2 Water, chloroform, ether, benzene, petroleum, andcarbon disulphide are suitable solvents for the original mixture, andthe same solvent with various small additions usually serves for thesubsequent elutriation of the various zones. Thus, chromatogramsfrom benzene or petroleum solution can usually be elutriated quitereadily with the same solvent containing a little alcohol, whereasthose adsorbed from aqueous solution can often be broken up byaqueous solutions of salts adjusted to a definite pH. The adsorbingmaterial most used is alumina, and its preparation for this purposehas been studied by H.N. Holmes, E. Delfs, and H. G. Ca~sidy.~Other adsorbents are : sugar for chlorophyll,* calcium hydroxide forcarotenoids and vitamin-A ,53 calcium carbonate for ~anthophylls,~~and magnesium oxide for carotenes and other plant pigment^.^Application of a potential to the adsorbing column is said toproduce a more rapid and effective separation, and this combinationof chromatographic and cataphoretic methods extends the utilityand application of both.1°More recently a micro-method for the separation of green-leafpigments from carbon disulphide solution has been devised in whichthe adsorbing material consists of a piece of white blotting paper orfilter-paper clamped between two glass plates, the upper one havinga hole in the centre. The test solution, followed by more solvent, ispoured through this hole and the component pigments pass outwardsin the paper in concentric rings, which may be examined by chemicalor physical methods.l1R. T. Arnold l2 has examined the relation of dipoles to chromato-graphic adsorption, and suggests that among isomeric moleculescontaining the same number and kind of functional groups, thosewith the larger dipole moment are more strongly adsorbed on polara J . Chem. Educ., 1939, 10, 88. J . Biol. Chem., 1933, 99, 417.I. M. Heilbron and co-workers, Biochem. J., 1934, 28, 1702.P. Karrer and F. M. Strong, Helv. Chim. Acta, 1936, 19, 25.H. Willstaedt and T. K. With, 2. physiol. Chem., 1938, 253, 40.7 L. Zechmeister and L. von Cholnoky, Annalen, 1934, 509, 269.A. Winterstein and G.Stein, 2. physiol. Chem., 1933, 220, 247.H. H. Strain, Science, 1934, 325; J . Biol. Chem., 1934, 105, 523.lo H. H. Strain, J . Amer. Chem. SOC., 1939, 01, 1292.l1 W. G. Brown, Nature, 1939, 143, 377.12 J . Amer. Chem. SOC., 1939, 61, 1611414 ANALYTICAL CHEMISTRY.media such as alumina. Where no permanent dipole exists, thosecomponents with highest polarisability should be the most stronglyadsorbed. Basicity and acidity of the substances are unimportant.This view is supported to a certain extent by the observations of W.Freundlich and W. Heller l3 on the adsorption of cis- and trans-azobenzene on various adsorbents from Werent solvents.Hitherto, the method has found most application in the analysis ofplant and animal products. As a qualitative method it can be verysensitive, as in the analysis of blood serum for pro-vitamin A andother carotenoids.14, l5 After hydrolysis of the serum with potassiumhydroxide the petroleum extract is passed through a column ofalumina and 0.5 pg.of a- or p-carotene or of cryptoxanthene may bedetected in the resultant chromatogram as definite rings, and thesemay be removed in sequence with definite mixtures of light petroleumand benzene.15 Carotenoids in milk have been determined in asimilar way, alumina or, less effectively, calcium hydroxide beingused as adsorbent , followed by elutriation with benzene-petroleumor benzene-me t h yl alcohol.With a view to separating aliphatic amines and the products ofprotein hydrolysis by these methods, the adsorption isotherms of themore important of such products and of ammonia, ammonium salts,mono- and di-amines have been studied.16 A preparation of aluminawas found to give the best results.It appears that ammonia isadsorbed better at low than at higli p,, and that adsorption ofammonium salts is but little affected by the valency of the anion.The adsorption of the hydrochlorides of primary, secondary, andtertiary amines increases with molecular weight, and is not influencedby branching of the carbon chain. The adsorbability of homologousamino-monocarboxylic acids decreases with increasing molecularweight, and among the dicarboxylic acids lysine dihydrochloride,tyrosine and 3 : 5-di-iodotyrosine are not adsorbed at all and can,therefore, be separated from all other products.Adsorption ofheterocyclic amino-acids decreases in the following order : histidine> tryptophan > oxyproline > proline. Considerable purificationof callicrein 1 7 from human urine is achieved by dialysis againstrunning water, followed by adsorption on alumina and subsequentelution with aqueous N-sodium hydrogen carbonate.Chitinase can be separated from emulsin and may then itself bel3 J . Arner. Chem. SOC., 1939, 61, 2228.l4 A. G. van Veen and J. C . Lanzing, Proc. K . Akad. Wetensch. Amsterdam,15 J. C. Lanzing, Med. Dienst. Volks. Ned.-Indiie, 1938, 27, 213.l6 A. Lottermoser and K. Edelmann, Kolloid-Z., 1938, 83, 262.1 7 E. Werle and A. Marcus, Biochem. Z . , 1938, 296, 275.1937, 40, 779GRIBFITHS, GULL, AND WHALLEY.415separated into two components by further chromatographic treat-ment.18 Separation of the opium alkaloids morphine, codeine,narcotine, and papaverine has been accomplished on a specialdecolorising earth from benzene s01ution.l~In inorganic analysis the method has been applied to the detectionof traces of impurities.20 Cations are adsorbed on alumina fromneutral aqueous solution in the following order : Sb***, Bi***, Cr'**,Fe"', UO,", Pb", Hg", Cue', Ag**, Zn**, CO.', Cd'., Ni", Mn". Thisorder is unaffected by any combination of cations or anions. Insome parts of the series the separation is very narrow, but thedistinction between the bands due to two ions can be made moreobvious by the addition of an element lying between them. Thehydrogen ion is adsorbed like a metal.The above order is quitedifferent when adsorption takes place from an aqueous ammoniacalor alkaline tartrate solution. Anions may be separated anddetected21 by adsorption on a column of alumina which has beentreated previously with dilute nitric acid. The order is OH', PO4"',F', CrO," and Fe(CN),"", SO4", Cr,O," and Fe(CN)6'", Cl', NO,',MnO,', ClO,', S".The ions Na', K , Cl', and Br' are adsorbed by pure kaolin and maybe separated in this way from Mg", Ca**, and whilst certainhydrogen-ion saturated base-exchange substances such as thesulphonic acid permutits 23 may be used to separate Na' from Cl',SO,", NO,', and HPO," ; Cu" from NO,' ; Cr"' from SO," ; Fe"'from C1' ; V"" from SO,'' ; Fe"' from alum solutions, and Fee** andMg" from SO," + PO,"' + H,S04.The passage of a mixture offerric sulphate, cupric chloride and hydrochloric acid through asodium permutit completely removes Fe"' and Cu" and leaves thefiltrate neutral. Treatment of the permutit with N-nitric acid thenyields a solution free from chloride ion. The possible advantages tobe gained by application of these separations to ordinary analyticalprocesses are apparent, for example, in the determination of sulphatein the presence of ions that interfere with the precipitation bybarium chloride or in the removal of hydrogen ions and heavymetals before a Mohr titration of chloride. Other workers 245 25I R L. Zeichmeister and G. Toth, Naturwiss., 1939, 27, 367.l9 G.R. Levi and 3'. Castelli, Gazzetta, 1938, 68, 459.2o G. M. Schwab and K. Jockers, Naturwiss., 1937, 25, 44.21 G. M. Schwab and B. Dattler, Angew. Chem., 1937, 50, 691.22 V. I. Nikolaev and E. I. Rudenko, Compt. rend. Acad. Sci. U.R.S.S., 1938,23 I. 0. Samuelson, 2. anal. Chem., 1939, 116, 328.24 S. S. Bhatnagar, A. N. Kapur, and M. S. Bhatnagar, J. Indian Chem. SOC.,2 5 G. Broughton and Y. N. Lee, J . Physical Chem., 1939, 43, 737.21, 237.1939, 16, 249416 ANALYTICAL CHEMISTRY.have examined the adsorptive properties of synthetic resins of theaniline- and m-phenylenediamine-formaldehyde types towardsorganic and inorganic anions.A considerable volume of work on adsorption phenomena bearingindirectly on this subject has been carried out but is outside the scopeof this report.A review of work on chromatographic analysis priorto 1936 has been given by A. H. Cook.267. SULPHIDE PRECIPITATION.Though systems of analysis without the use of hydrogen sulphidehave been devised,l it is not likely that a reagent which precipitatescompletely or in large part 22 elements from acid solution alone willreadily be replaced. In all, 33 elements form insoluble sulphides,and the means of separating qualitatively and quantitatively some ofthe millions of possible combinations continue to receive attention.Recent work increasingly reveals that it is one thing to defineprecisely the conditions for obtaining a sulphide precipitate, quanti-tatively and in manageable form, from a solution of MA, where M isthe metal ion and A the anion, and quite another when the cationsN, 0, P, Q, .. . and the anions, By C, D, E, . . . arealso present,together with, possibly, residues of organic reagents, colloidalmaterial, and, not unusually, small quantities of elements which onlyin theory have been completely removed by preceding analyticalprocesses.In devising an analytical separation based upon the precipitationof sulphides three factors have, broadly, to be considered. The firstis the condition of the solution from which precipitation is made :its temperature, concentration, pE, the presence of other anions andcations, etc. ; the second is the mechanism of precipitation, underwhich heading the possibilities of co-precipitation and post-precipita-tion have to be included; and thirdly, the physical state of theprecipitate ultimately obtained.The well-known separation in acid solution (of approximately0 .3 ~ ) of copper, germanium, arsenic, selenium, molybdenum,ruthenium, rhodium, palladium, silver, cadmium, tin, antimony,tellurium, rhenium, osmium, iridium, platinum, gold, mercury, lead,bismuth, and polonium from vanadium, iron, cobalt, nickel, zinc,indium, thallium, gallium, tungsten, manganese, and uranium isillustrative of the first factor, as are the separations of arsenic fromantimony, and copper or bismuth from cadmium by acidity adjust-26 Chem. and I n d . , 1936, 55, 724.1 J. Cornog, J. Chem. Educ., 1938,15,420; J. T. Dobbins, E. C. Markham,and H.L. Edwards, ibicl., 1939, 16, 94; C. J. Brockman, ibid., p. 133GRIFFITHS, GULL, AND WHALLEY. 41 7ment. G. E. F. Lundell and J. I. Hoffman give a useful survey ofthe conditions of sulphide precipitation. A recent example is theseparation of cadmium from zinc by the precipitation of cadmiumsulphide quantitatively in an easily filtrable form from a solutioncontaining 15 ml. of concentrated sulphuric acid per 100 ml. in theabsence of chloride^.^ H. Kato has examined in some detail theseparation of zinc from nickel and from cobalt. He states that sincezinc may be precipitated completely with hydrogen sulphide if thefinal pH exceeds 1, irreFpective of the initial p , of the solution, and asnickel sulphide is completely precipitated if the initial pH exceeds 3,the separation of zinc from nickel is obtained in a solution of pH 2.4,buffered to neutralise the strong acid liberated.Similarly, for theseparation of zinc from cobalt a separation p , of 2.3 exists. Thedevice of complex formation to prevent precipitation by hydrogensulphide is well known, and a recent application is reported by N.L ~ v g r e n . ~ Antimony may be detected in the presence of excess ofquadrivalent tin by precipitation with hydrogen sulphide in thepresence of phosphoric acid and concentrated hydrochloric acid, thetin remaining in solution as a complex with the phosphoric acid.The systematic investigation of precipitation processes by Kolthoffand his co-workers has included work upon sulphide precipitation.It has been shown that ferrous sulphide is post-precipitated withcopper sulphide from acid solutions, slowly at ordinary temperaturesand rapidly at 70-95".The amount of iron sulphide so precipitatedincreases with the concentrations of iron and of copper sulphide, andwith the p,. The promoting effect of the copper sulphide was foundto decrease on ageing at 90". I. M. Kolthoff and F. S. Griffith alsoobserved that the post-precipitation of zinc sulphide with bismuthsulphide can be avoided if the hydrochloric acid concentration is notless than 0 . 3 ~ after precipitation, and the solution is filtered within afew minutes. The rate of post-precipitation with bismuth sulphide,as with copper sulphide, rises with the time of contact and itsefficiency increases with short periods of ageing, falling after longerperiods.The addition of sodium chloride to the solution inhibitedthe post-precipitation of zinc sulphide. The zinc sulphide can beextracted from mixtures with the sulphides of bismuth and copperby 2~-hydrochloric acid, but when it is post-precipitated with1938, p. 49 et seq.2 " Outlines of Methods of Chemical Analysis," Chapman and Hall Ltd.,3 C. Zollner, Z. anal. Chem., 1938, 114, 8.Sci. Rep. TGhoku Imp. Univ., 1938, 26, 714, 733.Svenslc Kem. Tidskr., 1939, 51, 2 .6 I. M. Kolthoff and F. S. Griffith, J . Amer. Chem. SOC., 1938, 60,7 J . Physical Chern., 1938, 42, 531.REP.-VOL. XXXVI. 02036418 ANALYTICAL CHEMISTRY.mercuric sulphide, mixed crystals are formed from which the zinc isnot removed by this acid treatment. The same workers have in-vestigated the post-precipitation processes of nickel sulphide with thesulphides of copper, bivalent mercury, and zinc, and their variationswith acidity and temperature. B. Sagortschev examined certainprecipitation processes in which bismuth and lead ions take part bymeans of the corresponding radioactive isotopes thorium-B andthorium-0, and found that bismuth sulphide is completely precipi-tated even in 1.5~-hydrochloric acid. In Kato's separation of zincfrom nickel referred to above, precautions must be taken to preventthe induced precipitation of nickel sulphide, which catalyses its ownprecipitation. loThe physical state of a sulphide precipitate is frequently ofimportance : a coarsely crystalline precipitate, for example, facili-tates filtration, whilst a colloidal sulphide may be desirable forcolorimetric estimation. E. A. Ostroumovll has found that byadding 30 ml. of neutral 20% aqueous pyridine to 75 ml. of hotcobalt or nickel salt solution from which the sulphide is to beprecipitated, a coarsely crystalline precipitate is obtained. He hasalso observed l2 that the presence of 2 g. of hexamethylenetetraminein 100 ml. of slightly acid solution of a manganous salt ensures thatthe manganous sulphide subsequently precipitated is crystalline.The colorimetric determination of bismuth 13 as sulphide is aided byusing gum arabic or polyvinyl alcohol as a stabiliser. Cadmium hasbeen determined colorimetrically 14 in a solution by making italkaline with ammonia, adding potassium cyanide, ammoniumsulphate, and 1% gelatin solution, and adding the mixture to asaturated solution of hydrogen sulphide. The effect of varying theamount of gelatin used in the colorimetric determination of arsenic(0-1-1 mg.) as sulphide has been examined.15The determination of an element by weighing its sulphide is notusual, save in the case of arsenic, though this method may beemployed for molybdenum, mercury, and bismuth sulphides driedbelow 110" ; for antimonous sulphide dried at 280-300" in an inertatmosphere; and for the sulphides of copper, zinc, mercury, andbismuth by ignition under a layer of sulphur in an atmosphere ofJ . Physical Chem., 1938, 42, 541.2. anal. Chem., 1939, 116, 21.lo I. M. Kolthoff and F . S. G r a t h , loc. cit., ref. (8).l1 Zavod. Lab., 1938, 7 , 20.l2 Ibid., p. 1233.l3 T. Yamamoto, Bull. Inst. Phys. Chem. Res. Tokyo, 1937, 16, 1312.l4 R. Juze and R. Langheim, 2. anal. Chem., 1937,110, 262.l5 F. Gaudy and M . P. Antola. Anal. Assoc. Quim. Argentina, 1937, 25,76GRIFFITHS, GULL, AND -ALLEY. 419hydrogen. A recent addition t,o this series is lead, as determined by0. Brunck.16 The sample is dissolved in 10 ml. of nitric acid (d 1-18>,heated to expel nitric acid, diluted to 150-200 ml., and saturatedwith hydrogen sulphide. After Q hour the crystalline precipitate iscollected on a weighed sintered-glass crucible, washed wit4 coldwater, and dried at 110-120". If the zinc concentration is lessthan 0.2 g./100 ml. no precipitation of zinc occurs, nor are other ionsadsorbed on the precipitate.J. G. A. GRIFFITHS.H. C. GULL.H. K. WHALLEY.l6 2. anal. Chem., 1938,113, 385