CHEMICAL NEWS, March 31, 1911

147

The uranyl solution was prepared from Kahlbaum's acetate, and was carefully assayed gravimetrically by precipitation as phosphate; 50 cc. of the solution contained o 2566 grm. of the metal, which requires theoretically a consumption of 42.77 cc. of N/20 permanganate in the titration of the reduced solution.

The following figures are selected from a very large number of results obtained :

In each of these tests the colour of the reduced solution was certainly not that of pure uranous solutions, but brownish green, and the greater the degree of reduction the greater the suppression of the clear green of pure uranous solutions.

After aspirating a current of air through the reduced solutions for a minute or so, the brown tint was completely dispelled, and, on titrating, the lowest reading of the permanganate was 42'7 cc. and the highest 429, representing 0.2562 and 0.2574 grm. uranium respectively. The correct reading 428 cc. was given in the majority of the large

number of tests made.

The stability of cold uranous solutions having been established, the only factor which could determine this oxidation was evidently the temperature of the solutions. A further set of reductions was therefore made by heating the solutions as before, and, when a strong brown colour was developed, the flasks were rapidly cooled to ordinary temperature. In every case, after filtering and washing, and aspirating air for a minute, or even after allowing the solution to remain exposed to the air for twenty minutes, the readings of the permanganate were 42.7 or 42.8 cc.

The volumetric determination of uranium can therefore be made in about fifteen minutes by the following process: -Pour the solution of uranyl sulphate containing from 2 to 5 per cent by volume of free sulphuric acid into a flask containing about 50 grms. of pure finely divided and amalgamated zinc (20 to 30-mesh) and heat until a dark stream of water, and then pour the cold solution through brown colour is developed. Cool the flask rapidly under a a small pulp-filter, wash with cold water, aspirate air for a minute through the filtrate, and titrate with permanganate.

The zinc may be used for a dozen determinations at least before it requires further amalgamation. The reduced solution can be poured through the filter without carrying more than a few particles of the metal with it.

Since uranyl solutions are not reduced by sulphurous acid, a mixture of ferric sulphate and uranyl sulphate can be assayed by first reducing the iron with sulphurous acid, titrating with permanganate, and then treating the titrated

solution as above.

Metallurgical Department,

The University, Sheffield.

ΙΟ

It is evident from the above figures that the reduction of cold uranyl solutions to the uranous condition is more than completed during the short time (about five minutes) occupied by their passage through the column of zinc, but the results show varying degrees of reduction, no one of which corresponds to any definite chemical composition. It is further evident that cold uranous solutions are much more stable than is generally supposed, since the aspiration of air which in several experiments was continued at a brisk rate for twenty minutes, did not result in a measurable amount of oxidation. An attempt was made to carry the reduction to the condition corresponding to the formula U2(SO4)3, which would have involved 64.15 cc. of N/20permanganate in the titration. This was done by first passing the solution through the short reductor as above, and then passing the reduced solution through a much longer reductor into a receiver containing an excess of ferric alum so as to eliminate atmospheric oxidation. The titration figure was 50.2 cc., from which it is seen that zinc in sulphuric acid solution will not effect the reduction to U2(SO4)3, a conclusion also reached by Kern (loc. cit.). As already stated, cold solutions were used in the above experiments, but when, instead of passing the uranyl solutions through a reductor, they were brought into contact with a large excess of amalgamated zinc in an open vessel and the solutions were heated, it was at once obvious that a very rapid reduction took place, even in solutions of 24 per cent acid. In less than five minutes, that is, before the solutions reached boiling point, the change from yellow through the clear green to dark brown had taken place. After passing the hot solutions through a small filter to strain off tiny particles of zinc, and washing, the following extraordinary readings were obtained on titration with permanganate:-415 CC., 408, 41°1, 41°6, &c. Oxidation had thus taken place during the filtration and subsequent

THE following details for the rapid volumetric determination of tin and antimony in soft solder are adapted from A. H. Lowe's method (Fourn. Am. Chem. Soc., xxix., 66). Antimony.-On a counterpoised watch-glass weigh exactly 2 grms. of filings, which should be fine enough to pass a 30-mesh sieve. With a quill brush transfer the filings through a stemless funnel into a 300 cc. Jena Erlen.. meyer flask. Add 5 grms. of KHSO4 crystals and 10 cc. of sulphuric acid, specific gravity 1.8. By means of corklined tongs or test-tube holder manipulate the flask over a bare Bunsen flame until most of the free acid is expelled and no sulphur remains on the walls of the flask nor in the liquid. Do not attempt to take to dryness. Place the hot flask on a piece of asbestos. The tin is now all a stannic and the antimony all an antimonous salt. When the flask cools sufficiently add 25 cc. of cold water and 5 cc. of hydrochloric acid, specific gravity 12 (P. H. Walker and H. A. Whitman also use less hydrochloric acid than A. H. Low, Journ. Indust. and Eng. Chem., i., 519). Manipulate over a free flame for half a minute to complete the solution of the tin and antimony salts and to expel any sulphuric dioxide. Cool the flask under running water. Add 100 cc. of cold water, and titrate rapidly with N/20 potassium permanganate. From the volume of the latter required to give the first pink colour calculate the per cent of antimony. Dry to constant weight some highest purity sodium oxalate, made according to Sorensen, and use this to standardise the N/20 potassium permanganate solution.

Tin.-Weigh exactly o‘2 grm. of the filings and transfer as before to a 300 cc. Jena Erlenmeyer flask. Add 5 cc. of 15 per cent sodium carbonate solution. Add 20 cc. of hot water. Add 25 cc. of hydrochloric acid, 1.2 specific

gravity. Add I drop of 5 per cent antimony chloride solution from a dropping bottle. This solution should be strongly acid with HCl. Close the flask with a 1-hole rubber stopper carrying a capillary U-tube of 1 mm. bore. The short arm of the U-tube should just reach through the stopper, while the long arm should almost reach the surface on which the flask stands. Place the flask on a hot plate where it will boil very slowly but not suck air back through the tube. The solder will dissolve in about fifteen minutes, leaving a small black precipitate of antimony. As soon as this occurs, and without interrupting the slow boiling, bring a test-tube of 15 per cent sodium carbonate solution under the U-tube. Carry to the sink and cool the flask under running water, allowing the carbonate solution to suck back into it. When cold, add 5 cc. of 15 per cent sodium carbonate solution and 5 cc. of cold fresh starch liquor, and titrate immediately with N/20 iodine solution. From the volume of the latter required to give the first deep blue colour calculate the per cent of tin. Standardise the N/20 iodine solution by titrating in the same way o'I grm. portions of filings made from a stick of Kahlbaum's highest purity tin.

This method can be used to advantage for soft solder containing o to 2 per cent of antimony, 30 to 60 per cent of tin, 40 to 70 per cent of lead, and not more than traces of any other metals. Duplicate determinations of tin and antimony can be made in one hour.-Journal of Industrial and Engineering Chemistry, iii., No. 1.

METHODS OF ANALYSIS AND TESTS OF FATS
AND OILS SUGGESTED BY THE SPECIAL
COMMITTEE OF COMMITTEE ON
THE UNIFORMITY OF ANALYSIS OF FATS AND
FATTY OILS.*

2. Moisture. The term "moisture," as here used, refers to the chemical

substance "water" physically incorporated in the fats and fatty oils.

Standard Method.-(Similar to that mentioned by Ubbelohde).

100 grms. of the sample are mixed with 100 cc. of xylene in a suitable distilling flask, and about 50 cc. slowly distilled off over a free flame. The distillate containing the water is collected in a tube o 75 cm. diameter graduated in 1/10 cc., and the percentage of water read off directly from

the volume of water contained in the tube. Correction is to be made for the solubility of water in xylene. Details to be supplied. (The boiling-point of xylene is 138° C.).

3. Suspended Impurities.

Definition.-Suspended impurities in fats and fatty oils are those non-fatty solid substances physically incorporated therewith and insoluble in hot petroleum ether, such as particles of wood, coal, fibres, and mineral matter. They are determined by the standard method given below.

Determination: Standard Method.-A sufficiently large sample to be representative should be weighed out and dried, or the residue from a moisture and volatile matter determination may be used. Usually from 5-20 grms. should be used. The sample is dried in a beaker over asbestos board, keeping the beaker in motion by hand to prevent sputtering. The sample is then dissolved in hot petroleum ether (boiling-point 50-70° C.) by gentle boiling on a water-bath, filtered on a Gooch crucible or porous crucible, washed thoroughly with a boiling petroleum ether (boiling point 50-70° C.), dried to constant weight, and weighed.

4. Free Fatty Acids.

Preparing Neutral Alcohol.-Take commercial 95 per cent alcohol, add 50 grms. powdered caustic soda per litre, boil with reflux condenser for a period of six hours, allow

1. Moisture and Volatile Matter. Weighing out Sample. By the application of gentle heat soften, but do not meit, the sample, and emulsify thoroughly by means of a mechanical egg beater (or other suitable device). Of the thoroughly emulsified sample weigh out for the standard methods from 5 to 20 grms. according to the method used from weighing bottle into a watch glass or shallow glass dish whose sides are not more than 1 cm. high.

Standard Method No. 1, Moisture and Volatile Matter at 110° C.-A 5-10 grm. sample weighed out as above is heated in an oven held at a constant temperature of 110° C. until constant weight is attained. Constant weight is attained when successive weighing thirty minutes apart show a loss of not more than 0.05 per cent.

Standard Method No. 2, Moisture and Volatile Matter. -A 5-10 grm. sample weighed out as above is heated in a vacuum oven held at 50° C. under a pressure of not more than 30 mm. of mercury for four hours. (The tension of water vapour at 50° C. is 92 mm.).

Routine Method, Moisture and Volatile Matter.-The sample weighed out in a glass or aluminium beaker as above is heated on a heavy asbestos board over burner or hot plate, the sample at no time being allowed to reach a temperature greater than 130° C. During the heating the beaker is rotated gently on the board by hand to avoid sputtering or too rapid evolution of moisture. The proper length of time of heating is judged by absence of condensation on a cold watch-glass held over the beaker, by the absence of rising bubbles of steam, by the absence of foam, or by other signs known to the operator. Cool in desiccator and weigh. Report loss as moisture.

* American Chemical Society,

method.

Determination.-From 5-15 grms. of the sample are weighed into an Erlenmeyer flask (100 cc. capacity) and melted on the steam-bath, if solid at ordinary room temperature. Add 100 cc. of hot neutral alcohol. Titrate with N/2, N/4, or N/10 sodium hydrate, using phenolphthalein as indicator. Where the fat is known to have a mean molecular weight of 282 or thereabouts, that figure is to be used in calculating the percentage of free fatty acids. Report also the acid number: mgrms. KOH rerequired to neutralise I grm. The percentage of free fatty acids is to be calculated on the basis of sample freed from moisture and volatile matter.

5. Titre.

Method Proposed by L. M. Tolman as follows (Bureau of Chemistry, Bulletin 107, revised) :-"Weigh 75 grms. of fat into a metal dish, and saponify by using 60 cc. of 30 per cent sodium hydrate (36° Baumé caustic soda) and 75 cc. of 95 per cent by volume alcohol, or 120 cc. of water. Boil down to dryness, with constant stirring, to prevent scorching. This should be done over a very low flame or over an iron or asbestos plate. Dissolve the dry soap in a litre of boiling water, and if alcohol has been used boil for forty minutes in order to remove it, adding sufficient water to replace that lost in boiling. Add 100 CC. of 30 per cent sulphuric acid (25° Beaumé sulphuric acid) to free the fatty acids, and boil until they form a clear transparent layer. Collect the fatty acids in a small beaker, and place on the steam-bath until the water has settled, then decant them into a dry beaker, filter, using a hot-water funnel, and dry twenty minutes at 100° C. When dried, cool the fatty acids to 15° or 20° C. above the expected titre and transfer to the titre-tube, which is 25 mm. in diameter and too mm. in length (1 by 4 inches) and made of glass about mm. in thickness. This is

CHEMICAL NEWS,

March 31, 1911

Placed in a 16 ounce salt-mouth bottle of clear glass, about 70 mm. in diameter and 150 mm. high (2 x 6 inches), fitted with a cork which is perforated so as to hold the tube rigidly when in position. The thermometer, graduated to or C., is suspended so that it can be used as a stirrer, and the mass is stirred slowly until the mercury remains stationary for thirty seconds. The thermometer is then allowed to hang quietly with the bulb in the centre of the mass, and the rise of the mercury observed. The highest point to which it rises is taken as the titre of the fatty acids.

"The fatty acids are tested for complete saponification as follows:-Three cc. of the fatty acids are placed in a test-tube and 15 cc. of alcohol (95 per cent by volume) added. The mixture is brought to a boil and an equal volume of ammonia (0.96 sp. gr.) added. A clear solution should result, turbidity indicating unsaponified fat. The room temperature must be reported."

Determination.

149

Standard Method No. 1.-The filtrate from the suspended impurities determination (including the hot petroleum ether washings) is evaporated to a bulk of about 200 cc., and allowed to stand over night or twelve hours in a cool place (18-20° C.). It is then filtered from the separated insoluble metallic soaps on a Gooch crucible, washed with cold petroleum ether (boiling-point 35-50° C.), dried, and weighed.

Standard Method No. 2.-The filtrate from the suspended impurities determination is evaporated, burned, and ignited to constant weight. (a) The weight of the ash from metallic soaps thus obtained is to be reported as such. (b) Considering the ash to consist entirely of calcium oxide, it is to be calculated to normal soap, using 281 as molecular weight of fatty acid and reported as lime soap.-Journal of Industrial and Engineering Chemistry, iii., No. 1.

Standard Wet Method. Three grms. of the sample, free from moisture and volatile matter, are weighed into a 150 cc. flask, and a 30 per cent excess of strong colourless or nearly colourless alcoholic potash solution added. The contents are boiled for one hour under a reflux condenser, and then transferred to a stoppered 100 or 150 cc. cylinder and made up to 50 cc. with cold water. Add 30 cc. of re-distilled petroleum ether (boiling point 35-50° C.), and agitate vigorously. Draw off the petroleum ether layer by means of slender glass syphon. Repeat this operation with five separate portions of petroleum ether. Place the 150 cc. of petroleum ether into a Squibb's pear-shaped separatory funnel (250 cc.), and wash three times with 20 cc. of 50 per cent alcohol. Pour the contents of the funnel into a tared flask, and distil off the greater part of the petroleum ether and complete the drying on a steambath and in an oven, the latter held below 105° C.

Standard Dry Method.-Approximately 5 grms. of fat or oil are weighed into a 200 cc. capacity Soxhlet or Erlenmeyer flask, and saponified with sufficient alcoholic sodium hydrate solution to give 50 per cent excess of sodium hydrate by boiling under a reflux condenser for half-an-hour to one hour or until saponification is complete. The solution is then transferred to a 4-inch porcelain evaporating dish (the flask being rinsed with hot alcohol) and dried on a water-bath. The drying is completed in the oven at 120° C. for one to two hours. Grind the dried soap in an agate mortar with 10-15 grms. granular an hydrous sodium carbonate, and place the whole in a 33 x 80 mm. S. and S. extraction thimble, using a fat-free plug of cotton to cover the charge. Place the thimble and contents in oven, and re-dry for one hour at 110-120° C. The extraction is made with re-distilled light petroleum ether boiling from 35-50° C. During the extraction if a Soxhlet or Knorr apparatus is used the open end of condenser must be protected against atmospheric moisture by a CaCl2 tube. All connections of the apparatus must be tight. The extraction is allowed to proceed for about ten hours (if a Soxhlet apparatus is used at least 50 discharges). Transfer the extract to a weighed beaker, evaporate solvent on the water-bath and dry in an oven to constant weight. Test the extract by re-dissolving in light petroleum ether; a clear solution should be obtained if the determination has been properly carried out.

7. Metallic Soaps. Definition. The insoluble metallic soaps in the present sense are the fatty acid compounds of bases other than the alkalies. They are insoluble in water.

STRUCTURE.*

By Prof. WILLIAM J. POPE, M.A., F.R.S

LARGE numbers of chemical substances occur on the earth's surface as definite geometrical forms bounded by plane faces; these polyhedral shapes are called crystals. Inspection of the crystal forms assumed by mineral substances shows that, roughly speaking, each crystalline substance affects some specific geometrical shape which is characteristic for the material; further that, whilst crystals of any particular mineral attain vastly different dimensions and are bounded by planes which vary greatly in relative area, one geometrical feature remains constant. The angles between corresponding pairs of faces on any two crystals of the same substance are the same, notwithstanding the existence of difference in size, or in relative face magnitude between the two crystals. The constancy of interfacial angle amongst crystals of the same substance is à law of nature, and has been amply demonstrated by the very careful crystallographic measurements made by Tutton during the last twenty years.

It is, however, not essential to study mineral substances alone in order to obtain a knowledge of the laws governing crystal growth. Great numbers of laboratory products can be caused to crystallise by condensation from some fluid condition; thus, the crystals of various alums exhibited were obtained by slow evaporation of aqueous solutions of these salts.

The examination of a crystal shows that many of its physical properties differ according to the direction in the crystal in which the property is determined; the hardness of crystals, the speed at which light travels through them, and many other properties, are commonly dependent on the direction in which the material is examined.

The dependence of crystal properties on direction indicates the most essential feature of the crystal to be a definite and orderly arrangement of its ultimate particles; this arrangement is referred to as the crystal structure. Further evidence that crystals possess an arranged structure is furnished by the observation that crystallisation is not necessarily a spontaneous process. Thus, on melting benzophenone and rapidly cooling the clear molten mass, the liquid state is retained for many hours at a temperature far below the normal melting point of the compound. But on inoculating the liquid with a trace of crystalline benzophenone crystallisation immediately commences and rapidly becomes complete. The introduction of a small particle of crystalline or arranged material into the liquid mass provides a nucleus upon which the molecules are able to deposit themselves in a similar crystalline arrangement; the process thus started quickly becomes propagated throughout

• A Discourse delivered at the Royal Institution, April 15th, 1910,

Many substances are capable of crystallising in two or more distinct crystalline forms of which one is, in general the more stable at any particular temperature. The physical properties of the several crystalline modifications of any one substance are quite distinct and characteristic for the particular crystalline form and, in many instances, even the colours of the several modifications are different. An example of this is afforded by pouring boiling water into a beaker coated with cuprous mercuric iodide; the brilliant scarlet crystalline form stable at ordinary temperatures, when heated in this way, becomes converted into another crystalline modification which is nearly black. The change is a reversible one, and the differences between the properties of the two crystalline modifications are to be attributed to differences in the mode of arrangement of the molecules in the two cases; the two modifications, in fact, possess different crystalline structures.

Although vast numbers of observations, such as the preceding, lead to the conclusion that crystals are arranged structures, it is not essential that the crystal should be a solid substance; during recent years large numbers of crystalline liquids have been discovered. On allowing melted cholesteryl chloride to cool rapidly a brilliant display of interference colours is seen, owing to the particles of the substance assuming crystalline or orderly arrangement whilst still retaining the liquid condition.

Having very briefly reviewed some of the many reasons for concluding that crystals are structured edifices, the nature of the architecture which they exhibit may now be considered. All the properties of crystalline solids harmonise with one simple assumption as to the manner in which the parts of the structure are arranged; this assumption is that the structure is a geometrically "homogeneous" one, that is, a structure the parts of which are uniformly repeated throughout, corresponding points having a similar environment everywhere within the edifice. The assumption of geometrical homogeneity as the characteristic of crystalline solids leads at once to the great problem solved by the crystallographers of the nineteenth century. This consisted in the inquiry as to how many types of homogeneous arrangement of points in space are possible, to the study of those types and to their identification, in symmetry and other respects, with the known systems into which crystalline solids fall. This work was commenced by the German crystallographer Frankenheim in 1830, and completed by the English geometrician Barlow in 1894. Briefly stated, the final conclusion has been attained that 230 geometrically homogeneous modes exist of distributing material, or points representing material throughout space, and that these 230 homogeneous types of structure, the so-called homogeneous "point-systems," fall into the 32 types of symmetry exhibited by crystalline solids. Models of a number of homogeneous point systems illustrating some of these types are exhibited.

It is, however, obvious that the limitation of the possibilities of solid crystalline arrangement to 230 types marks but one stage in the determination of the nature of crystal structure, and throws no direct light on the relation between crystal structure and chemical constitution.

learnt that corresponding points of the units of crystalline structures form homogeneous point-systems, the great problem still remained of determining what are the entities which become homogeneously arranged, for what reason they become so arranged, and in what way the conclusions drawn by modern chemistry are reflected in crystal structure. This problem was a legacy to the twentieth century, and it now remains to indicate briefly the extent to which it has been solved and the results of chemical importance which have accrued during its investigation. The problem may be most easily visualised in connection with some comparatively simple case, that, for instance, presented by the crystalline forms assumed by the elements themselves. It is generally admitted that an elementary substance consists of identical atoms, each of which acts as a centre of operation of attractive and repulsive forces. In a solid crystalline structure the atoms are obviously not free to travel through the mass, each, if not indeed fixed to a particular spot, being retained within a certain minute domain; each of these domains must be regarded as possessing a centre which marks the mean position of the atom.

The crystalline condition of an element may consequently be defined as one of equilibrium between forces of attrac tion and repulsion emanating from or referable to a flock of points homogeneously arranged in space, that is to say, of points of a homogeneous point-system. Under these conditions, the space occupied by a crystalline element, a homogeneous assemblage of identically similar atoms, may be partitioned into identically similar cells in such a manner that the boundaries of a single cell shall enclose the entire domain throughout which a particular atom exercises predominant influence. Since it is postulated that every point in the space is subject to the dominating influence of some next neighbouring atomic centre, it follows that the cells fit together so as to occupy the whole available space without interstices. Nothing is here said about the shape of the cells; but since, in the case of an elementary substance, the atomic centres are all alike, so too will be the cells. Before proceeding to discuss the actual shapes of the cells referred to, it will be convenient to illustrate more graphically the mode of treating the problem which is here introduced with the aid of a particular point-system connected with the crystalline structure of elementary substances.

The point-system in question may be derived in the following manner. Space is first partitioned into cubes by three sets of parallel planes at right-angles to one another (Fig. 1); a point is then placed at each cube corner and at the centre of each cube face. The cubes of the partitioning, having served their purpose, may now be removed, leaving one of the 230 types of homogeneous point-systems (Fig. 2). Imagine next that each point of the system expands uniformly in all directions until it touches its neighbours a system of spheres packed together in contact is thus obtained (Fig. 3), and, on examination, it is found that no way exists of packing these equal spheres more closely together than the one thus derived. The system is therefore termed the 'cubic closest-packed assemblage of equal spheres and, being derived in the manner described, still retains the high symmetry of the cube; the fragment shown, in fact, outlines a cube. Three directions at right angles in it, those which are parallel to the three cube edges, are seen to be identical in kind; this identity in kind in the three rectangular directions, a, b, and c, is conveniently expressed by the ratio, a: b: c = 1: 1: 1.

On removing spheres from one corner of the cubic closestpacked assemblage of equal spheres a close triangularly arranged layer is disclosed, and, by similarly treating each corner of the fragment of assemblage, the cube outline gives place to one of octahedral form. The assemblage is now seen to be built up by the superposition of the disclosed triangularly arranged layers, the hollows in one layer serving to accommodate the projecting parts of the spheres in adjacent layers. When this operation is per