34 stroyed by prolonged heating, but only when sufficiently small concentrations of copper were employed. When these colour. less beads were treated with water, a red precipitate was formed in the beads after a short time. This shows that the destruction of the red colour on prolonged heating is not a colloidal phenomenon, and points to the formation of a colourless cuprous pñosphate. The final test to be applied wes an at. tempt to precipitate the colloid particles (if any) by the addition of an acid. Sodium metaphosphate beads were worked with, and metaphosphoric acid which is stable at high temperatures was used as the precipitant. With cobalt and nickel no indications of any precipitation were obtained. Copper in the oxidising flame also gave 1egative results. With copper in the reducing flame, however, the addition of metaphosphoric acid caused the red colour of the bead to disappear, as long as the copper content was not excessive, and in this way, colourless beads containing comparatively large amounts of copper were obtained. These beads could be preserved indefinitely in a lightly corked tube, but were decomposed by water, a somewhat coarse red precipitate of copper, which was distinguished from cuprous oxide by the action of concentrated hydrochloric acid, being formed; the bead was thereby rendered opaque. Beads containing somewhat less copper did not lose their transparency entirely, and in these beads it could be seen that the sodium metaphosphate had acquired a light green tinge, indicating the formation of a divalent copper compound. શૈ These facts show that the colourless beads contain a colourless cuprous phosphate, which is decomposed by water into a cupric phosphate and metallic copper, decomposition exactly analogous to that observed when water reacts with cuprous sulphate (Recoura, Compt. Rend., 1909, CXLVIII., 1105; Cf. also V. Auger, Ibid., 1907, CXLIV., 199). It had now been shown that the colour produced in Borax and Microcosmic salt beads by the oxides of cobalt. nickel and copper was not colloidal in origin, and was therefore due to the formation of coloured metallic borates and phosphates. The nature of these compounds was investigated further. A number of sodium metaphosphate beads were heated with cobalt oxide till a clear blue glass was obtained. These coloured beads were powdered, dissolved in dilute nitric acid, and an excess of silver nitrate was added. The liquid was then carefully neutralised by adding dilute ammonia drop by drop. Under these conditions a white precipitate which did not possess the faintest tinge of yellow was thrown down, thus showing that no orthophosphates were present. Similar results were obtained with cupric oxide. When the concentration of the metallic oxide was increased, the same results were obtained as long as the beads formed were transparent; when, however, the concentration of the oxide was so great that solution in the sodium metaphosphate was incomplete, With sodium metaphosphate beads, no orthophosphates are produced, as is generally stated, the coloured salts being metallic pyrophosphates. SUMMARY. The salts of gold, silver and the platinum metals impart a colour to beads of fused borax and sodium metaphosphate, which is due to the presence of the free metals dispersed throughout the bead in the colloid state. In conformity with the principles of colloid chemistry, the colour of the beads varies with the concentration of the metal and the heat treatment; and the particles can be precipitated by the addition of suitable precipitants (e.g., acids )to the molten beads. An attempt has been made to produce similar changes in colour, using oxides of cobalt, nickel and copper, but no colour changes of a colloidal nature have been observed. These negative results confirm the view that the colour produced in the beads is due to the formation of coloured metallic borates and phosphates. In the case of borax beads, metallic metaborates are formed. In the case of sodium metaphosphate beads, the suggestion that only orthophosphates are formed has been investigated. It has been shown, by > testing for orthophosphates with silver nitrate, that pyrophosphates alone are produced and no orthophosphates are formed, as long as the beads are transparent. Colourless beads of sodium metaphosphate have been obtained, containing colourless cuprous phosphate in solid solution. These beads are stable in air, but are decomposed by water with the production of cupric phosphate and metallic copper. In conclusion, I wish to express my thanks to Professor H. Brereton Baker, F.R.S., for his kind interest in this work. Imperial College of Science and Technology, South Kensington, S. W.7. BY GEOFFREY N. RIDLEY. Many methods have been devised from time to time for the preparation of chromium dioxide. Some of these processes are very convenient for laboratory practice; others are inconvenient in that they take too long, and the quantity of oxide prepared is often useless. Wet Methods. (a) Perhaps the commonest method in this category is that devised by Schweiger, which consists in passing nitric oxide into a warm dilute solution of K,Cr2O,: 2K,Cr,O,+2NO = 2KNO, + K,Cro + 3CrO2. The precipitate is washed with water and alcohol and dried at 258° C. A great deal of time is required for the satisfactory precipitation of Cro2 in this process, and good results are largely dependant upon the conditions to which the dichromate is subjected while in solution. (b) Potassium dichromate may also be reduced by Na,S,O,, with similar results. (c) Chromium dioxide is produced when K,CrO is treated with a conc. solution of Cr,(SO), or CrCl2: pure a condition as may be desired, but is liable to be contaminated by other substances which have to be removed by suitable means. The dry methods are, nevertheless, very suitable for the production of chromium dioxide in quantity. (a) Moissan's method is to heat Cr,O, in air or oxygen up to 400° C. (b) Krüger advises the heating of Cr2O,.xH2O in air up to 250° C. (c) CrO2 is also formed at an intermediate stage in the heating of CrO, at 250° C. This procedure is about the most suitable way of preparing the dioxide, since the temperature conditions can be carefully regulated. Additional methods are: (d) To pass sulphur dioxide over heated CrO,. Quantities of SO, are formed, and a black or dark grey mass of CrO2 is left in the combustion tube: The solid is removed, and any sesquioxide or trioxide present may be extracted by washing the finely ground substance several times with dilute H2SO. (e) To gently heat a mixture of phosphorus and Cro, in a crucible until the whole takes fire. Unless the proportion of phosphorus to oxide is correctly maintained other substances are liable to be formed, including phosphides of chromium. PROPERTIES AND CONSTITUTION. With regard to the properties of chromium dioxide, there are several interesting points to be noted. It is a dark grey, almost black, solid. It is insoluble in liquids, such as ether, acetone, etc. When heated to 300° C. it evolves oxygen, being converted into the lower oxide, Cr2O,. It When heated in a current of chlorine at about 250° C., it forms a compoundCr,O,Cl2. Treatment with HCl or HCl and H2SO1 causes the evolution of chlorine (Moissan). It is unattacked by PC,. reacts quite vigorously with hydrogen peroxide, evolving oxygen very rapidly at 68° C. The addition of a few drops of dilute H2SO, produces the blue colouration of perchromic acid. This furnishes evidence in favour of the theory that chromium dioxide is expressed-Cr,O,, CrO,. When the powdered oxide is boiled for a long time with extremely concentrated sodium hydroxide solution, it breaks up, THE STUDIES IN THE REACTIONS OF THE This paper is essentially the continuation of the work in Parts VIII. and IX. of this series of papers, and deals with the azo dyes of dehydrothiotoluidine sulphonic acid and their bromination. It is well known that the introduction of a sulphonic acid group into a dye intermediate has usually the effect of enriching the colour of the resultant azo derivatives by deepening the shade and promoting greater fastness to light and washing. The monosulphonic acid of dehydrothio. toluidine is well known, and was first prepared by Green, by sulphonation of the thiazole in sulphuric acid solution with pyrosulphuric acid at a temperature below 50° C. Green converted the acid into the ammonium salt and analysed the latter. Up to the present there appears to be no means of obtaining the free pure thiazole from its ammonium salt, though if this were possible it would mean that a successful method of preparation of pure dehydrothiotoluidine had at last been found. sulphonic acid further forms the method of separation of primuline and dehydrothiotoluidine by the Kalle method (Meyer, loc cit.). The The formula assigned to the compound is that of dehydrothiotoluidine substituted with a sulphonyl group in the phenylating part of the molecule. The acid is no doubt formed in a manner analogous to the production of sulphanilic acid from aniline, namely, by conversion of the original thiazole into the sulphate and the migration of the sulphonyl group from the amino group. The para position is, of course, blocked in the case of dehydrothiotoluidine by the substituting 5 methyl benzothiazole system. It is clearly necessary to assume migration to another position, therefore. The sulphonic acid used in these experiments was prepared by dissolving dehydrothiotoluidine in pyrosulphuric acid, and keep ing the solution at a temperature of 50° C. for some time, drowning the liquid in cold water and filtering off the precipitated sulphate of the base; drying this, and diazotising suitable amounts of this with the theoretical sodium nitrite, and coupling with suitable couplers in the preparation of the thiazole dyestuffs previously discussed in these papers (loc. cit.). The four azo compounds prepared were those formed by diazotising dehydrothiotoluidine sulphonic acid with the naphthols, resorcinol and salicylic acid, that is to say, the couplers used were the same as those used in the cases of 4' amino 1 phenyl 5 methylbenzothiazole and nitrodehydrothiotoluidine, such that the properties of the azo compounds from these three intermediates might be compared under the same conditions of preparation, etc. The azo compounds should, of course, be (1), (2) and (3): The introduction of a sulphonic acid group greatly improves the quality of the azo dyes formed from the thiazole intermediate. The introduction of a sulphonic acid group, however, increases the solubility of the compounds, and it is not therefore recommended to digest the azo dyes with hot water for any length of time to remove the last traces of salt, as was done in the previous cases. The colours are better. The red, particularly that derived from resorcinol, is brighter, and the compounds are obtained in a better state than in the case of nitrodehydrothiotoluidine (loc. cit.). azo As has already been stated, one of the purposes of this series of investigations on 4' amino 1 phenyl 5 methylbenzothiazole is to provide new thiazoles for the study of bromination of the fused thiazole ring in benzothiazoles of the type of dehydrothiotoluidine and its homologues, with a view to definitely establishing the hypothesis which has been advanced by the author as regards the bromination of the thiazole ring in benzothiazoles, namely, that bromination and the addition of bromine in solvents such as glacial acetic acid and ethyl alcohol, consists in the normal splitting of the double bond between the nitrogen and carbon atoms of the thiazole nucleus, with the addition of two atoms of bromine at the nitrogen and carbon atom respectively (loc. cit.), and (Bromination of substasces containing a thiazole ring (this Journal). As has been indicated in past papers on this subject, the addition in the case of dehydrothiotoludine, the dehydrothioxylidines, acetyl dehydrothiotoluidine, and other substituted benzothiazole systems of this type, consists of two bromine atoms, but there is a somewhat peculiar observation on record as to the formation of brom addition products in the benzothiazole series, namely, the observation of Bogert and Abrahamson (Jour. Amer. Chem. Soc., 1922, XLIV., 826), that 1 phenyl benzothiazole, the parent substance of the dehydrothiotoluidine derivatives, adds four bromine atoms in glacial acetic acid. The suggestion was put forward by the author that in 1 phenyl benzothiazole, where the homocyclic six-membered ring of the bicyclic benzothiazole system is unsubstituted, a para bridge bond may exist in the homocyclic ring joining the carbon atom 5 and the para carbon carrying the fused nuclei by a slanting bridge bond, as would appear to exist in naphthalene, a fused homocyclic bicyclic ring system in some ways analogous to benzothiazole, for the thiazole ring is nothing more than the benzene ring in which three carbon hydrogen groupings have been replaced by a sulphur and nitrogen atom. The bridge bond under the action of bromine in glacial acetic acid solution rupturing to add four atoms of bromine in conjunction with the nitrogen carbon double linkage in the thiazole ring, addition taking place at position 5, which later is substituted by bromine if the solution is warmed, this being in accordance with the rule of addition preceding substitution. In view, then, of what has been stated above, it was decided to examine the bromination of benzothiazoles substituted by various groups in various positions, with a view to obtaining some information as to how the substitution affects the addition of halogen, and two of the azo compounds. prepared from the sulphonic acid of dehydrothiotoluidine were therefore dissolved in glacial acetic acid by warming, and brominated in the usual way by adding bromine drop by drop from a burette, when the usual type of products separated. This was, of course, only done on a very smail scale, and the quantities of products isolated much too small or analysis, or even any real examination. The observation is, however, of interest, and it is hoped to repeat it at a later date on a somewhat larger scale, and analyse the products of the reaction. It is to be expected, of course, that dehydrothiotoluidine sulphonate azo naphthols will add bromine in the normal way at the double bond of the thiazole ring to give substances of the constitution (4). |