LXXVII1.-A Revision of the Atomic Weiglzt Oj* Rubidium

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776 ARCHIBALD: A REVISION OF THE Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42.

LXXVII1.-A Revision of the Atomic Weiglzt oj* Rubidium. By EBENEZEBHENRY ARCHIBALD. TEE results obtained from the investigation oE the atomic weight of cmium (Richards and Archibald, Proc. Anier. Acad. Arts and Sciences, 1903, 38, 443) seemed to show tbat the analysis of the chloride, bromide and nitrate of this element could be carried out with considerable accuracy, It was thought that the methods elaborated for the analysis of the cEsium salts could be applied View Article Online

ATOMIC WEIGHT OF RUBIDIUM, 777

with almost equal advantage to the case of the salts of rubidium. If then pure rubidium compounds could be prepared, it would be desirable to subject the atomic weight of this element to the same rigorous test which had been applied in the case of cesium.

Review qt Earlier Investigntions. Bunsen, who first determined the atomic weight of rubidium (Pogyendorfs Ann., 1861, 113, 339), estimated the amount of silver chloride precipitated by a weighed amount of rubidium chloride, and his weighings, when corrected for air displacement, gave the following values for the atomic weight of rubidium: 85.36, 85.36, 85.59, 85.28, giving a mean value of 85.40, these being calcnlated on the basis of 0 = 16, with C1= 35.455 and Ag = 107.93. To remove traces of potaseium, Bunsen precipitated his rubidium several times as the platinichloride. Whilst this treatment would eventually remove the potassium, it would nevertheless require a very large number of precipitatione. The next observer to study the atomic weight of rubidium was Piccard (J. p~.Chem., 1862, 86, 454; Zeit. anal. Chem., 1862, 1, 519), who used the same methods of purification aud analysis as those employed by Bunsen, but appears to have subjected his rubidium carbonate to many more fractionations with alcohol in order to get rid of any trace of cesium; this treatment would tend to raise the percentage of potassium present, whereas, as already indicated, the previous precipitations its the platiuichloride would hardly be sufficient to remove the last traces of this metal. Moreover, in correcting the observed weights of silver chloride, Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. this experimenter assumed that the portion of the precipitate finally adhering to the filter was entirely reduced to silver during ignition, and as total reduction is highly improbable this assumption may have vitiated the results to an appreciable exteat. The results of Piccard’s four determinations gave a mean value of 85.44 (0=16.0, &c,). In 1876, Godeffroy (Ann. Chim. Phnrm., 1876, 181, 185) published the results of four determinations of the chlorine in rubidium chloride, the method of analysis being practically the same as that used by the earlier investigators. His method of purifying the rubidium material was based on the work of Redenbatcher (Sitxungsber. Wiener Acad., 1865,51,247), who pointed out the difference in solubility of the alums of rubidium and caesium, and showed how this fact might be made use of in separating these elements. As the caesium alum is the less soluble, the impossibility of getting rid of the last trace of cesium from a sample of rubidium alum by recrystallisation is apparent; and this might account for the higher values obtained View Article Online

778 ARCHTBALD: A REVISTON OF THE

by this observer His analysis gave the mean value of 85-50 (0= 16.0). The results of a number of analyses of rubidium chloride and bromide were published by Heycock (British Association Report, 1882, 4993, but as the details regarding his experiments were not given, one cannot form an opinion as to their accuracy, He found the ratio of silver to rubidium chloride and of silver to rubidium bromide. The mean of seveu analyses of the chloride gave the ratio Ag :RbCl : : 107.93 : 120.801, indicating an atomic weight for rubidium of 85.344. From the analysis of the bromide, the ratio found W~SAg : RbBr : : 107.93 : 165.390, or an atomic weight of 85.389. If an average is taken of the mean resrilts calculated from the data given by the earlier investigators, the value 85.42 is obtained.

The A ncilysis of Rubidium Chloride. In preparing a pure salt of rubidium from material which contains traces of both potassium and czesium, one must keep in view the relation of rubidium to these two elements. As regards the properties of its conipounds, it stands midway between potassium and cmium, and any treatment which will lower the proportion of one in the material may tend to increase the percentage of the other. For example, when the last traces of potassium have been removed from a rubidium compound by recrystallisation, the rubidium compound being more insoluble than the corresponding compound of potassium, then if any czesium was originally present, the final portion of rubidium salt will contain a much higher percentage of this impurity. It is also evident that a rubidium compound free from czsium cannot be prepared by taking a middle portion from which several fractions of crystals Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. have been deposited, as the mother liquor from these crystals will always contain a quantity of cmium, depending on the solubility of the czsium salt in this medium. It will therefore be necessary to convert this salt into that of another acid, the rubidium salt of which is less soluble than the czesium ; recrystallisation of this compound will then give a rubidium compound practically free from czesium. This principle was kept in view in the preparation of the pure rubidium salts. The material used was rubidium iodide obtained from two sources, one specimen coming from Haen,of List, Germany, the other from Eimer and Amend, of New York. Whatever be their origin, these samples differed to a considerabIe extent as regards purity; the former containing a large percentage of potassium and traces of cmium, whilst the latter was spectroscopically free from both these metals. It was thought that the best method of removing the potassium from the rubidium material would be by recrystallising one of the trihalide salts. This method has been suggested and used by Wells View Article Online

ATOMIC WEIQHT OF RUBIDIUM. 779

(Amer. J. Sci., 1901, [iii], 43, 17; Chem. Yews, 1901, 84, 2184), and was found to give very pure material in the case of caesium. As the rubidium dichloroiodide is nearly ten times less soluble than the corresponding potassium compound, and crystallises quite readily on cooling a saturated solution, this salt was first tried. About 150 grams of the German sample of rubidium iodide mas converted into the dichloroiodide and subjected to many fractional crystallisations. An approximately equal amount of the New York sample, which showed in the spectroscope no indication of the presence of potassium or czsium, was also converted into this salt, and likewise recrgstallised. After each crop of crystals had been separated from their mother liquor, they were carefully drained and washed with a little aqua regia. The iodine, a little of which had to be added during these fractionations, had been carefully resublimed. When ten crystallisations had been made of the first portion of salt, 35 grams were set aside for treatment which would remove the cmium present. It was noted tbat after five crystallisations the potassium line could not be found in the spectrum of the salt when a good single prism spectroscope was employed, and hence the five subsequent fractionations should give a salt practically free from this impurity ; nevertheless, after the 35 grams had been removed, the remainder was thrice recrystallised, and the product converted into another salt in order to remove the caesium. As another test for the presence of potassium, the mother liquors from the last four fractionations of the dichloroiodide were combined, the trihalide salt converted into the normal chloride, and fractionated by precipitating with hydrochloric acid, but no potassium could be detected in the final precipitate. Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. The second 150 grams of rubidium iodide were treated like the first sample, except that, as the salt was purer at the beginning, onIy eight crystallisations were made before a portion was set aside, and the remainder received three additional crystallisations. The portion which had been fractionated eleven times gave a very faint cdum line when tested in the spectroscope, whereas the caesium lines could not be found in the original salt or even in the sample which had been fractionated eight times. It will be seen that four different samples of rubidium dichloroiodide have thus been obtained. If the potassium has not been entirely removed from the portion which has received the least number of recrystallisations, it will be present in different proportions in all four samples, and therefore, if all are treated in the same manner when removing the msiurn, four different values for the atomic weight of rubidium should be obtained on analysis, the first portion giving the lowest value and the last the highest. View Article Online

780 ARCHIBALD: A REVISION OF THE

The removal of the cmium mas the next point to be considered, Whereas the methods used by Bunsen and Godeffroy for separating rubidium aDd casium can be employed with advantage in preparing pure caesium material, they are not suitable for removing the last traces of caesium from a rubidium salt ; Johnson and A!len (Amer. J. Sci. rcnd Arts, 1863, [ii], 35, 94), however, have shown that rubidium hydrogen tartrate is about eight times less soluble than the caesium salt, and fractional recrystallisation of this salt should therefore remove all traces of casium. It was also thought advisable to see whether the csesium could not be removed by precipitating the chlorides with hydrochloric acid. As the rubidium chloride is much less soluble in hydrochloric acid than the cssium chloride, this might prove a simple and convenient method of removing this impurity. For this purpose, the two portions of dichloroiodide first prepared, which had been recrystallised eleven and eight times respectively, were converted into the normal chloride by heating in a porcelain basin in an electric drying oven at a temperature of 75O until all the iodine and excess of chlorine had been expelled. These preparations were then dissolved separately in platinum basins in the least amount of hot water, and precipitated with hydrogen chloride, generated by boiling a moderately concentrated hydrochloric acid solution. The salt which separated was washed on a platinum Gooch crucible with a little hydrochloric acid, redissolved in water, and again precipitated. This process of precipitation was repeated ten and fifteen times respectively for the above samples of salt ; the portion which, as the dichloroiodide, had been recrystallised eight times mas now reprecipitated fifteen times, whilst the portion which had received eleven recrystallisations as the trihalide salt was reprecipitated ten Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. times with hydrogen chloride. These two fractions of rubidium chloride will be referred to as samples I and 11. It is evident that from the treatment sample I has received, both as the trihalide salt and also when precipitated as the chloride, it should contain the higher percentage of potassium ; whilst according to the same reasoning a greater percentage of cssium should be found in sample 11. The tendency for the formation of the platinichloride when the chloride is in contact with platinum in the presence of an excess of hydrochloric acid is not so marked here as in the case of czsium ; nevertheless, the temperature was not allowed to rise above 60' or TO" when precipitating with hydrogen chloride. The two other portions of rubidium dichloroiodide referred to above were treated differently for the removal of the csesium present. After converting them into the normal chloride in the same manner as before, each portion was dissolved in water and evaporated with pure redistilled sulphuric acid. Barium hydroxide, which had been repeatedly crystal View Article Online

ATOMIC WEIQHT OF RUBIDIUM. 781

lised, was then added to the solution of rubidium sulphate, until only a trace of sulphuric acid remained. After the barium sulphate, tartaric acid was added in double the quantity necessary to neutralise the rubidium hydroxide, when crystals of the hydrogen tartrate at once separated. The tartaric acid employed, which had been crystallised several times, was perfectly free from ammonia, and left no residue on ignition. The portion of the hydrogen tartrate which had separated was now fractionally crystallised five times, then converted into the chloride, precipitated twice with hydrogen chloride, and set aside for analysis. This fraction will be referred toeassample 111. The second portion of the hydrogen tartrate, which received eight fractional crystallisations, was then converted into the chloride and precipitated with hydrogen chloride. This treatment furnished sample TV. While these fractionations for the removal of the caesium were being carried out, portions were tested from time to time by means of the spectroscope to ascertain whether the treatment was having the desired effect. In the case of samples I and II it was found that after four precipitations as tbe chloride the cesium lines had entirely disappeared from the spectrum, although they were quite distinct when the original sample of chloride was tested. After two crystallisations as the hydrogen tartrate, the cesium lines were no longer present in the spectrum of portions of sample IT, but in this case the percentage of cesium present in the original salt was much smaller. The last three mother liquors from each of all these samples were evaporated to dryness, the residues converted into the dichloroiodide, and repeatedly fractionated by recrystallising, until a very small quantity remained. Portions from each of these four end-products, Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. when examined in the spectroscope, did not exhibit any czesium. It would seem that a few more fractionations-perhaps in the ratio of three to two-are required when the czesium is being removed from the rubidium compound by precipitating the chloride with hydrogen chloride, than when the separation is effected by recrystallising the hydrogen tartrate, but the ease with which the former process can be carried out will often render it preferable, whilst there is much less likelihood of introducing impurity. Shortly, we have now four samples of rubidium chloride to be analysed. The first two, prepared from the German salt, have been crystallised ten and fifteen times as the dichloroiodide, and fifteen and ten times respectively as the chloride. The last two preparations, from the salt obtained in New York, have been crystallised eight and eleven times as the dichloroiodide, and eight and five times respectively as the hydrogen tartrate. The method of preparing the silver was practically the same as that View Article Online

782 ARCHIBALD: A REVISION OF THE

pursued byRichards and Parker(Proc. Amer. Acccd, 1896,32,55)in their investigation on the atomic weight of magnesium. After precipitating the silver as chloride, the precipitate was thoroughly washed with several litres of distilled water, treated for several days with aqua regin, and then reduced with a solution of invert sugar and sodium hydroxide. After thorough washing, the reduced silver was redissolved in nitric acid, reprecipitated, and washed as before, and then, having been reduced from the chloride and again washed, fused on sugar charcoal before the blow-pipe. A portion of this fused silver was now dissolved in nitric acid, the excess of acid removed, the silver nitrate dissolved in water, and the solution diluted until it contained about twenty per cent. of silver. The greater part of the silver was deposited electro- lytically from this solution, using a very weak current,, when silver crystals of considerable size were obtained. The anode was wrapped in filter paper to prevent the anode dust from contaminating the rest of the solution. The silver crystals were now removed from the solution, thoroughly washed, and fused in a boat made by igniting a mixture of pure lime and anhydrous calcium nitrate. The fused silver was cut into pieces of the proper weight with a clean steel chisel, and these, after successive treatment with hydrochloric acid, ammonium hydroxide, and nitric acid, were kept under distilled water. The acids used were carefully redistilled, rejecting the first and last portions of the distillate; the nitric acid was always tested in the nephelometer for the presence of chlorine ions, (Richards, Proc. Awer. Acad., 1894,30, 385). The water used in this research, which had been redistilled and condensed in block tin vessels, had a very low electrical conductivity, Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. and showed no evidence of the presence of ammonium compounds or chlorides. Method of Analysis. It has been shown by Richards (Proc. Amer. Phil. Soc., 1903,42,28, 42,172) that the fusion of such a salt as rubidium chloride is necessary before one can be certain that the last traces of moisture have been expelled. It has also been shown by Johnson and Allen (loc. cit.) that fusion of cesium or rubidium chloride in a moist atmosphere was likely to cause decomposition ; as in their analysis of casium chl.oride, where the fusion took place in the air of the laboratory, a solution of the fused salt was always found to be slightly alkaline. In the present instance, the fusion of the substance was effected in an atmosphere of pure dry nitrogen, the product being subsequently bottled in an atmosphere of dry air. To secure the first conditions, air was forced through a strong solution of ammonia, then over heated copper, and the View Article Online

ATOMIC WEIGHT OF RUBIDIUM. 783

resulting nitrogen passed successively throngh dilute sulphuric acid, two wash-bot tles containing strong sulphuric acid, an eprouvette containing caustic potash, a tube packed with freshly sublimed phosphoric oxide, and a combustion tube where the fusion and subsequent, bottling took place. A detailed account of this bottling apparatus will be found in Richards and Parker’s paper already cited, and the following figure will give an idea of its construction.

D D A rpp -----\ --F- I e3.E ,/z= \cIII) -_ -- IJfXJ- FIG. 1.--Bottliny Apparatus, Hori-oiitnl Sectio??. A = tribe leading from phosphoric pentoxide ; B = weighing bottle ; DD = hard glass tube ; 3 = platinurn boat containing fused rubidium chloride ; F = tube containing phosphoric oxide to prevent any moist air froin diffiisiiig inward.

By means of a Y-tube, a stream of dry air can, at any time, be sub- stituted for the nitrogen. A platinum boat of known weight was placed in a weighing bottle provided with a tightly-fitting glass stopper. Another weighing bottle was prepared having the same weight as the first and containing a quantity of platinum equal in weight to the platinum boat. After prolonged desiccation, the platinum boat and its bottle were carefully weighed, using the other bottle and its platinum as a counterpoise. A few grams of the rubidium chloride were weighed into the platinum boat, the boat then pldced in the combustion tube, the weighing bottle pushed into position, and the Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. tubes connected. A current of dry nit,rogen was now passed through the apparatus for about half an hour in order to sweep out any moist air, and while this stream of gas was still passing, the salt was gradually heated until it melted. After it had been fused for a few minutes, it was allowed to cool slowly, and the apparatus then swept out with a stream of dry air. When the air had completely displaced the nitrogen, the boat was pushed into its bottle, the stopper pushed home, and the bottle and contents transferred to the desiccator. The bottles were left in the desiccator for perhaps two hours; a careful weighing was then made before the salt was dissolved in about two-thirds of a litre of water contained in an ErIenmeyer flask, fitted with a ground glass stopper. These solutions were always found to be neutral and quite free from any insoluble residue. The platinum boat lost only 0.3 milligram nft.er about twenty determinations, and never showed any sign of having been acted on by the fused salt, View Article Online

784 ARCHIBALD: A REVISION OF THE

A quantity of silver, slightly in excess of the amount necessary to precipitate the chlorine in the rubidium chloride taken, was now weighed out and dissolved in nitric acid, Every precaution was taken, while the metal was dissolving, to prevent any of the silver being carried away with the escaping vspours. After the metal had dissolved, the solution was diluted to about 500 c.~. The silver nitrate was slowly added to the solution of rubidium chloride in a dark room. The contents of the flask were now vigorously shaken for half an how, in order to coagulate the precipitate and shake out any occluded silver nitrate, the mixture being then left in the dark for several hours, A Gooch crucible fitted with a mat of carefully selected asbestos, which had been thoroughly purified by treatment with acids, was dried in an electric oven for an hour at 125' and then tared against another platinum crucible having approximately the same weight. The mother liquor from the precipitated silver chloride was filtered through the crucible and the preripitate washed twico by shaking it vigorously each time with about 150 C.C. of water, these washings being then added to the main filtrate. An additional 300 C.C. of water were shaken up with the precipitate, and after several hours poured through the filter, this filtrate being kept separate. This treatment was repeated with about the same amount of water, and the silver chloride then washed on to the filter, dried for several hours at 125", desiccated for two or three hours, and then weighed. The foregoing operations were carried out in either a very subdued or a ruby-red light ; the precipitate was also protected from the light while drying and was never exposed to strongly actinic rays. The amount of silver chloride which had been dissolved by the 600 C.C. of washing water was estimated by the method worked out Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. by Richards (Zoc. cit.). By means of a nephelometer, a solution containing a known amount of silver was compared with the unknown solution, hydrochloric acid being added to the former and silver nitrate to the latter. The amount of silver chloride found in the washing water was of course added to the observed weight of the precipitate. The asbestos carried away in the filtrate was estimated when necessary by filtering the solution through an ashless filter and vreighing the residue after ignition ; it never amounted to more than 0.25 milligram. The amount of moisture still remaining in the dried pellet of silver chloride was estimated 5y Stas's method, namely, by heating the precipitate just to fusion in a tared porcelain crucible, the loss being then subtracted from the observed weight of silver chloride. Before the ratio of silver to rubidium chloride could be determined, it was necemary to know the €mess of silver in the filtrate from the View Article Online

ATOMIC WEIGHT OF R WBIDIUM. 785

silver chloride precipitate, This was estimated by adding hydrochloric acid in slight excess to the filtrate, washing and weighing the precipitated silver chloride in the manner previously indicated. In this case, a little hydrochloric acid in the washing water prevented any appreciable amount of the silver chloride from dissolving. In order to correct the observed weights of rubidium chloride for the buoyancy of the atmosphere, it was necessary to know the specific gravity of this salt. For this purpose, pure dry redistilled toluene was employed (b. p. llo"), since rubidium chloride mas found to be quite insoluble in this liquid. The specific gravity of the toluene at 20" referred to water at 4" was found to be 0.8650, whilst Richards found the value 0.8646 (PTOC,Amer. Acud., 1895, 31, 158).

1.1170 fused RbCl displaced 0,3511 toluene at 20". Sp. gr. =; 2.752. 1.8080 ,, ,, 0.5678 ,, 30" Sp. gr. =2.755. The mean specific gravity of the rubidium chloride is therefore 2.753, this being a little lower than the value 2.807 given by Setter- berg (Oefvers. Stockh. Acad. Porh., 1882, 39, 23). The specific gravity of silver chloride has been carefully determined by several observers; for the purpose of this research it was taken to be 5.55. The long-armed Troemner balance, which was employed in this work, was sensitive to 0.02 milligram with a load of forty grams, and hadan aluminium beam, the rest OF the metal being gold-plated. The weights were carefully compared among themselves, according to the method suggested by Richards (J.Amw. Chsm. Soc., 1900, 22, 144). During the course of the investigation on the atomic weight of cmium (Zoc. cit,), several analyses were made of a sample of very pure Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. potassium chloride in order to test the method and also the purity of the silver used. This precaution was taken in the present case with equally satisfactory results. The value found for the atomic weight of potassium agreed closely with the number given in the former paper, which in turn was in accord with the value found by Stas. The results do not appear here, as the observations are being extended, and will form the subject of a future communication.

Results of the Analysis oj' Rubidium Chloride. The ratios of silver chloride to rubidium chloride obtained from the analysis of the four samples of material are shown in Table I. In computing these values, and any others given below, the atomic weights of silver, chlorine, and bromine were taken to be 107.93, 35,455, and 79.955 respectively ; oxygen being 16.000. View Article Online

786 A.HCEIBALD: A REVISION OF THE TABLEI.

Weight of Weight of Ratio 1 Atoiiiic: RbCl AgCl AgCl : ItbCI= weiuht of in z'acuo. ii~cacz~o. I 100*000: x;. i rubhnn.

I 199966 2.37070 1 84.349 1 85.489 21 1 2.06480 2'44778 54.354 ~ 85'496 3 Ii2 *29368 a.ilgtjo 84.33'3 85'476

1 ~ Average (a)... 85.457 I

4 I1 ~ 1.09495 1.29796 ' 81.360 85 -502

5 ~ 11 2.14381 2'54118 84,364 85.507 6 I1 ' 2 *89 T 0 0 3-43475 84.344 86'482 i Average (b).. 85'497

7 ~ ill ' 2-19692 2.60452 54.350 85'491 8 2.14543 2'54386 Y 4 '338 85.473 9 2 '1 2164 2'51557 84'341 86'477 10 j 111 2.25777 2.67685 84'344 85.482 I Average (c).. 85.481

11 I\' 2.1 8057 2 58528 84.346 85.484 2.32699 2.75878 84.348 85.488 13 4 -00 0 35 4'74233 s4.354 95.495 14 IV 2 '434 40 2.88613 84.348 85'458 Average (d) .., 85'488

The following table contains the ratios of silver to rubidium chloride. TABLE Ir.

I I Weight of A toiiiic No. of' I Sample RbC1 weight of analysis. of' RhC1. ~ in V~CZLO. rttbicliiiin .

Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. I

I I 15'1' 1-99966 1'78454 112.054 85.485 16 I 2.06480 1'84241 112-070 85'503 17 I 2.29368 2 '0 47 10 112.046 85.478 Average (e). 85.488

18 I1 1.09495 0'97702 11 2'070 85'503 19 IL 2,14381 1'91316 112.056 85'488 20 I1 2 -89700 2.58550 112.047 85'479 Average (f). 85.490 21 I1 I 2-19692 1.96076 112.044 85-475 22 111 2 -145 4 3 1'91462 112.065 85'486 23 I11 2'12164 1'89346 112.052 85'483 24 I11 2,25777 2.01515 112.040 85-471 Average (9). 85'479

2.18057 1.94594 111.051 85'489 2.32699 2.07668 112-0513 85.484 4.00035 3 *56998 112.055 85'486 2'4344a 2.17233 112.064 85-496 Average (h) 85'488 View Article Online

ATOMIC WEIGHT OF RUBIDIUM. 787

In the foregoing experiments, 28*69865 grams of silver yielded 38.12529 grams of silver chloride, giving the ratio

AgCl : Ag = 100*000: 75.274.

The ratio found by Stas was 75.276. This agreement serves to show the purity of the silver used, and also indicates that the amount of occlusion of alkali chloride or silver nitrate by the precipitated silver chloride must have been inappreciable. From the above average values it will be seen that the different samples of salt have given the following results :

Sample I mean of (a) and (e) 85.487 Sample I1 ,, ,, (b) ,, (f)85,494 Sample 111 ,, ,, (c) ,, (9) 85.480 Sample IV ,, ,, jd) ,, (h) 85.488 The varied treatment to which the different samples have been subjected does not seem to have altered them as regards their purity, and one is therefore justified in assuming that the potassium or cssium has been practically all removed, even From that sample which received the least number of crystallisations. Further treatment of the chloride or additional analysis of this salt mould seem at present to be of little use.

The Analysis of Rubidium Bromide. It now appeared desirable to obtain another ratio from which to calculate the atomic weight of rubidium, as this would necessitate a certain amount of variation in the experimental conditions of the Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. analysis, and thereby render it possible to eliminate any constant error which might be present in the analysis of the chloride. A number of analyses of rubidium bromide were accordingly carried out by precipitating the bromine as silver bromide and estimating the amount of silver necessary for complete precipitation, as well as the amount of silver bromide produced. The bromine used in preparing the rubidium bromide and hydrobromic acid was purified by dissolving in calcium bromide and reprecipitating by adding a large volume of water, The bromine was then redistilled six times, the first and last portions of the distillate being rejected in each operation. The hydrobrornic acid was prepared from the rectified bromine and thoroughly washed red phosphorus. It was washed, collected, and redistilled five times, the first and last portions of the distillate being discarded in each case. The rubidium material for the bromide determinations, which con- sisted of the residues from the previous analyses together with a View Article Online

788 ARCHIBALD: A REVISION OF THE portion of sample IV, was converted into the hydrogen tartrate, and after three crystallisations had been made of this salt, one portion was analysed, whilst the remainder received two more fractionations. These two portions were converted successively into the bromide and tribromide, and the latter thrice crystallised in order to remove any sodium or potassium which might have found its way into the material during these operations ; as in the case of the chloride, the final mother liquors, both from the hydrogen tartrate and tribromide fractionations, were examined in the spectroscope for the presence of caedium or potassium, but there was no indication of the presence of either of these metals. The normal bromide was now prepared from the trihalide salts by gently heating in an electric oven, dissolved in a little hot water, twice precipitated with hydrobromic acid, and dried. The two samples will be referred to as I and 11. The method of analysis was essentially the same as in the case of the chloride, with this exception, that dry hydrogen bromide was mixed with the nitrogen which passed through the apparatus while the bromide was being fused. The solutions of the fused salt were always found to be neutral and the platinum boat was not attacked. In each case, the amount of silver necessary to precipitate the bromine from the portion of salt taken was estimated by precipitating the excess with hydrobromic acid, filtering, and weighing in the manner already described in the case of the chloride. The correctiou for the solubility of the silver bromide in water was of course much smaller than that for the chloride. In order to reduce the weighings of rubidium bromide to the vacuum standard, the following specific gravity determinations were made. The bromide was found to be insoluble in pure toluene, Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. which, as before, had a sp. gr. 0.865 at 20".

1.2401 fused RbBr displaced 0.3271 toluene at 20". Sp. gr. -3.279

09795 )3 9) ,, 05?580 ,, ,, 20". Sp. gr.=3-284 Mean ...... 3.282

This mean result is appreciably smaller than the value 3.358 found by Setterberg (Zoc. ck!.). The results of the analysis of the two samples of rubidium bromide given on p. 789 are expressed in the units already employed in the case of the chloride. From these results, we see that 13,97953 grams of silver produced 84.33539 grams of silver bromide ; therefore the bromide must have contained 57.445 per cent. of silver. This is identical with the View Article Online

ATOMIC WEICIKT OF RVBIDIUM. 789

TABLE111. Ratios of Silver Bg.oaiide to Rubidium Bromide.

Ratio Atomic NO. of I sample ' AgRr : RbBr= weight of analysis. (of RbBr. 100*000 :x. rubidium.

88.047 1 85.471 2'35401 2'07280 88 054 85.486 2.38589 2.10086 88.053 85.485 Average (i) ... 85.480

2 '9 6462 2'61044 88'053 85.484 ;; I :: 4 -36215 3'84082 88.049 85.475 34 11 4'29084 3.77852 88.061 85-499 35 I I1 4.93210 4 -3429 9 88.056 85'488 Average (j)... 85'486

Ratios of Silver to Rubidium Bromide. I Weight of Weight of Ratio 1 Atomic No. of i Sample RbBr weight of auslysis./of RbBr Ag Ag:RbBr= VCCCUO. 1 in vctcz1.0. in 100~000: x. rubidium,

1'74930 2,68170 153.301 85502 1'35230 2.07'280 153.280 85-479 38 I 1'37061 2'10086 15 3 -27 8 85,478 Average (k).. . 85.486

39 I1 1'70300 2'61044 153.285 85.486

Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42. 40 I1 2'50590 3,84082 153.272 1 85.471 2 '46502 3 -77 85 2 85.488 2,83340 4.3 4 299 153.278 85,477 Average (I) . .. 85-480

value found by Stas. Bringing together these average values, we obtain a mean value, thus: Avernge (i) ...... 85.480 ,, (j)...... S5.486 ,, (k)...... 85-486

,, (I) .....I ...... 85.481 Mean value ...... S5.483 The mean value of the chloride determinations = 85-487

The final mean value = S5.485 VOL. LXXXV. 3a View Article Online

790 MACKENZIE AND JOSEPH : BENZOPHENONE CHLORIDE

It seems most unlikely that, after such varied treatment, an appreciable quantity of potassium or cesium can be present in all these samples in the same proportion, or in such proportion as to give identically equivalent weights for the rubidium, If an appreciable amount of caesium still remains in this material, there is not much hope of its being removed by any known method of separation. The lower value obtained by former investigators must have been due chiefly to the presence of potassium in their material. This metal is almost as difficult to remove as the cxsium, whilst its detection by means of the spectroscope gives a great deal more trouble. In fact, the amount of either cesium or potassium which may be present in the rubidium material before its presence can be detected by a good prism spectroscope is much larger than is generally supposed, so effectually are the lines masked by the bright rubidium spectrum. The foregoing results seem to show that the atomic weight of rubidium cannot be far from 85.48. It is hoped that a study of the decomposition of rubidium nitrate in the presence of silica will be taken up before long and made the subject of a future comniunication.

In conclusion, I wish to thank Dr. B. J. Harrington, director of the chemistry building, for his kindness in placing valuable apparatus and material at my disposal. MACDONALDCHEMISTRY BUILDING, McG ILL USI VEILSITY, M o N T REA L . Published on 01 January 1904. Downloaded by Brown University 26/10/2014 22:48:42.