T H E ATOMIC W E I G H T OP CADMIUM BY T H E I N VES'I'IGATIOX OF CAL>?rlIULI CHI,ORIIIE A N D C A L ~ I I U ~BR( I nirr)p BY ELTOX .I
QUI"
ASU G E O R G E A . HULET'r
Among the large number of determinations of the atomic weight of cadmium made since the time of Stromeyer (1818)' we can find almost any value between 111.6 and 1 1 2 . 7 . ~ A close concordance between the early determinations is hardly t o be expected, but among those more recently made and where a better agreement would naturally be looked for practically none is found. The present accepted value of I 12.4 is the mean of seven values all above I 12.37 ~ h i l ea i l average of all the values as given by Clarke in his * ' Constants of Nature" gives us the value of 1 1 2 . 3 2 . ' ) Our interest in the matter was awakened by the work of Perdue and Hulett,:' wherein a method was devised which makes i t possible to determine cadmium electrolytically in such a manner as to avoid inclusions. This was accomplished b y means of the mercury cathode, the usual method of procedure being so modified t h a t inclusions were precluded and very exact results obtained. Their analyses of very carefully purified cadmium sulphate crystals gave a percentage of cadmium measurably smaller than that calculated when the accepted atomic weight of cadmium is employed. The atomic weight indicated was I I 2.30. I n the work by Laird and Hulett,' on inclusions in electrolytic deposits, a cadmium coulometer was devised in which it was possible to deposit the cadmium in mercury, in much the same manner as in the Perdue-Hulett method. This, run in series with a sil\Ter coulometer, gave I 12.31 as the atomic weight of cadmium. ~
* The work described in this :irticl[, \vas made 1 ~ 1 s 4 l ) lI)y e :i grant from the Elizabeth Thompson Science Fund, which placed cit our disposal some ol' the special apparatus and materialb tieeded lor this iiivestigatioti, :inti the :iuthors t a k e pleasure in :icknuwIedging their i i i d r l ) t c d n e ~ st o thr. 'I'rriiter..: of this Funtl.
These two sets of rwults derived in such a different manner agreed so well t h a t it seemed advisable to extend the investiqation so as to include some determinations of cadmium in cadmium chloride and cadmium bromide. Thew salts have been much u v d in determining the atomic n eight of cadmium ; indeed. the accepted \-,due depends largely upon the analyses of these compounds. Since our results with these salts confirm those obtained t y the methods mentioned abol-e, i t would seem t h a t the real value for this constant is measurably 1 x 1 0 ~the accepted value of 112.4 Preparation a n d Purification of Materials ‘cidi?iiiiiii --The iulphate is probably the most ca5ily purified salt of cadmium and for this reason it was used as the starting point of our research. By the method of recrystallization i t is possible to get cadmium sulphate in a n exceptionally high stare of purity, as i t is not isomorphous with an\- known substance. Kahlbaurn’s best cadmium sulphate was dissolved in distilled water and the Liquid diluted until a I O percent solution was formed. BJ- treating this solution with hydrogen sulphide i o as to precipitate part of the cadmium as cadmium sulphide, digesting the whole on the water bath for twelxe hours and filtering, n-e were able t o obtain a solution practically free from other heavy metals present in the original ialt. The filtrate was again tre:tted with hydrogen sulphide and in a quantity sufficient to transform almost all of the cadmium into the sulphide This n a s filtered off and, after T-ery careful washing, dissolved in nitric acid which had been especially purified by distillation in a quartz distillation apparatu5 After removing the free sulphur a calculated excess of sulphuric acid, purified by distillation in a \-acuum, was added to the liquid, and the solution placed on the water bath where we permitted it to remain until all of the volatile acid had been expelled. ’The excess oi sulphuric acid was next poured off, the CclSO, heated to redness in B platinum evaporating dish
tllc, form oi C'tlC'(
):i ~ v ; t s 1)r'~ci1)itatctlfroin :I wlution of our 1)11 I-i fi C Y 1 ct i (111I i t i i I 1 s11 11I 1I ;it . 'I' tic' CY i t l i i i i LIn 1 earl) on a t e n-a s tiItc.rc'cI off, \v:tslithtl, tlric.tl aiitl l i c t p t i i i gl:iqs-stolqiered hottles ( 3
uiitil itscd.
.After ; I sitfticieiit aiiiouiit of cadmiuin had lxen olitained, it \vas rc'inoved from t h c . l)eakcr, separated from the electrolyte a n d washed maii?- titnrq with distilled ivater. ll'hether the mc.1.aI is dcpositcd i n a platinum dish or as alioi-e descrilxxl, i t is im1)ossil)lc.to rc>ii.io\.e a11 of [lie c~lecti-olq.tc 11y simply ~vaslii i i x , :is 1)ai-t o f i t is 'iiiix, to inc.lutlcd i i i the crystals. '1'0 i x > t i i o \ . c L :ill oi tlie iiicl*.isioiisancl get tlic metal into a Iiai~lcoin])act iorin, we incltcd the. crystals uiidcr 1m-e res;ul~lirnetlam111o i i i II 111 ch 1or id e . '1'11 e :II 11111o 11i It m ch 1or i de acts as a re du ci iig a,gent and ivould teiid to reduce any oxide or sulphate which might have lieen dissolved in the. metallic cadmitimi. T h e ingot thus obtained was clean and bright and could be casily .;eparnti.d from the ammonium chloride which formed a crust on the suriace. It was then washed and placed in a hard :-lass tulle so arranged t h a t after the highest possible vacuum had lieen made 01-er the molten cadmium, i t could be distilled from oiie compartment into another. l17hen most of the cadmium had passcd o\.er, the molten distillate was run into a (Iran-11-out end of the titlie which was sealed up until used. 'I'hc cadmium with which our determinations \\-ere made rvas clistilled tn-ice b y thi:; same process. ( 'tltiiiliillil 'hlori ic. Part of our p ~ i r edistilled cadmium \vas cut i n t o small liieces with a clean pair of steel shears, placed under condueti\-ity water in a Jena glass flask and pure hydrogen chloride gas run in from a generator which will 1x clcscrihed later Fig:. I ) . After all the cadmium had (lissol\-ed, the solution \\-as transferred to a platinum evaporating dish, covered with a Tratch glass and el-aporated to dryness 011 a water liath. 'I'lie salt was dried as far as possible on the water bath, and the:i put into an ol-en and heated t o about -?oc)' C for sc.\.eral hours. I t was finally lirokcn up, removed ivith a 1)l:itiiiuiii s!xtula and preserved in a glass-stoppered t,ot tlc. 1%
J l c t l i o d ol A - l u d l 'I'he general method of melting cad~ chloride was the mium Chloride uiiclcr rt1i-c dr! l i drogen same as t h a t used I)! I k t c r and Hitici in their work on the atomic weight of cadmium in 190~5 'I'he h\ droken chloride was generated and used in t h e iollon.ing apparatus in the large two-liter fa3k 1 ~ I 2 Fig I I v 'i5 placed the purest grade of Baker's hydrochloric acid T h e li! drogen chloride was driven from this b y heating the Ha& gentl?- I h e gas ~ r a 5 dried by first passing i t through the condenser placed in the Dewar tube c ) and surrounded I>! the freezing mixture ( P i The freezing mixture was made 1 ) ~ miliiig finely ground ice P
I
arid ground crj-stdlized calciutn cliloi ide T h e temperature was kept a t about 20' C' I)\ tl!i\ mixturc and practicall> all of the water n as reinoi ed 'I he gas then pa5sed through the calcium chloride tulle hi) and from there into the fusion apparatus 1 l'hii consisted of a large, liard, glass tube prustopper The itopper vided with a 1-ei-y careiull! qrountl g1 was similar to t h a t of a Ilre\el TI a h 1x)ttle and the tube pa-inq through ai ground 011 to 1' irii.ill { l u , i i - t L tube ihon n a t [ i which conducted the 111tlro:.cn chlor irle 01 er t h e cadmium *I. e\ceis of gai then chloridc in the ytl;krt/ cri1cihle i passed out through t h e tubc i I. Iiicl! TI a \ connectcd \lit11 .illowed air to ctittr, suction pump '1 1 1 ~ 3 miall openink hr3
13)
so the pressure in the apparatus \vas not reduced. T o prevent the tube from breaking on cooling from a high to a low temperature a wad of asbestos was placed a t ( 1 ) . The tube was heated in a platinum resistance furnace with a I I O volt alternating current. B y means of an atljuitable resistance the temperature could be regulated very well The connection between the fusion tube and the generator was always sealed before a fusion was made so t h a t all joints were of glass or ground together. I'v.ucihle.-Our first idea was to r s e a platinum Smith crucible, b u t we soon found t h a t platinum would not do, as the molten cadmium chloride a t t h a t high temperature dissolves quite an appreciable amount of this metal Baxter and Hines.' in their ivork made corrections for the dissolved platinum. However, the large amount lost by our crucible a t each heating made it necessary for us to look for some other material. Table I gives t h c loss in weight of the crucible during four heatings. TABLE I Loss in weight Gram
TVeight before heating Grams
\Veight after Grams
2 1
24 S2SSS
(1
1
2 1 S2\31()
24 S2,j92 2 4 s2431 24 h 2 i . p )
0 002()0
>
82894 24 S 2 5 8 S 24 825')'
I 7
(1
ooooh 0 0 1 ~ 1 1
0 (IO(lS2
From these figures we see that the total loss (luring four fusions was somewhat over 5 mg It seemed t h a t platinum would be useless for this work, so we tried quartz. At first the special quartz crucible we used lost weight, and sometimes the loss was quite appreciable The loss, honever, did not take place when we washed the crucible carefully and took special care that our hands did not afternards comc in contact with its surface Our crucible was a closed quartz tube almit 8 cm long, with a platinum cap L-er? accurately ground on thc top. -4cross-section of the tube is shown a t (gI 1 Fig I ) The next step was to find an exact and tlelic'itc test for
cadmiuni in ai1 acitl solutiori such a i ~voultlresult upon the evaporation of our electrolytc~ :iftc.r c~lcctrolysis Potasiiuni ferrocyanidc was first tritd m t l , i l l ‘I iicautral solution, it worked very well Cadmium may he detected l)y this means wheii the concentration is a i inial1 ‘is o 008 nig per cc H o w v e r , when this test w a i applic>d to our evaporated electrolyte it was found to be useless The solution usually turned dark blue and sometimes gave a precipitate even when there was 110 cadmium prewnt This t i a\ proved by running several blank tests. I t is posiible t1i:tt pu-iulphuric acid (H,S,(I,) was formed in the clectrolytc., aiid this produced I’ruisiaii 1)luc~ l)y iti action on the ferrocyanide \‘e next made determinations by treating small concentrations of cadmium sulphate with hydrogen sulphide gas The tests were carried out in tubes of the Sessler form but holding, when filled. only about 2 cc of solution. Blanks were run a t the same t h e in similar tubes and under like conditions. \\-e tried the following concentrations (1 0843 mg Cd to I cc n-ater o 042 mg Cd t o I cc nater o 0 2 1 rng Cd to I cc nater o ooS+ mg Cd to I cc water The cadmium in the first three could tie detected a t once, while the liquid in the iourth was slightly colored after standing a day. Since the blank used for comparison in the latter case was treated in the same manner, and allowed t o stand the same length of time, the difference in color must have been due t o cadmium sulphide and not to free sulphur. TT7e evaporated all of our electrolytes to I cc, and tested each in this manner so there was little chance for any cadmium to escape deposition without being detected All of our weighings were made in an especially designed balance room The Borda method of substitution was used, and the readings of the pointer taken by means of telescope. The temperature of the room was alwa? s under control, being kept practically constant The moisture content of the room was also under control. The air which entered the room wai;
forced through a large, upright cylinder of calcium chloride 1)y means of an electric fan I n all our weighings the pressure. temperature and humidity were recorded, so a t any time we could calculate the weight of I cc of air The average weight was I . I j mg per cc and the variations from this were so dight as to be negligihle The object was :dit a? s allowed to stand inside the balance room a t least one hour before a weighing was attempted, and after placing the object on the halance pan we always waited a t least ten minutes for the temperature, to become constant hefore the readings were taken A11 of our weights were 1 ery carefully calibrated The 1-acuum correction Iraq calculated from the density oi cadmium chloride, foiind by Bnvter and Hines' to lie 4 049 V'e found the correction to lie +o I j 3 nig for each gram of cadmium chloride n.cighed T h e correction for tlic cadmium dissolved in mercur!- was taken from the nork of I'crclue and Hulett who found it to lie o 014 m g for each gram 01' cadmium dissolvcd iii mercury ' ,, 1 he problem of clc~ctroly~itig :I cliloi-icle w l i i t i o i i i i i :I vruvi1)le such as we proposed to LIW wa5 a very difficult oiic' t o sol\ e'. T h e chlorine lilmated at thc anode acted on the rnercurv i i i such a manner that the electrolyte as well as the mcmur? was 1)lackcned '1'0 pre\ crnt this we tried electrolyiis i i i '1 \xeutiin, also inclosing our aiiode in a glass cup 11011~' oi these iiic.tliods J v c w succc.ssful we uiidertooh to cli,tnge the clilol-idc, to the sulphate lxfvre electrolysis This plali ivorkcd \ c'i J satisfactorilq- -A solution of the cadiniuin chloride i i i i z :I t c ~ was transferred to a platinum evaporating dish which wai co\-ered with two watch glasses. One was small enough to be placed in the dish with the coma\-e surface down X small hole n7as drilled through its center for the escape of vapors. The other was larger and placed ox-er the evaporating dish with the conca1.e surface up. Two holes nere drilled through this glass, one on each side of the center \T7ith these watch glasses over the solution it was impossible for an?- to escape 11y spattering. Hoicever, a free passige for the water vapor was provided for and the evaporation proceeded at a fair
rate '1'0 the cadmium chloride was added a slight exceSS of the calculated amount of iulphuric acid and the wholc evaporated to dryness on the water hath I t was then transferred to a hot air oven and heated slowly to 220' C After the sulphuric acid had fumed for iome time it was cooled and transferred to the crucihle for electrolysis The electrolysis apparatui consiTted of a specially designed platinum crucible of about 100 cc capacity The diagram in Fig. 2 shows the arrangement of this crucible with the rest
n
I
+
of the apparatus Fitting in the crucible ( L Z ) is the carefully ground glass funnel ( i / , which served as a cover and allowed the anode i d , to pais through the stem The crucible was amalgamated with mercury 11y heating a small amount in the bottom to its boiling point and allowing the vapor t o condenie on the sides u p to ahout I m i from the top, so t h a t by rotating the crucible on its side a continuous film of mercury would wet its sides and bottom It was then placed in the small gla5s cup i t i a n d surrounded with distilled water, the cup resting in a larger glais jar i / i , and the space between being filled with ice water A split glass plate covered the top of
thc jar so that iiothiriq biit t h e funnel cteni protruded The electrical connections were made 1 ) ~means of a platinum wire through the glais jar< to the crucible inside -411 the mercur!' used in t h e v determinations was distilled three time\, and n as exceptionally pure .lfter the crucible had beeii amalgaindted, and before it was neighed. it was waqhed on the outside, first with dilute nitric acid and then n i t h distilled water. A\fterwards it n a s placed in a desiccator arid allowed to dr? The actual operation carried out during each determina tion was as follows X definite amount of cadmium chloride Ivai placed in the q u x t z crucible (after it had been weighcdr a i d the charged crucible introduced into the fusion apparatus After this apparatus had been iealed t o the hydrogen chloride qcnerator it was heated in the furnace to about 6o03 C and a t the same time a sloir stream of dry 11)-drogeii chloride was passed over the qalt melted and
