I S D U S T R I A L A 5 D ESGIA'EERI*VG CHEMISTRY
July 15, 1930
filled with 50 per cent potassiuni hydroxide solution. Should this test give a n unsatisfactory result, repeat the process just described taking care not to allow air to enter through H. I t is advisable to have a plug of cotton a t the lower end of B. C is of use in filling and emptying A . The functions of D , a capillary manometer, and E , a mercury bubble trap, are self-evident. To obviate a possible intake of air through H , maintain at all times a pressure in F aboye atmospheric. Since A must be heated whenerer carbon dioxide is generated any clamps supporting it should be cushioned with asbestos. K h e n the 1)icarbonate is to be renewed, break the connection at a point ahow side arm R. I n each operation of storing a supply of gas in F , carbon dioxide is trapped before the mercury seal in the generator d , arid, as the contents of d cool this carbon dioxide is re-absorbed, producing a partial vacuum in this part of the system. Sliould a pinhole leak be overlooked in the construction of the apparatus, t h i s allowing an intake of air. this is easily detected by a drop in the mercury column of the manometer. Obviously a leak on the other side of E would not allow a n entrance of air before detection, because the gas in F is under pressure. Type I1 Generator
The construction of the apparatus is the same as for Type I, except' that G of Type I is replaced by K (3-liter flask) and L (safety trap). By varying the amount of mercury intro-
251
duced into L , it can be regulated to give any maximum pressure in K up to 300 mm. Temporarily connect H to a T-shaped, three-way stopcock, the perpendicular arm of which is filled with and extends into de-aerated water. After charging A with sodium bicarbonate and filling E and L with the proper amounts of mercury, flame the entire generator system and pump with an oil pump for at least 15 minutes to remove occluded air from the inner surfaces of the glass. It is necessary to evacuate L partially and close JI during the pumping of generator &ern. X o w allow approximately 550 cc. of deaerated water to run in. Close H and then generate carbon dioxide until it bubbles through L when 31 is open. Caution should be exercised in opening so as not to suck air back into the storage systems. If the stored gas gives a satisfactory test for purity, the generator is ready for use. Obviously, if the occluded air in K is thoroughly swept' out, the water seal between K and F can be dispensed with. . Acknowledgment The author takes this opportunity to express his appreciation for the many valuable suggestions and criticism offered by J. R. Bailey, director of Project' 20. Literature Cited (1) Alarck, B i t L SOC. chim., [41 46, 559 (1929). (2) Pregl, "Quantitative Organic Xlicroanalysis," translated b y Fyleman, p. 79, Churchill, 1924.
Evaluation of Stibnite' 11-Determination of Antimony Wallace M. McNabb and E. C. Wagner UKIVERSITY OF
I
S A previous paper (30)
there was d e s c r i b e d a procedure for the determination of sulfide sulfur in stibnite by evolution of hydrogen sulfide, this being received in ammoniacal cadm i u m s o l u t i o n and determined iodometrically. .Vole--Von Bacho ( I ) reported low results b y a similar procedure a t t r i b u t i n g t h e error t o loss of hydrogen sulfide, during decomposition of t h e cadmium sulfide, because of either volatilization or air oxidation T h e loss m a y be explained b y t h e f a c t t h a t t h e cadmium sulfide was treated with hydrochloric acid before t h e iodine solution was added.
PENKSYLVAKIA, PHILADELPHIA, P A
The titrations of trivalent antimony by bromate, iodine, or permanganate, under properly adjusted conditions, yield practically identical results. In presence of ferrous iron the titration of trivalent antimony with iodine, in a solution buffered with bicarbonate, is affected by a negative error. A method for determination of iron present as impurity in stibnite is described. The Frankford Arsenal method for determination of antimony in stibnite was found to yield results about 0.5 per cent too low, apparently owing to loss of antimony by volatilization. The evolution procedure has been extended to include both the iodometric determination of sulfide sulfur and the titration of antimony with permanganate. The analysis for both constituents is made on a single sample, and is rapid, easily executed, and accurate.
The present paper gives a n account of the extension of the procedure to include the determination of antimony, both elements being determined volumetrically and in the same sample. Volumetric Methods for Determination of Antimony
Methods which involve reduction of pentavalent to trivalent antimony include Weller's application of Bunsen's evolution method (40), a similar method (36) in which iodine is liberated from hydriodic acid by pentavalent antimony in the cold, with titration with thiosulfate in either case, Received J a n u a r y 31, 1930.
and reduction by t i t a n o u s chloride (24), the end point being located potentiometrically. M e t h o d s in which trivalent antimony is oxidized to the pentavalent condition include most prominently the titrations with iodine (32), with permanganate (19), and with bromate (35). Willard and Young (42) applied their ceric sulfate method to the determination of antimony, t h e aT7eraged results being very close to the theoretical. Knop (21) titrated antimony with standard dichromate, with diphenylamine as internal indicator, and Fleyscher ( I O ) -deteknined the end point electrometrically. Jungmichl and Hack1 ( I @ , and also McMillan and Easton (29), titrated with dichloramine-T. Kakosono and Inoko (33) modified the bromate method by titrating in presence of potassium bromide. h4anchot and Oberhauser (98)described an excess method using standard bromine and arsenite solutions. Iodate titrations have been proposed by Jamieson (17) and by Lang ( 2 5 ) . Winkler (49)revived the method of Gooch, and Bertiaux (3) titrated antimony with permanganate using methyl orange as internal indicator. Accuracy studies of several methods for determination
ANALYTICAL EDITI0,V
252
of antimony may almost be regarded as part of the history of the atomic weight of this metal. Dumas' early value 121.8 was replaced by the value 120 determined in 187781 by Cooke with what appeared to be extreme precision. During the interval ended by the acceptance in 1925 of the value 121.77 determined by Willard and RlcAlpine (41) in 1921, the atomic weight was 120.2. The fact that this is more than 1 per cent too low was pointed out repeatedly as a result of quantitative studies by Beckett (Z), von Bacho ( I ) , Zintl and Wattenberg (44), Knop (ZO), Collenberg and Bakke (8, 7), and Koenig (29). Atomic weight values calculated from their results are close to 122, the best values being slightly below this. Otherwise stated, the methods investigated yielded correct results only by using values near 122 as the atomic weight of antimony. This is a n interesting case in which a painstaking atomic-weight determination was actually much less accurate than several ordinary methods of analysis. The methods which were given consideration for use in stibnite analysis are the titrations with iodine, bromate, and permanganate. These are among the five methods examined comparatively b y Collenberg and Bakke ( 6 ) , who reported all to be satisfactory. The bromate and permanganate titrations, since they are conducted in hydrochloric acid solution, are directly applicable to the liquid remaining in the evolution flask after distillation of the hydrogen sulfide. These titrations, however, are affected by ferrous iron, and this metal is present as an impurity in stibnite. For this reason the iodometric analysis waq a priori preferred, as it was thought that in a solution buffered with bicarbonate the iodine titration mould not be influenced by small amounts of ferrous iron, an assumption which proved to be incorrect, as will appear. C o m p a r a t i v e Trials of T h r e e Volumetric M e t h o d s
TITRATIOX AND S U L F I D E l\IETHOD HEsz-The accuracy of the iodine titration n-as tested first by analysis of a solution of tartar emetic (recrystallized, free from pentavalent antimony) whose initial antimony content was determined b y the gravimetric method of Henz ($78). The solution was prepared with addition of tartaric and hydrochloric acids, which according to Gruener ( I S ) renders it stable for periods up to a year. This solution, however, developed a copious bacterial gro'iT t h in about G weeks. Aliquots of the solution, after approximate neutralization and treatment with sodium bicarbonate, \\ere titrated with 0.1 A' iodine xhich had been standardized against Bureau of Standards arsenic trioxide. The results are given in Table I. COllPARISOK O F I O D I N E
OF
Table I-Comparison of Iodine Titration and Grariinetric Method for Determination of Antimony ~~
R~ETHOD
~
WEIGHTOF AKTIMOSY I& 2 5 cc Gram
Gravimetric method of Henz Iodine titration
0.1642, 0.1638, 0 . 1 6 3 6 ; a v . 0.1639 0.1641, 0 , 1 6 3 4 , 0,1635, 0 1635, 0,1637, 0.1636,O. 1635; av. 0 1636
The difference of 0.18 per cent in the averaged results cannot be attributed solely to error in the iodometric method, for the doubtful effectii eness of washing the precipitated sulfide in the method of Henz is apparent during the analysis. Both Beckett (2) and von Bacho ( I ) reported that precipitation by hydrogen sulfide from solution containing hydrochloric acid yielded always a sulfide contaminated TT ith chlorine. Beckett reduced the chlorine content to 0.18-0.3 per cent by heating the sulfide in carbon dioxide a t 300" C., and to 0.03 per cent by heating in hydrogen-free hydrogen sulfide. After careful trials he recommended as a satisfactory analyti-
Vol. 2, s o . 3
cal procedure the precipitation according to S'ortmann and Metzl (58)followed by heating in carbon dioxide. Collenberg and Bakke's analysis of antimony metal ( 7 ) seems to confirm this opinion, their error being positive but apparently less than 0.1 per cent. Nofe-The metal analyzed contained 0.06 per cent impurity ( P b , F e , Cu S n , trace of X i ) , of which a large p a r t would b e precipitated a n d weighed with t h e antimony sulfide. T h e purity indicated b y analysis was 100.05 per cent.
The method of Vortmann and hletzl, however, is not always convenient or even fails owing to difficulty in converting the precipitated sulfide into the black form before filtering ( I ) , and as Henz's method is unconditionally endorsed by Treadwell it was used, the results indicating that the iodine titration is probably a t least equally accurate. COMPARISONS OF BROMATE,I O D I N E , AND PERMAiVGANATE TITRATIONS OF TRIVALENT ANTIMONY-Because of the doubtful keeping qualities of tartar emetic solution there was used for comparative trials of the methods a specimen of antimonous oxide. Reference to a primary standard was judged unnecessary, as the accuracy of the bromate titration has been est'ablished by Zintl and Wattenberg (44), and that of the permanganate titration by Knop (20) and by Collenberg and Bakke ( 7 ) . Primary Antimony Standards-Artificial Black Antimony Sulfide, obtained by precipitation from hydrochloric acid solution of lead-free tartar emetic, was used a standard by Beckett ( 2 ) . After heating in hydrogen sulfide the impurities were 0.03 per cent chlorine and 0.01 per cent carbon. Von Bacho's sulfide ( I ) , precipitated from sulfuric-tartaric acid solution, was apparently less pure than this. Metallic Antimony-Groschuss ( 1 2 ) tested specimens of antimony variously prepared, the best commercial varieties being refined electrolytic metal (impurity 0.013 per cent) and Kahlbaum's antimony (0.019 to 0.073 per cent impurity). Other specimens contained up to 0.35 per cent impurity. Antimony prepared by the method elaborated by Groschuss contained no detectable impurity. Kahlbaum's antimony was used as primary standard by Zintl and Wattenberg, by Knop, and by Collenberg and Bakke, the determined impurities being 0.019, 0.02 per cent, and 0.063 per cent, respectively. Tartar Emetic-Hale (15) reported that medium-sized crystals, air-dried a t about 25" C. for several hours, had almost the theoretical antimony content (iodine titration, arsenic standard). Unfortunately a recalculation of Hale's data using 121.77 as the atomic weight of antimony, instead of 120, shows that his specimens averaged 36.61 per cent antimony instead of the theoretical 36.47 per cent. Moreover, loss of crystal water occurred on keeping, especially with fine crystals, and after several weeks in stoppered bottles the antimony content rose to nearly 36.9 per cent. This behavior was confirmed for a specimen of tartar emetic recrystallized by one of the writers in 1920. The initial antimony content of 36.67 per cent (iodine titration, arsenic standard) increased on storage, and in 1929 was 37.0 per cent. The recommendations of Metzl (31) and Lutz ( 2 7 ) that tartar emetic be used as a standard substance were also based upon results obtained by use of the atomic weight 120. Zintl and Wattenberg commented on the variable water content of the salt and on the preserxe of lead, an impurity not removable by crystallization. Beckett, however, had earlier freed tartar emetic of lead by partial precipitation with hydrogen sulfide water. The use of anhydrous tartar emetic as primary standard, suggested by Metzl, and also by Sutton (371, has little to recommend it, as Hale showed that neither the definiteness nor the permanence of composition is improved by drying. I t is clear that tartar emetic does not possess the qualifications required of a standard substance.
Bronzute Titration. This titration is conducted in a solution containing much hydrochloric acid, and the end point is determined either by the decoloration of methyl orange, methyl red, or indigo sulfonic acid (added near the end of the titration, as located by an approximate preliminary trial), or electrometrically. Collenberg and Bakke ranked it first among the five methods examined by them (6). Zintl and Wattenberg (44) studied the method carefully and stated it to be one of the best available, being both accurate and rela-
July 15, 1930
I S D GSTRIAL AND ESGINEERING CHEMISTRY
tively independent of conditions. Results were the same whether titration was conducted a t room temperature or a t 50-60” C., the latter being preferable because the oxidation was more rapid and the end poirt sharper. Results ryere not affected by variations in concentration of hydrochloric acid from 5 to 20 per cent (by weight, at the end of the titration), and nere the same JThether the end point was determined electrometrically or n ith a chemical indicator. Exposure of the solution to air led to Flight oxidatiou of trivalent antimom-, especially with lower acidities, so that the titration should be fairlv rapid, though nith effective stirring, until near the end. I n any case the last several cubic centimeters of bromate must be added by separate drops, avoiding local excess. The presence of tartaric acid (5 grams) was found not to affect the results. The conditions of acidity for this titration as described by various authors-e. g., Kolthoff, Low, Scott, Sutton-differ considerably, but are generally \?-ell within the limits stated abore. The rewlts listed in Table I n-ere obtained by the procedure given by Scott (36). The potasqium bromate used m s the best obtainable salt tvice recrystallized from water. It was analyzed by the method of Kratsclnner ( 1 1 , %$), using hydrochloric acid; the purity n a s 99.95 per cent. Iodine Titration. Available evidence as to the accuracy of the iodine titration is not ~vhollyreassuring. Though Sutton (5’7) states it to be “both convenient and exact,’, Beckett ( 2 ) obtained results ah-ays somenhat low, the average error being -0.25 per cent (extremeq -0.08 and -0.42 per cent). Ton Bacho (1) obtained results 0.46 per cent lorn for an artificial sulfide apparently pure (though 0.23 per cent low in antimony by bromate titration), attributing the inaccuracy to presence of salts formed during neutralization of the initially acid solution. The later n-ork of Collenberg and Bakke (0‘) showed the satiqfactory character of the iodine titration, and its accuracy in micro-analysis was shown by Bruckl (4). Kolthoff (24) states, however, that the reaction proceeds very slowly toward the end; it is best, therefore, to titrate without starch and to drop in iodine solution until the color no longer fades after 2 minutes.” This opinion may be based in part upon the unsatisfactory end color often obtained with starch, the color varying from reddish blue to almost full red. Hale ( I C ) studied this end point in 1902, and found the color to be normal (free from red) when pure potato starch was used. K i t h impure starches the familiar reddish hues resulted, and the end point appeared to be the first permanent color, whether red or blue. The faintly yellow solution obtained by titration without starch, when treated with a solution of impure starch, gal-e a pure blue color, whereas in presence of the same indicator added a t the start of the titration the color was red. Hale concluded that the interfering erythrodextrin was formed by hydrolysis of amidulin (soluble starch), an action induced by the conditions existent during titration. It therefore seems advisable, as suggested by Fales (9), always to add the starch indicator just before the end point is reached. The writers, using Kahlbaum’s soluble starch, obtained distinctly reddish end colors when the starch was introduced a t the start, and much improved colors n h e n i t was added near the end point. The results were the same, however, and the end points in both cases sharp. I n trials of the iodine titration samples of antimony oxide were dissolved in hydrochloric acid, the solution nearly neutralized, and treated with excess of sodium bicarbonate in the form of a saturated solution just previously treated with 0.1 N iodine until a portion gave a faint blue color with starch. Titration was begun immediately thereafter, and TI as carried to a moderately strong color, a blank for which was determined as accurately as possible (the colors being ne7 er identical) and deducted. The iodine solution was prepared according to the suggestion of Chapin ( 5 ) . I t s strength was deter-
263
mined by thiosulfate standardized against pure potassium iodate (SO). Permanganate Titration. This titration was carefully investigated almost simultaneously by Knop (20) and by Collenberg and Bakke ( 7 ) , their work definitely establishing it? accuracy under properly regulated coditions. I n both studies antimony metal of knovn purity was the standard substance. I t was dissolved in sulfuric acid, hydrochloric acid added, and the solution suitably diluted. Knop obserretl that the solution thus prepared contained apparently a trace of pentavalent antimony, and therefore reduced TI ith sulfur dioxide before titration. He took the further precaution of collecting and determining nephelometrically traces of antimony which sometimes escaped with the steam during expulsion of excess sulfur dioxide by boiling. This loss never exceeded 0.4 mg. and n-as generally zero, but the ponsibilit>of such loss i q to be considered in preparing antimony solutions for titration, as viill appear later. I n both investigations titrations were made a t room temperature (Knop, 18” C‘.), and the permanganate was standardized against oxalate, an empirical standardization being unnecessary when the atoniic weight 121.77 is used. Knop found the permissible acidity to range from 10 to 20 per cent by volume of concentrated hydrochloric acid (initial volume 150 cc.), and Collenberg and Bakke placed the limits a t 10 and 19 per cent (final volume 100 cc.). In larger volumes, even up to 900 cc,, results were satisfactory when the proper acidity was maintained. I n any case less than the specified minimal acidity led to turbidity (hydrolysis) and poor end points. K i t h maximum acidity the end point v a s sharp, but the color persisted only a few seconds. With too much acid the end color was fugitive, the odor of chlorine was frequently noticeable, and the consumption of permanganate was excessive. Knop found that in the presence of only enough hydrochloric acid to prevent hydrolj by volume) the greater part of the required acidity could be supplied by qulfuric acid. A satisfactory combination was 6.7 per cent by volume of each of the coiicentrated acids. As a general average of numerous satisfactory titration., Knop obtained an atomic weight of 121.9 for antimony. From ten titrations using the 6.7-6.7 acidity the calculated atoniic weight n-as 121.84 (minimum 121.72, maximum 121.95). This indicates that his results averaged 0.06 per cent low n-it11 maximum variations of *0.1 per cent. Collenberg and Bakke obtained r e v l t s slightly too high (0.1 to 0.3 nig. in 0.1249 gram). The quantity of tartaric acid which may be present in this titration is limited, and even n i t h permissible quantities It is necessary to increase the acidity to the maximum or results will be too high. According to Collenberg and Bakke there may be preqent not over 0.05 gram of tartaric acid; Knop placed the limit a t 0.12 gram. Clearly it is better to avoid the presence of tartaric acid if possible. The tn o researches just discussed hare been considered a t some length because, in spite of the internal evidences of their north, they have thus far apparently been ignored. Kolthoff’s directions for the Permanganate titration are wholly inadequate, the selection of correct conditions being a matter of chance. Hillebrand and Lundell (16) specify an acidity which is so high (10 to 25 per cent of concentratecl hydrochloric acid in addition to 10 per cent of concentrated sulfuric acid, both by l-olume) as to impose the necessity of chilling the solution to 5-10’ C. in order to avoid error due to excessive acidity. Trials of the permanganate method were conducted in the presence of hydrochloric acid alone and of both acids. The initial acidities approached the maximum; titrations in presence of both acids nere made under Lorn’s conditions, which approximate those of Knop. Solutions were cooled n i t h
254
ANALYTICZ4LEDITION
Vol. 2, No. 3
tap water before titration, and blanks for the end color were run and deducted. The permanganate solution had been well aged, and was standardized against Bureau of Standards sodium oxalate by McBride's procedure. Analytical Results. In Table I1 there are collected results obtained by the three methods discussed above, applied to the analysis of a specimen of antimonous oxide. This material was of c. P. quality, selected from among several specimens because of its uniformity and complete solubility in hydrochloric acid. It contained pentavalent antimony, which did not interfere with its usefulness for the present purpoye. Separate weighed samples, approximating 0.2 gram, TT ere analyzed by the bromate, iodine, and permanganate titration.. All volume measurements were made with calibrated apparatus, corrections for temperature being applied T\ henever necessary.
Table 111-Influence of Ferrous Iron upon Titration of Trivalent Antimony w i t h Iodine, in Solution Buffered w i t h Sodium Bicarbonate
Table 11-Comparative Results for Trivalent Antimony in A n t i monous Oxide b y Titration w i t h Bromate, Iodine, a n d Permanganate
I n presence of ferrous iron the titration of antimony in a solution alkaline wit'h sodium bicarbonate is shown to consume too little iodine, the interference being roughly proportional to the quantity of iron. As a consequence of the inapplica,biliby of the iodine titration and the decreased usefulness of the bromate titration in presence of iron, the permanganate titration was adopted for the determination of antimony in stibnite. This necessitates a separate determination of iron present, and suitable correction of the apparent antimony content of bhe sample.
P e r cent 62.40
Byomale Titralion-0.2 gram Sbz03 dissolved i n 20 cc. concd. HCI, diluted with 40 cc. water, heated t o boiling, and titrated w i t h 0.1 N KBr03, with addition of 4 drops 0.1 per cent methyl orange near end. Final volume about 85 cc.
82.49 82.55 82.47 82.49 82.44 82.49 82.45
Zodine
2'itration-0.2 gram Sb20a dis- ( a ) solved in 20 cc. concd. HCI, 20 cc. of SOY0 Rochelle salt solution added, and solution cooled. Neutralized w i t h PiaOH, a t once acidified slightly, 75 cc. of satd. NaHC03 solution added, a n d titrated immediately with 0.1 S iodine, ( h ) using o cc. of O.5yO starch indicator. Final volume about 200 cc. (a) Starch added at beginning ( b ) Starch added near end point Blank deducted P e r m a n e a n a l e TiWafionI Hydrochloric Acid Solution-0.2 gram Sb203 dissolved in 2 5 cc. concd. HCI, diluted to 150 cc., cooled under tap, and titrated with 0.1 KMnOl. Blank deducted. Final volume about 175 cc. ~
1
H y d r o c h l o r i c - S u l f u r i c Solution-0 2 eram SbgO? dissolved in 20 cc concd kCI, a solution of 5 cc. concd. H?SOli in 120 cc. water added liquid cooled under tap, and titrated with 0.1 KMnOi. Blank deducted. Final vol-, ume about 165 cc. 1
.\-I
I
82.48 82.47 82.40 8 2 54 82 49 82 59 82.47 82 5 j 82.53 82 54
82 4 1 82.49 82 42 82 44 82.40
Per cenl
P e r cenl
+0.0s 82.47
-0.03
Gram 0.1651 0,1653 0,1632 0.1634 0 1652 0.1653 0 1652 0.1653 0 1650 0.1631 0.1634 0.1650 0 1653 0 1612 0 1651 0 1650
MgAr;s
FeSOa. -
ADDEO
ADDED
i"20
Gram None Sone 0 0062 0 0099
>-one h-one ?;one Sone Sone Sone Sone Sone
None hTone h'one hTone
Gram h-one None None None None None None h-one None 0.0012 0,0014 0.0014 0.0017 0 0019 0.0029
0.012s
FEpR"ooN"s
ANTIMONY
ERROR IN
ADDED
FoUKD
Sb*fT
Gram None None 0 0009 0 0014 Xone
Gram 0.1650 0.1652 0 1646 0.1646 0.1650 0.1652 0.1652 0.1651 0 1648 0 1650 0 1648 0 1646 0.1646 0 1606 0 . 1643 0.1621
Gram -0.0001 -0.0001 - 0.0006 -0.0008 -0.0001 -0 0002
Sone
Sone Sone Sone 0 0002 0 0003 0 0003 0 0003 0 0004 0 0006 0 0026
0.0000 -0.0002
- 0.0002
-0.0001 - 0.0006 -0 0004 -0 0007 -0 0006 -0.0006 -n 0029
Determination of Iron i n Stibnite 82 48
82 32
82 4 3
+O.ll -
.08
+0.03 -0.05
+0.06 -0.03
82 28 82 44
82.40
81.34 82 32 82 50 82 40 82.41 82 38
TAKES
82.39
+0 -0
11 11
Results by the bromate and iodine titrations are practically identical. The permanganate titration yielded slightly lower values. To judge from these trials there is little to choose among the three methods. Influence of Ferrous Iron Stibnite contains iron as an impurity, and this is preient in the ferrous condition a t the time the antimony is titrated. In both the bromate and the permanganate titrations this iron will consume some of the standard solution and will be calculated as antimony. In the bromate titration, moreover, presence of more than a trace of iron interferes with the end point, and much iron renders it so indefinite as to exclude the satisfactory use of this method. It was believed that the iodine titration, conducted in bicarbonate solution, would be unaffected by ferrous iron. I n order to test this point, there were conducted a number of trials in which antimonous oxide was analyzed by iodine titration (as in Table 11) both with and without addition of ferrous iron in the form of Mohr's salt or crystallized ferrous sulfate. The results are listed in Table 111.
The following method is recommended as simpler than that of Cusliman (8). I t is based upon the solubility of antimony sulfide in hot concentrated sodium hydroxide or sodium sulfide solution, and the insolubility therein of sulfides of iron and lead, both of which can be determined in the insoluble residue. JIETHOD-BOil gently 1 to 5 grams of finely powdered stibnite (100 mesh) with 25 to 50 cc. of 25 per cent sodium hydroxide or sodium sulfide, with occasional stirring, until only a small black residue remains. Filter the extract through a Gooch crucible, wash with hot sodium hydroxide or sulfide solution and then with water. Discard the filtrate and washings. Dissolve the residue from the filter in hot 3 AVnitric acid, followed by hot 3 A' hydrochloric acid, and wash with water. Saturate the cold solution with hydrogen sulfide, filter off any precipitate, and discard it. Boil the solution to expel hydrogen sulfide, add a little concentrated nitric acid to the hot solution, and precipitate iron with ammonia water. Filter off the precipitate and either dissolve in dilute hydrochloric acid and reprecipitate with ammonia, weighing as Fe203, or dissolve in dilute sulfuric acid, reduce with zinc, and titrate with permanganate.
The tlirec specimens of btibnite u;ed in later work viere all analyzed for iron, the results (in each case the mean of two analyses) being, respectively, 0.042, 0.152, and 0.222 per cent. The deductions to be made from the apparent antimony results (Sb Fe) are by calculation, respecti>-ely, 0.046, 0.166, and 0.242 per cent, the con\-ersion factor being 1.09. Frankford Arsenal Method for Determination of Antimony i n Stibnite
+
I n this method (8,39) the sample is decomposed by concentrated hydrochloric acid, first in the cold and then with heating. There are then added in sequence concentrated hydrochloric acid, concentrated sulfuric acid, and Tvater, and t,he mixture is boiled to expel sulfur dioxide and hydrogen sulfide. The solution is diluted largely, cooled, and titrated with standard permanganate. This procedure, if directions are followed, is unpleasant, as hydrogen chloride is given off in large quantities. If the boiling is vigorous and prolonged, so much hydrochloric acid
ISDCXTRIAL AAVDEXGINEERISG CHEMISTRY
J ~ l 15, y 1930
may be lost as to favor air oxidation of antimony (Knop, Collenberg and Bakke) or to cause irregularity in the titration due to deficiency of hydrochloric acid. Furthermore, the boiling, accompanied by vigorous evolution of acid, may lead to lo- of antimony by \-olatilization (Knop), even though tlie flask is covered and the boiling is gentle. Evidence for loss of antimony by volatilization during the boiling, or by air oxidation subsequently, appears in Table IV, which records the results of analyqes of antimony oxide by thi, procedure. Table IV-Determination of A n t i m o n y in A n t i m o n o u s Oxide by Frankford Arsenal Method ( S b - ? + b y direct t i t r a t i o n with K h f n O d = 8 2 , 4 3 5 :
Determination of Antimony and Sulfur in Stibnite by Evolution Procedure (Final Method)
Determinations of both antimony and sulfur in the same sample were made as a final test of the proposed method. The determination of sulfur was conducted as described previously (30). The acid was barely boiled during the decompo-ition and transfer of hydrogen sulfide, the volume of liquid in the flask decreaqing little if a t all. This is probably important, even in presence of the partial condenser, both to avoid loss of antimony by ~olatilizationand t o prevent distillation of so much hydrochloric acid as to interfere later with the titration of antimony.
see T a b l e 11) A A II 'M0K Y (Sb+--)a
PROCEDURE
Per cent 0.2 gram SbzOa treated with 35 CC. concd. HC1 a n d heated 20 rnin. on water b a t h . Liquid cooled, treated x i t h 20 cc. concd. HCI, 20 cc. concd. HzSOI, and 100 cc. H20, and boiled gently 20 minutes. Liquid diluted t o 600 cc., cooled quickly under t a p with air excluded a n d titrated with KhInOa.
82.22 81.79 80.97 82.39 82.35 82.03 82.30 82.18 Av. 82.03
At. wt. 1 2 1 . i i
These re-ults are all too low, averaging 0.4 per cent lower than those obtained by direct titration ith permanganate, and their range of variation (1.4 per cent) is excessive. Determination of Antimony in Stibnite by Permanganate Titration following Decomposition by Evolution Procedure
Samples of stibnite were decomposed in the evolution apparatus as described in the previous paper (30). To the residual liquid in the flask vas adtled a previously cooled mixture of 120 cc. of water and 5 cc. of concentrated sulfuric acid. The solution was cooled under tlie t a p and titrated with 0.1 S pernianganate. As a further test of the Frankford Arsenal method, check analyses were run by this procedure also. The samples in all ewes approximated 0.2 gram. The collected results appear in Table T-.
Procedure for Determination of Antimony
After distillation of the hydrogen iulfide keep the reoidual liquid in the flask in an atmosphere of carbon dioxide until the determination of antimony can be made nithout interruption. Prepare previously, and cool well, a solution containing 40 cc. of concentrated sulfuric acid per liter. Add to the liquid in the flask 120 cc. of this dilute sulfuric acid, cool the solution rapidly under the tap, and titrate with 0.1 S pernianganate until the first pink tint pervade3 the liquid. Although this color is quite transitory, and will seldom persist more than about 30 second, at most, it correctly and sharply indicates the end of tile titration. If directions have been properly followed, the liquid a t the beginning of the titration nil1 contain hydrochloric acid equivalent to not less than 10 per cent by xolume of tlie concentrated acid, in addition to about 4 per cent by volume of concentrated sulfuric acid, this being an acidity close to the optimum determined by Knop. Each cubic centimeter of 0.1 permanganate represents 0.006089 gram of antimony.
STIB-
ASTI\iOSY
.kntimony (Sb Fe)
+
Iron
Fz:Fe.
Antimony .
