January 15, 1930
1SDC:iSTRIALA X D ENGINEERISG CHEMISTRY
45
Microscopic Examination of Precipitates as an Aid to Precise Analysis' I-Estimation of Sulfates as Barium Sulfate Stephen Popoff and E. W. Neuman2 ARALYTIC.4L LABORATORY, STATE UNIVERSITY OF I O W A , I O W A CITY, I O W A
The quantitative determination of sulfates by precipitation as barium sulfate was studied, employing various recommended methods. It was shown t h a t t h e reverse method of precipitation -i. e., adding t h e sulfate t o t h e acidified barium chloride solution-gives results which are nearer to t h e theoretical t h a n t h e usual and most of t h e recommended methods. The usual method, as well as most of t h e recommended methods, gives results t h a t are very m u c h lower (over 1 per cent in t h e case of potassium sulfate) t h a n t h e theoretical. The lower results may be attributed to t h e preferential adsorption of t h e sulfate ions rather t h a n of t h e chloride ions. The reverse order of precipitation of barium sulfate is also to be preferred i n t h e presence of salts including large concentration of a m m o n i u m chloride or ferric chloride.
Neither t h e reverse nor regular methods can be employed in t h e presence of potassium nitrate. The chloride contamination is greater in t h e reverse method t h a n in t h e usual methods. This greater contamination does not condemn t h e reverse method, as t h e contamination is practically offset by t h e solubility of barium sulfate. There appears to be progressive loss on heating up to 950' C. t h e barium sulfate obtained by either t h e regular or reverse method. The photomicrographs of barium sulfate obtained under varying conditions and methods show t h a t t h e reverse method gives in practically all cases more uniform and more nearly t h e same kind of crystals t h a n other recommended methods. This feature is not surprising in view of t h e fact t h a t t h e environment is more nearly alike in t h e reverse method.
.............. HE aim in analytical work, whatever its object, is to obtain as pure a precipitate as possible. I n most gravi-
T
metric methods it is next to impossible to obtain pure precipitates because coprecipitation plays havoc with many, if not all.,. DreciDitation . reactions. A method whereby the purity and uniformity of the materials obtained in precipitation r e a c t i o n s might be ascertained would help to solve many interesting problems, such as analytical methods, the determination of activity (ion) product constants, and the relation of size of particles to solubility. The predicted order of precipitation involving several p r e c i p i t a t e s often is contrary to the experimental facts. This may mean either that the law of fractional p r e c i p i t a t i o n Figure I-Distillation Apparatus for fails to hold or that Chloride Determination the activity Droduct constants are incorrect. To solve this problem satisfactorily, the materials employed must be pure and must contain only one kind of crystals with the same energy content. A microscopic examination of precipitates not only makes it possible t o determine the general appearance of crystalline precipitates, but also affords the opportunity of ascertaining 'Received August 29, 1929. * Holder of J. T.Baker Chemical Company Research Fellowship in Analytical Chemistry.
many of the physical properties of the crystals, thus helping to establish the number and kind of constituents. Sulfates are usually estimated by precipitation as barium sulfate. The barium sulfate precipitate bas in most cases a definite crystalline appearance which may be studied both analytically and microscopically. It would be an almost endless task to give a complete historical description of the various methods recommended for precipitating barium sulfate and to set forth the opinions and explanations pertaining to the phenomena of coprecipitation. The experimental results here reported show that theoretical values for sulfates are possible when barium sulfate is precipitated by the reverse order of adding the sulfate solution to the acidified barium chloride solution,rather than by the usual order of adding the barium chloride solution to the acidified sulfate solution. Photomicro- I I under varying condi- I d tions show that the re\verse order of DreciDitation gis'es Figure 2-Calomel Cell and Salt Bridge which a r e uniform and differ in general appearance from those obtained by the usual order of precipitation. This uniformity of crystalline appearance of the precipitates may be attributed to the relatively greater constancy of the environment during precipitation in the reverse method. The kind and concentration of the metallic ions, other factors remaining constant, ~~
A S A L Y T I C A L EDZTIO-lT
46
Figure 3-Effect
Vol. 2, No. 1
of HCI Concentration on Crystal Form of BaSOl Prepared from K2S04 by Reverse Method with Digestion a t 1 Hour (a) none; ( b ) '2 cc.; (c) 4 cc.; ( d ) 8 cc.; ( e ) 16 cc.; (f)32 cc.
appear to influence not only the analytical results but also the general crystalline appearance. Materials
A11 chemicals used \yere J. T. Baker's c. P. grade. All water was redistilled from alkaline potassium permanganate in an all-Pyrex specially-constructed still. This water was used for all crystallizations and solutions. All salts were recrystallized three times. ,411 solutions were made u p by weight. Hydrochloric acid was purified by distilling approximately 6 N HC1 to obtain a constant boiling product. The first and last 300-cc. portions of the distillate were rejected. The middle portion was again distilled, rejecting the first and last 200 cc. Concentrated sulfuric acid was purified by two distillations, rejecting the first and last portions. These acids were stored in silica bottles. Sodium hydroxide was prepared carbonateand chloride-free by the method of Cornog (3). Ammonium hydroxide was prepared a? needed by distilling a concentrated solution of ammonia into water in a wax bottle which was packed in ice. S o htions
Potassium sulfate solutions and sodium sulfate solutions were prepared from the purified salts which had been heated a t about 230" C., ground in an agate mortar, and finally
l o o o C. for
ignited a t about 700" C. for an hour. The salts were allowed to cool in a desiccator and then made up by weight into solutions, 40 grams of which gave about 0 8 gram of barium sulfate. A check on the concentration of these solutions was made by evaporation to dryness in platinum vessels, with subsequent ignition to constant weight a t about 700" C. The potassium sulfate solutions were evaporated in the presence of about 5 drops of dilute sulfuric acid. Concentrated solutions of the residues and of the original salts were neutral. Barium chloride solution, containing 21 grams of the anhydrous salt per liter, was prepared from the purified crystals dried for several hours a t 300" C. Concentrated solutions of the salt were neutral. The concentration of the solution was checked by precipitating the chloride in a known volume of the solution as silver chloride, according to the usual method. Sulfuric acid solutions Tyere prepared by weight from the purified stock solution which was stored in silica bottles. This stock solution was standardized against Bureau of Standards benzoic acid, and potassium acid phthalate ('?'), and against constant boiling hydrochloric acid prepared according to F o u k and Hollingsworth ( 5 ) . The maximum deviation with these standards was less than 0.05 per cent. All titrations were done by weight with carbonate-free sodium hydroxide solution, and all necessary precautions were taken to prevent access of carbon dioxide to the titrating solu-
I S D C S T R I A L An'D EA-GISEERISG CHEMISTRY
January 15, 1930
F i g u r e 4-Effect
of HCI C o n c e n t r a t i o n o n C r y s t a l F o r m of BaSOa P r e p a r e d f r o m K ~ S O by I Reverse M e t h o d w i t h Diaestion a t 100' C. f o r 24 H o u r s
( a ) 4 cc.; ( b )
F i g u r e &Effect
6 cc.; (c) 8 cc.
of BaClz Excess on C r y s t a l F o r m of B a s 0 4 P r e p a r e d f r o m K z S O I by Reverse M e t h o d
(a) 5 cc. excess BaClr, 4 cc. HCI; ( b ) 10 cc. excess BaCIz, 4 cc. HCI: (c) 15 cc. excess BaCIz, 4 cc. HCI; ( d ) 20 cc. excess BaCI?, 4 cc. HCI. (e) 10 CC.
excess BaCI?, 16 cc. HCI;
U)20 cc. excess BaCh,
16 cc. HCI
47
48
A S A L Y T I C AL ED1 T I O S
Vol. 2 , No. 1
Figure 6-Effect of T i m e of Addition of Precipitating Reagent o n Crystal Form of BaSOa Prepared from K&Oa by Reverse Method (a)
Figure 7-Effect
1 minute; ( b ) 4 minutes: ( c ) 8 minutes
of Certain Factors on Crystal Form of Bas04 Prepared f r o m K2SOd by Reverse Method (a)
precipitating in the cold: ( b ) filtering at once;
tions. Vacuum corrections were made whenever necessary. Three normal hydrochloric acid was prepared from the purified hydrochloric acid. The concentration was checked against the sodium hydroxide mentioned. For a further comparison of results obtained by precipitating the sodium, potassium, and ammonium sulfates as barium sulfates, solutions of the foregoing salts were prepared by neutralizing the diluted stock solution of sulfuric acid with the various purified hydroxides. Apparatus
Except where otherwise stated, precipitates were prepared in 600-cc. Pyrex beakers. These were covered with a glass disk, having 3 holes, serving as a projection for the weight pipet containing the sulfate solution, for the stirrer, and, in certain instances, for the pipet containing the barium chloride solution. The electrically-driven stirrer was made of silica. Silica was found best suited for this purpose. Glass soon became etched and was consequently very difficult to wash free from adhering barium sulfate particles. A 40-cc. weight pipet was used for weighing all sulfate solutions. An electric oven nTas used for ignition of the precipitates.
(c)
acid equally divided
Table I-Reproducibility of Weight of Platinum Gooch Crucibles NO. 1 NO. 2 Grams Grams 18.1788 20.1763 18.1786 20.1762 18.1i86 20.1764 20.1760 18.1784 18.li82 20.1761
Perforated platinum crucibles with asbestos mats were used throughout. An attempt to use Llonroe crucibles proved futile, as the platinum sponge tended to crack upon ignition. The asbestos fiber used in the preparation of the crucibles was purified by heating with a mixture of hydrochloric and nitric acids. After rinsing in distilled water and in dilute sodium hydroxide it was further washed by digesting several times in distilled water. The coarse fibers were then roughly separated from the fine fibers. I n the preparation of the filter base a 2-mm. layer of the coarse fibers was followed by a 3-mm. layer of fine fibers. This was covered with a perforated disk, which, in turn, was covered with a 1-mm. layer of the fine fibers. An asbestos filter was never used twice. Crucibles so prepared filter rapidly and thoroughly. The data in Table I show the reproducibility of the weight of crucibles prepared in this manner. I n these trials
49
50
A S A L Y TICAL EDI T I O S
Vol. 2, No. I
Figure %Effect of Presence of Various Salts i n Sulfate Solution o n Crysreg.; Cf) NHICI, rev.; (g) KNOs,
(a) NaCl, reg.; ( b ) NaC1, rev.; (c) KC1, reg ; ( d J KC1, rev.; (e) "4C1,
the crucibles were washed between each ignition with a solution similar to the mother liquor from certain barium sulfate precipitates and finally with water. The temperature of the ignition was 800" C. for 1 hour.
saturated calomel electrode with a potassium nitrate bridge was used as the other half element of the cell. A special type of electrode was prepared (Figure 2) t o eliminate as far as possible a diffusion of chloride ion from the calomel half cell.
Prevention of Creeping of Precipitates
Methods of Analysis
To prevent loss due to creeping in the case of certain precipitates a fine film of vaseline about 5 mm. wide was placed near the top of the crucibles. This treatment was found to have no effect on the final weight of the crucibles. Apparatus for Chlorine Determination
The contamination of the various precipitates by chlorides was determined by a method similar to that described by Hulet and Duschak (8). The distillation apparatus was modified to prevent loss of hydrochloric acid through joints. The apparatus was tested with known quantities of potassium chloride. (Figure 1) The precipitates were dried a t room temperature and weighed in glass tubes open a t both ends. They were then transferred to the still and dissolved in redistilled concentrated sulfuric acid, and the solution was heated a t 200" C. for about 1*/*hours. The hydrogen chloride formed was passed with the aid of a stream of air into a n excess of 0.02 N silver nitrate solution. The excess silver nitrate was titrated electrometrically with 0.02 N potassium chloride, using silver-silver chloride electrodes. A
By the reverse method of precipitation is meant the addition of the sulfate solution to the barium chloride solution, as opposed to the usual or regular method, in which the precipitating agent, barium chloride solution, is added to the sulfate solution. I n the reverse method, about 40 grams of the sulfate solution were added dropwise t o an acidified solution containing an excess of barium chloride and enough water t o bring the final volume of the mixture to 350 cc. During the addition of the sulfate, the barium solution was kept just below the boiling point and stirred continually. The precipitates were digested for 1 hour just below the boiling point, with occasional stirring. The precipitate was washed by decantation with four 15-cc. portions of hot water and then washed in the crucible with small portions of hot water until 5 cc. of the wash solution failed to give even the faintest traces of turbidity when mixed with 1 cc. of a 0.1 N acidified silver nitrate solution. The precipitates were heated for 1 hour at 800" C. Heating for 1 hour a t 800" C. was found to be sufficient. I n the case of overnight or prolonged digestion, loss due to evaporation was prevented by covering the beakers with evaporating dishes kept full of water.
January 15, 1930
ISDUSTRIAL AND ESGINEERING CHEMISTRY
51
tal Form of Bas04 Prepared by Regular a n d Reverse Methods f r o m H & 0 4 reg.; ( h ) KSOa, rev.; (i) FeCla, reg.;
(3 FeC13, rev.; ( k ) iYH4CI and KaCI, reg.: ( I ) NHaCl a n d XaCl, rev.
Conditions Studied and Experimental Data
Various factors were studied in the precipitation of barium sulfate. Tables I1 to XI and Figures 3 to 10 give the experimental data and photomicrographs obtained. ,411 precipitations were carried on a t the boiling point. The final volume was 350 cc., containing 4 cc. of 3 N hydrochloric acid and 5 cc. exceSs of barium chloride (equivalent to 0.1 gram of barium chloride). The time of addition of precipitating reagent xvas 4 minutes. The precipitates were digested for 1 hour near the boiling point and heated a t 800" C. for 1 hour. Exceptions to the general procedure are noted in the tables. EFFECT O F VaRYING HYDROCHLORIC A\CID CONCENTRATION -The effect of varying the concentration of the HCI (Table I1 and Figures 3 and 4). in the precipitation of BaSOl by the reverse method, v a s studied under the following conditions: precipitating reagent, K2S04; yarying concentrations of 3 N HC1; varying times of digestion; and x-arying temperatures of ignition of the BaPOd. The inaccuracy (the deviation from the true yalue) and the mean deviation are given. The inaccuracy indicated indirectly the accuracy, the mean deviation the precision. All subsequent tables give the accuracy and precision of the experimental data in similar fashion. The effect of increasing the concentration of HC1 from 2 to 8 cc. of 3 A' in 350 cc. final yolume is comparatively small
with both the 1-hour and 24-hour digestion. In the same acid concentration, but heating the BaS04 to 150" instead of t o 800" C., the results are invariably higher than the theoretical. If the concentration of acid is increased from 8 to 16 or to 32 cc. the results are lower if BaS04 is heated to 800" C. The lower results may be explained on the basis of increasing solubility with increasing HC1 concentration. The best HC1 concentration appears to be from 4 to 8 cc., if the precipitates are subsequently ignited a t 800" C. Table 11-Effect of Variation of HCI Concentration o n Determination of SO4 Ions i n KzSO1 by Reverse Method N O OF
HC1 DETXS
cc
Bas04 Calcd Found Gram Gram
DIGESTED 1 H O U R A T 1oo0
0 2
1 4
4 6
4
8 16 32
1 3 3
4
0.8040 0 8037 0.,989 0 8037 0.8121 0.8029 0.8091
c.;
2
4
I
8 16 32
2 2 2
0.8089 0.8039 0.8049 0 8041 0 8057
C.:
0.8118 0,8068
0.8069 0.8073 0.7995
DIGESTED 24 H O U R S .41 100' C.;
4 6 8
2 0.8113 2 0.8102 3 0.8108 4a 2 0 8090 a H e a t e l a t l5O0 C.
0.8112 0 8097 0.8101 0.8135
Per cent
Mg
DEVIATIOV Per cenl
c. -0.21 -0 04 0.01 0.1 0 08 0.16 0.96
HEATED 1 HOUR AI 8 w 0
0 8057 0.8040 0.7988 0.8029 0.8115 0.8016 0,8014
D I G E S T E D 1 H O U R A T 100°
2
I\IEAV Ih4CCUR4CY
--1.7 --0.3 0.1 0.8 0 6 1.2 I
,
HEATED 1 H O U R AT 150'
--2.9 -2.9 -2.0 -3.1 6.2
0 , -4 5
0' 02 0.02 0 08 C.
-0.36 -0.36 -0.25 -0.40 0.78
H E A T E D 1 H O U R A T 800'
0 1 O.p
0:03 0.03
0.01 0.06 0 08 -0.52
0 05 0.04 0.00 0.14 0.16 C.
0.04 0.02 0.01 0.00
52
A S A L Y TICB L EDI TI 0-V
Figures 3 and 4 give the photomicrographs obtained. The magnification in all photographs is 450 diameters. Rapidly formed crystals appear to be thin and with apparent symmetrical fringes. The effect of increasing the acid concentration above 8 cc. of 3 N HC1 is to increase the thickness of the crystals and to eliminate entirely the rough edges. EFFECT O F EYCESS OF B-4RIUhX CHLORIDE-The effect Of an excess of BaClz (Table I11 and Figure 5 ) in the precipitation of Bas04 by the reverse method was studied, using K2S04 and 4 and 16 cc. of 3 N HCl. T a b l e 111-Effect
cc
Gram
Gram
15a 205
0.7989 0.8070 0,8034 0,8082 0.8052 0,8016
0,7988 0.8083 0.8048 0.8101 0.8042 0.8024
lob
20 b a
HCI.
The effect of increasing the excess of BaC12 beyond 5 cc. in a final volunie of 350 &, containing 4 cc. of 3-&VHCl, is to increase the weight of the precipitate. With 10 cc. excess the results may be lowered by increasing the acid concentration to 16 cc. Evidently both the acid concentration and the excess of BaClz must be taken into consideration. The crystals appear to be less uniform with greater excess of BaClz than with 5 cc. EFFECTOF TIMEO F ADDITIONOF SULFATE-The effect of time of addition of the sulfate (Table IV and Figure 6) was studied by the reverse method of precipitation of BaS04. employing KzS04. T a b l e IV-Effect
No. OF
of R a t e of P r e c i p i t a t i o n of B a s 0 4 f r o m KzSOa by
by Two M e t h o d s w i t h Different S u l f a t e s hfE.4N S O . OF BaSOa DEVIASULFATZ DETNS. Calcd. Found INACCURACY TION Gram Gram Mi. Per cent Per cent RIGULAR
HzS04 KzSOI NazSOa (3Ha)zSO4
4 5 2 2
0.8022 0.8031 0.7797 0.8531
HzSOi
6 4 9 8
0.8064 0.7989 0.7991 0,8237
4 4 2
i n P r e c i p i t a t i o n of Bas04 f r o m KzSOa by Reverse Method ME 41.1 SO. OF BaSOa DEVIAFACTOR DETNS Calcd. Found INACCURACY TION Grams Grams Mg. Per cent Per cent Pptd. in cold 0.8066 0.8086 -2.0 -0.25 0.10 Filtered a t once 0.8127 0,8128 -0.1 -0.01 0.01 Digested 1 hour a t 6.50 r -. 0.00 0 06 0.8105 0.8105 0.0 Stirring rods instead of -0.1 -0.01 0.05 mechanical stirrer 0,8028 0,8029 Half usual weight of 0,4034 0.4021 0.1 Bas04 1.3 0.32 Twice usual weight of 1,5000 1.5010 -1 0 -0.06 0.02 BaSOi Si02 beakers instead of -0.2 0 . so04 0. SO06 -0.03 0.04 Pyrex Filter paper instead of -0.1 0 . 7793 0.7994 -0.01 0.01 Gooch Acid eauallv divided betwe'en Ba and Sod' solns. 4 0.8080 0.8072 0.8 0.10 0.01 T a b l e V-Factors
--
++
From the data in Table V it can be concluded that precipitation in the cold is not advisable, but that the following procedure will not appreciably affect the results: Filter a t once, or digest for 1hour between 65" and 100' C.; use either glass stirring rods or a mechanical stirring device. Pyrex beakers may be used without objection; filter paper may be used if precautions are taken to keep the precipitate as far from the top as possible and if the paper is ignited very slowly with free access to the air. The HC1 may be evenly divided be-
5.0 10.0 4.9 5.8
0.8066 0,7987 0.8009 0.8249
0.58 1.25 0.60 0.74 .
-0.2 0.2 -1.8 -1.2
.
-0.02 0.02 -0.24 -0.16
0.1 0.04 0.01 0.06 ~
~
0.01
0.03 0.04 0.04
CHECK ON REGULAR VERSUS RETERSE14ETHOD OF PREC I P I T A T I O N - ~ a check on the concentrations and on the purity of the solutions employed to obtain the data in Table VI, a similar series of determinations was made, using solutions prepared from standard HzS04 neutralized by the various hydroxides. The conditions of precipitation were identical with the conditions fixed in Table VI. Table VI1 gives t h e resulting data, which show that the solutions in Table VI were prepared accurately. Table VII-Comparative Values f r o m S a l t Solutions P r e p a r e d Directly f r o m HzSOa a n d Various Hydroxides NO. OF BaSOa MEAN HYDROXIDE DETNS. Calcd. Found INACCURACY DEVIATION Gram Gram Mg. Per cent Per cent
Reverse M e t h o d Bas04 MEAN Found INACCURACYDEVIATION KOH Gram MP. Per cent Per cenf XaOH 0.7918 0.7891 2.i 0.35 0.06 XHaOH 0.7989 0.7988 0.1 0.01 0.03 0.7884 0.7867 1.7 0.18 0.00 KOH 4-minute addition of KzS04 gives the best NaOH NHIOH
Evidently a results. EFFECT OF OTHERFACTORS STUDIED-A number of other factors were varied (Table V, Figure 7 ) in the reverse method, employing KzS04.
METHOD
0.7982 0.7931 0.7748 0,8473
REVERSE h1KTHOD . ~ ~
KzSOI IiazSOa ("4)zSO4
TIME DETNS. Calcd. Mznules Gram 1 4 8
~
T a b l e VI-Values
MEAN I X A C C U R A C Y DEVIATION Mg. Per cent Per cewl 0.1 0.01 0.02 -1.3 -0.16 0.05 -1.4 -0.1s 0.02 -1.9 0.00 -0.26 1.0 0.12 0.05 -0.8 -0.10 0.00
4 cc. HCI.
b 16 cc.
tween the BaCl? and the sulfate solution. With the conditions mentioned, the final weight of BaS04 should not deviate appreciably from 0.8 gram. REGCLAR VERSUS REVERSENIETHOD O F PRECIPITATION EMPLOYING T T SULFATES-T~~ ~ ~ results~ in Table ~ VI and ~ Figure 8 show a comparison between the results obtained by the regular and reverse methods of precipitation when using the sulfates of hydrogen, potassium, sodium, and ammonium. The reverse method of precipitation of Bas04 is to be preferred in every case.
of Excess BaClz o n D e t e r m i n a t i o n of SO4 I o n s i n KzSOa by Reverse M e t h o d
EXCESS r'io OF Bas04 BaClz DETNS. Calcd. Found
1-01. 2, KO. 1
R E G U L A R METHOD
4 4 6
0,8003 0.7966 0.803%
7 4
0 8012 0.8009 0.7996
8.5 5,l 5.4
1.06 0.61 0.68
0.07 0.05 0.04
0.4 -2.3 -1.1
0.05 -0.29 -0.14
0.02 0.02 0.04
REVERSE METHOD
4
Table VIII-BaSOa SALT
0.7918 0.7915 0.7978 0.8008 0,8032 0,8007
P r e c i p i t a t e d f r o m HzSO4 i n Presence of S a l t s MEAN DEVIAINACCURACY TION ME. Per cent Per cent
KO. OF BaSOa DETNS.Calcd. Found Gram Gram
REGULAR METHOD
KCI NaCl NHlCl NHiCl XaCl KxOs KSO3 4- HCl (4 cc ) FeCh FeCh (filtered a t once)
+
0.8106 0 8506 8302 7988 7973 8015
0.8008 0 8461 0 8014 0 8172 0 79Sd 0 7939 0 7692
9.8 4.5 5.0 13.0 3.3 3.4 32.3
1.62 0.41 0.42 4.04
0.01 0.01 0.01 0.07 0.06 0.07 2.51
0.8031
0.7661
37.0
4.61
0.31
0.8064
0 0 0 0
1.22 0.56 0.82
RKYBRSE XETHOD
h'aCI KCl KHaCI NHaCI KaCl KiiO3 K?;Os HCl (4 cc.) Feel3 FeCh (filtered a t once)
+ +
0.8020 0.8047 0.8071 0.8197 0.7994 0.8001 0.8050
0.8022 0,8032 0.8080 0,8007 0.8182 0.8166 0.8076
- 0 2 0.5 0.9 - 1.0 -18.8 -16.4 2.6
-
-0.02 -0.06 -0.1 -0.12 -2.6 -2.07 -0.3
0.01 0.01 0.01 0.02 0.00 0.16 0.14
0.8007
0.8019
-
-0.15
0.02
-
1.2
EFFECTOF S.4LTS I N THE REGULARAND REVERSEMETHODs-The effect of various salts in the regular and reverse methods of precipitation, employing HzSOa and conditions similar to those used t o obtain the results shona in Table VII, is given in Table VIII. A weight of NaC1, KC1, or NH4C1equivalent to 0.8 gram of Bas04 was added to the standard H2SO4solution. In the rest of the cases the following weights were employed: KN03, 1gram; FeC4.6Hz0, 0.35 gram; NaCl, 0.8 gram; NHdC1, 8 grams. When 4 cc. of 3 N HC1 were used the iron tended to go in the colloidal state on digestion. Figure 9 gives the photomicrographs obtained.
~
~
I S D L - S T R I S L A S D EiYGISEERISG CHEMISTRY
January 15, 1930
Figure 10-Crystal
Form of B a s 0 4 Prepared by Regular a n d Reverse Methods a n d b v Several Standard Methods f r o m KzS04 (a) reverse;
( b ) regular,
(c) Hahn and Otto,
Here again, as in all former cases, the reverse method of precipitation is to be preferred over the regular method of precipitating BaSOd. I n view of the fact that all results by the reverse method are higher than the theoretical, it may be advisable to employ 8 cc., rather than 4 cc., of 3 N HCl. The presence of KNOa, and probably of any nitrate, is objectionable. COMPARISON O F LrARIOUS hIETRODS O F PRECIPIT.4TINC BARIUMSULFATE-Table Ix gives the results obtained when various recommended methods of estimating sulfates as Bas04 were followed. KzS04 was employed. Figure 10 gives the photomicrographs obtained. The results show convincingly that the reverse method of precipitating BaS04 is to be preferred to all the other methods compared. Table IX-BaSO4
METXOD~
Prepared f r o m K S O 4 by Various Methods MEAN No OP Bas04 DEVIADETNS Calcd. Found INACCURACY TION Grams Grams MR. Per cent P e r cent 4 0,7989 0.7988 0.03 0.1 0.01
Reverse 4 0,8034 Regular Hahn and Otto ( 6 ) 2 0,5840 Blasdale ( 2 ) 4 0.9776 Allen and Johnston (: J ) 2 2.0929 Fales ( 4 ) b 2 0.4793 Fales ( 4 ) c 0.5218 2 a Italic numbers in parentheses refer b Heated a t 800' C. c Heated a t 110' C.
0.7935 9.9 1.24 0.5877 1.3 0.16 13.8 1.72 0,9638 2.0821 10.8 1.35 0.4738 5.5 0.69 0,5189 2.8 0.34 t o Literature Cited.
0.01 0.02 0.09 0.09 0.02 0.10
CHLORIDE CONTAMINATION UKDER T'ARYIKG CONDITIONs METHODS-Table x gives the chloride contaminations
AND
53
( d ) Fales,
(e) Blasdale;
if, Allen and Johnston
under varying conditions and methods. All the precipitates were dried a t room temperature in a desiccator instead of being ignited to 800' C. The following factors increase the chloride contamination: (1) increasing the concentration of HC1; (2) decreasing the time of addition of KzS04; (3) increasing the excess of BaC12 (beyond 15 cc.); (4) increasing the time of digestion from 1 hour to 24 hours; ( 5 ) employing the reverse, and the Hahn and Otto methods of precipitating BaS04rather than the regular or other recommended methods. The chloride contamination is less with H2S04than with salts when employing the reverse method. The chloride contamination is small by the regular or recommended methods, except by t h a t of Hahn and Otto. Whcbn employing KzS04 with 4 cc. of HCl the chloride contamination was decreased from 0.17 to 0.09 per cent when the BaS04was heated at 800' C. EFFECT O F TEMPER.4TURE OK ]?-EIGHT O F BARIUM SULFATE PRECIPITATED BY DIFFERENThfET€iODS-Table XI gives the effect of heating BaSOl precipitated by different methods. Bas04 was obtained by the usual and reT-erse methods and by dissolving the precipitated sulfate from H2S04and Ba(OH)2in concentrated H2S04and reprecipitating it by adding the solution to water. All the precipitates \\-ere dried a t 100" C. for 1 hour. It can readily be seen that Bas04 continues to lose weight on successive heatings from 100' to 950" C. After heating the sulfates to 950" C. they were shaken with water
ANALYTICAL EDI TIOX
54
Vol. 2, No. 1
free from COz and containing phenolphthalein. The solution remained colorless, showing that practically no Bas04 was decomposed to yield BaO.
ommended. Filtration shortly after precipitation in the case of iron is recommended.
C o n t a m i n a t i o n of B a s 0 4 P r e c i p i t a t e s P r e p a r e d u n d e r Varying C o n d i t i o n s CONDITIONS OF PRECIPITATION TABLE CHLORIDE Per cenl I1 0.15 Reverse &SO4 2 cc. HC1 I1 0.17 4 cc. HC1 8 cc. HCI I1 0.18 24-Hour digestion (4 cc. HCI) I1 0.21 V Acid divided 0.17 V N o digestion 0.17 5 cc. 111 Excess BaClz 0.17 10 cc. 111 0.15 111 15 cc. 0.17 20 cc. 111 0.23 R a t e of addition of sulfate 1 minute 0.19 IV 4 minutes IV 0.17 8 minutes IV 0.10 BaSOa from various sulfates HzSOa reverse 0.13 VI VI HzSOa regular 0.015 KzSO4 reverse VI 0.17 0.01 KsSOa regular VI NazSOI reverse VI 0.16 NazSOa regular 0.03 VI (NHaIzSOa reverse 0.15 VI 0.01 (NHdrSOd regular VI K ~ S O by I various methods Reverse IX 0 17 Regular IX 0.01 IX 0.15 H a h n and O t t o IX 0.07 .411en and Johnston 0 01 IX Blasdale IX 0.01 Fales
TEMPERATURE TIME
T a b l e X-Chloride
T a b l e XI-Ignition
C. Hours 150 1 150 1 230 1 240 1 240 1 240 1 240 1 240 1 350 1 350 1 350 1 470 1 470 1 470 1 470 1 470 2 470 2 470 1 650 1 650 1 650 1 650 1 750 1 750 1 950 l'/r 950 1 950 1 Total Loss (54) Time (hours)
Loss of Bas04 P r e p a r e d u n d e r Varying
Conditions REGULAR REVERSE METHOD METHOD 0.1 0.1 0.3 0.0
...
-0.1 0.1 0.2 0.0
0.8 0.0 0.1 0.0
... ...
...
...
, . .
... ...
0.6 0.2 -0.1 0.4 0.3 0.1
...
0.8 0.2 -0.1 0.2 0.1 0.0
... ... ...
I . .
...
... ...
. . ,
1.3 0.2 0.0
2.6 0.2 0.0
...
. . ,
0.3 0.3 -0.3 -0.2 0.1 0.4 0.3 0.1 0.1 0.1 4.3 4.9 0.42 0.48 18.5
... ...
0.4 0.0 0.1 -0.1
...
0.2 0.1
.,
.
...
... ...
...
0.4 0.0
...
0.1
0.0
...
..,
... ... ... ...
0.2 0.6 0.1
0.8 0.4 0.0
... ..,
... ... ...
...
... ...
1.4 1.1 0.0 0 1 0 6 0.8 0.4 0 3 0.2 0.1 4.5 4.3 0.39 0.38 15.5
RECRYSTALLIZED FROM HISO, 0 4 0 0 17.7 0.4 0 4 0.0 1.0 0.8 13.5
... 1.9
0.8 0.0 16.2 0.4 0.5 5.0 0.8 -0.1 13.8
... ...
5.2 0.6 -0 9 1.0
0.8 -1.2 1 4 0 9 0,5 -0.4 -0.3 -0.2 -0.4 4.2 4.4 0.3 -0.5 0.3 -0.9 0.2 0.6 0.4 0.6 0.0 0.1 0.7 0.6 0.5 0.4 0.1 0.0 46.7 49.5 4.41 4 36 25.5
Conclusions
Acknowledgment
As a result of this study the following conditions are recommended for the estimation of sulfates: To the nearly boiling barium solution containing 4-8 cc. of 3 N hydrochloric acid and 5 cc. excess of barium chloride (about 0.1 -11) add the sulfate solution dropwise (about 4 minutes) and with constant stirring. Digest 1 hour near the boiling point with occasional stirring. Wash the precipitate by decantation, and, on the filter, with hot water until free from chlorides. The final weight of barium sulfate should not differ appreciably from 0.8 gram nor the final volume from 350 cc. Gooch crucibles may be used. Heating to 800" C. for 1 hour is rec-
Acknowledgment is made to B. E. Thomas for certain preliminary investigations in connection with this study. L i t e r a t u r e Cited (1) Allen a n d Johnston, J . A m . Chem. Soc., 32, 588 (1910). (2) Blasdale, "The Fundamentals of Quantitative Analysis," p 185, Van Xostrand, 1928. (3) Cornog, J. A m . Chem. Soc., 43, 2573 (1921). (4) Fales, "Inorganic Quantitative Analysis," p. 185, Century, 1925. (5) Foulk a n d Hollingsworth, J . A m . Chem. SOL.,46, 1220 (1923). (6) Hahn a n d Otto, Z . anorg allgem. Chem., 126, 25 (1923). (7) Hendrixson, J . A m . Chem. Soc., 37, 2352 (1915). (8) Hulet and Duschak, Z . anorg Chem., 40, 196 (1904).
An Improved Weight Buret' Harold B. F r i e d m a n arid Victor K. LaMer DEPARTMENT O F CHEMISTRY, COLCMBIA UNIVERSITY, h-zw YORK,hr. Y
AVIKG had frequent occasion to use a weight buret, the writers have developed the modification here described. It has the following advantages over the usual (Ripper) type with vertical protruding stopcock as in the early type of volume buret (Bur. Standards, Reprint 92) : (1) A horizontal stopcock like that in the ordinary volume buret, which makes manipulation and cleaning easier. (2) The guard which fits over the tip to prevent evaporation and mechanical loss while weighing is held by a ground joint to a fixed part of the buret, as contrasted with the attachment to the movable plug in the older type. This materially lessens the risk of breakage inherent in the older form. Thus, if the guard were not put on firmly it was likely to fall off and if pushed on too firmly the stopcock plug was likely to be forced out in the same operation, causing disastrous loss. (3) The knobs on the shoulders of the buret permit it t o be suspended vertically by a wire attachment from the balance arm, whereas in the older type it hung askew owing to the projecting stopcock.
A total volume of 60 cc., which is sufficient for three 20-cc. titrations with one filling and yet does not overload a balance I
Received August 16, 1929.
of Chemistry, Columbia University.
Contribution No. 612 from Department
of 100 grams capacity, has been found most convenient. It is desirable to hare the tip as fine as that on a standard volume buret to facilitate the delivery of small droplets, thereby increasing the precision with which the end point mag be determined when using concentrated reagents. It has been our experience that after the operator has become accustomed to carrying out weight titrations, which involves no more skill than weighing to a milligram on an analytical balance, the time consumed is but slightly more than that required by volume burets. The increased precision resulting from the ability to weigh to one more significant figure than it is possible to read a volume buret and the absence of such items as errors in drainage and unavoidable error in reading the meniscus are well worth this slight increase in effort. Furthermore, of course, a weight buret needs no calibration.
