ANALYTICAL EDITION
August, 1945
treated special graphite electrode, which is dried at 110" C. for approximately one hour. The spectrograph is adjusted to the 2600 to 3600 A. range, Eastman Kodak process plates are used, and two No. 2 cover slips are placed between the slit and the light source. The prepared electrode is used &s the lower electrode and a pointed electrode having a tip 1.2 mm. in diameter as the upper electrode. Samples are burned to completion, using 11 amperes direct current, and analyses are made in triplicate. After exposures are completed, the plates are developed for 2 minutes at 18' C. in D-19 developer. The cobalt line having a wave length of 3453.5 b. is used &s the internal standard line. Calculations for the various elements are established on the following lines: Fe Na Ca A1
3306.4A. 3302.3 A. 3179.3 A. 3082.2 A.
Concentrations are evaluated from the working curves (Figure 1) established by the use of the series of synthetic solutions. Sodium and calcium are calculated as chlorides, while iron and aluminum are calculated as oxides. ANALYSIS FOR POTASSIUM. To secure sufficient sensitivity for the analysis of potassium, the spectrograph i s adjusted to photograph the 5000 to 10,000 A. range. To secure satisfactory line density levels in this region, and to obtain suitable sampling, a Wratten Filter No. 57 is placed in front of the slit. Eastman Kodak IV N plates used for the analysis of potassium are sensitized for the wavelength region used by placing in '4% ammonium hyroxide for one minute. Strontium is a suitable internal standard for this determination. To 5 ml. of the original recovery acid sample are added 5 ml. of a solutioii containing 29.7 grams of strontium chloride per liter. These are rapidly mixed, and one drop of the mixture is immediately placed on a graphite electrode previously treated with
49 1
kerosene. The procedure from this point is similar to that outlined previously, except that a 15-ampere direct current arc is used to increase the precision. Either K 7664.9 1.or K 7698 A. may be used for the determination of potassium and Sr 7070.1 A. is used as the internal standard line. Potassium is calculated as the chloride. RESULTS AND DISCUSSION
A series of recovery acid solutions was analyzed spectrographically, and the results were compared with those of routine chemical methods, as shown in Table I. Spectrographically, the iron and aluminum oxide values obtained were converted t o R,O* in order to show comparisons with the chemical results in which RIOl was determined as the hydroxides. Qualitative spectrographic analysis of a number of these precipitates showed the presence of appreciable proportions of titanium, copper. and manganese. By slit-height adjustment and camera movement it is possible to secure triplicate exposures of 10 samples and one standard on each plate. The total time necessary for the analysis of 10 samples for sodium, potassium, iron, aluminum, and calcium should not exceed 7 man-hours. LITERATURE CITED
Breckpot, R.,Chimie & Industrie,Special No., 220-9 (1934). (2) Cholak, J., and Story, R. V., IND.ENQ.CHEM.,ANAL.ED., 10,
(1)
619-22 (1938).
(3) Eastman Kodak Co., Rochester, N. Y.. "Wratten Light Filters", 1940. (4) (5) (6)
Hasler, M. F., J. Optical SOC.Am., 31, 140-5 (1941). Owens, J. S., IND.ENO.CHEM., ANAL.ED.,10, 64-7 (1938). Ramsey, R. H.,Eng. Mining J., 144, No. 10, 61-7 (1943).
Optical and X-Ray Diffraction Studies of Certain Calcium Phosphates WILLIAM F. BALE, JOHN F. BONNER, AND HAROLD C. HODGE, The University of Rochester, School of Medicine and Dentistry, Rochester, N. Y., AND HOWARD ADLER, A. R. WREATH, AND RUSSELL BELL, Victor Chemical Works, Chicago, 111. The x-ray diffraction data, the melting points, the crystallographic systems, and the indices of refraction of 11 calcium phosphates are reported. These 11 calcium phosphates are divided into four groups: a primary calcium phosphate and 3 derivatives, a secondary calcium phosphate and 3 derivatives, P tertiary calcium phosphates, and hydroxylapatite.
M
fraction. All the compounds shown by fusion studies to exist in the CaO-P20s system except tetracalcium phosphate are given in Table I. PROCEDURES
In Table I1 are given the x-ray powder diffraction data for the 11 substances. These data are presented in the same way as the comprehensive data of Hanawalt, Rinn, and Frevel (Y)-i.e., spacing measurements and relative intensities are given with additional notations for the index lines. The procedures for determining the powder diffraction data in Table I1 are given in detail elsewhere (9).
ICKOSCOPIC characteristics and x-ray diffraction patterns of some of the calcium phosphates have been reported, but none of the lists is complete, even of the more commonly manufactured compounds. This paper attempts to present new data to fill some of the gaps in the knowledge of these important compounds (18). In 1938,lO diffraction patterns were published (9),which were taken as repTable 1. Data on Eleven Calcium Phosphates resentative of 10 calcium phosphates. Melting BireTh(1identity of these compounds was Diffraction P$t. fringRefractive Indices Pattern Type ( 9 ) C. System ence No,N ,N o Nm,"! IVS Formula sought by chemical and optical d. 153 Triclinic 1. I primary 1.528 analyses and x-ray spectrography. 1.518 1,501 2. d. 268 Triclinic I1 primary 1.590 1.564 1.552 The diffraction patterns have been 3. 111 primary .1000 . . Amo;phous 1.578 1.518 Ignited primary 4. 1.540 associated with formulas, and by the Tetragonal 1.588' 1.5955 5. CaHPO4.2HL> I11 secondary ... Monoclinic 1.551 present work eleven compounds have 1:5i5 1.540 Triclinic 6. CaHPO4 I secondary 1.63 1.61 1.60 now been identified. They are pre1.6285 7. R-CaxPz07 Ignited secondary t. 'ii70 Tetragonala ... 1,624'' 1350 8. a-CalPzOr ... 1.605 1.60 1.585 sented by formulas in Table I, in t. 1350 9. B-CaaPz01 1gnited't'e;tiary ... 1.620a ... 1.622" 1720 10. a-Car PzOs 1.588" which the following data are also .. 1 .591° 1540 Hexagdnal 11. Ca~a(POds(0Hh Hydroxzapatite 1.610 .. ... given: the x-ray diffraction pattern d. Decomposition temperature. types as published in 1938, the meltt. Transition temperature. ing points, the crystallographic Values reported from the literature. syst,cms, and the indices of re-
INDUSTRIAL AND ENGINEERING CHEMISTRY
492
Table 1I.d
Vol. 17, No. 8
Interplanar Distance and Relative Intensities of Lines in the X-Ray Diffraction Patterns of Eleven Calciulrr Phosphates (Index linea indicated in each case)
Position,
Relative Intensity
11.8 5.9 5.7 4.9 4.4 4.2 3.9 3.7 3.37 3.18 2.96 2.82 2.68 2.58 2.45 2.40 2.15 2.09 2.00 1.93 1.84 1.79 1.75 1.70 1.66 1.57 1.53 1.47 1.42 1.40 1.37
b
A.
Position,
A.
Relative Intensity
Position,
A.
Relative Inteasity
1. Ca(HsP0tb. Hi0
Index Linea
Hanawalt 220. A.8.T.M.. iimj
... ...
0.10 0.08 0.08 1.00 0.35 0.18 0.23 0.32 0.30 0.10 0.33 0.18 0.20 0.20 0.30 0.05 0.05 0.18 0.25 0.15 0.22 0.08 0.17 0.05 0.10
4.45 3.76 3.35 3.18 2.70 2.50 2.32 2.21 2.05 1.93 1.84 1.72 1.66 1.585 1.48 1.39 1.365 1.335 1.295 1.255 1.205
0.15 0.70 1.00 0.80 0.16 0.12 0.12 0.22 0.10 0.18 0.11 0.18 0.10 0.20 0.12 0.10 0.08 0.10 0.07 0.04 0.07
0.10 0.11 0.11 0.10 0.10 0.10 0.10 0.05 0.04
1.48 1.44 1.41 1.37 1.35 1.275 1.220 1.180 1.140 Index Linea 3.63 3.48 2.58 3.05
6.1 6.6 5.3 4.45 4.00 3.55 3.40 3.20 3.00 2.83 2.50 2.44 2.30 2.02 1.93 1.87 1.77 1.69 1.65 1.58 1.50 1.47 1.41 1.34 1.320 1.280 1.270 1.210 1.180
1.00 0.35 0.33 0.32
7.7 4.20 3.35 3.02 2.90 2.82 2.60 2.42 2.26 2.15 2.07 1.98 1.86 1.80 1.70 1.60 1.54 1.51 1.44
a
0.05 0.05 0.05 0.06 0.06 1.00 0.06 0.06 0.06 0.60 0.15 0.06 0.17 0.20 0.05 0.25 0.20 0.04 0.17 0.20 0.10 0.16 0.04 0.04 0.04 0.10 0.17 0.06 0.06
Index Lines 3.55 2.83 1.87 2.02
1.00 0.60 0.25 0.20
... 0.80
0.10’ 0.80 0.80 0.10 1.00 0.30 0.10 0.30 0.25 0.25 0.40 0.40 0.20
CaHzPrOr
Index Lines 3.35 3.18 3.76 2.21
1.00 0.80 0.70
Index Linea 2.60 4.20 3.20 2.90
0.10,
0.20 0.20 0.20
Hanawalt, 219; .S.T.M., 1293)
3.
1.00 0.80 0.80 0.80
(Hanawalt, 218. A.S.T.M., 888)’
6. CaHPOd 6.89 3.48 3.35 3.11 2.95 2.74 2.62 2.50 2.23 1.98 1.91 1.84 1.78 1.71 1.67 1.64 1.60 1.56 1.53 1.50 1.45 1.41 1.36 1.31
...
...
1.00 0.22 0.93 0.72 0.12 0.24 0.27 0.06 0.10 0.22 0.06 0.22 0.06 0.06 0.05 0.06 0.07 0.04 0.06 0.05 0.07 0.04
Index Lines 3.35 2.95 2.74d 2.23
1.00 0.93 0.72 0.27
Hanawalt 217. A.S.T.M., i617j
0.22
a The data in this table have been compared with certain published diffraction data. The atterns of (1) Ca(HaPOdz.Hr0, 2) Ca(HaPOda, CaHP04.2Hz0, a n s (6) CaHPO4 were corn ared with t6ose of Hill and endricka (8). The correspondence is goo$ The patterns of (9) 8CarPpOg (10) a-Ca,PzOg, and 11) hydroxyapatite were compared w t h those given b; Bredi Franck andbuldner (4). Although many more lines were reported for a-8a*PnOg than, are given here (Table 11. lo), the authprs’ lines correspond with the moreintense linea in their pattern. The diffraction pattern of the material designated as “a-phase (oxyapatit)” corresponds to (1 1) hydroxyapatite.
g)
Relative Intensity
5.
9. Ca(HaP0th 7.1 6.4 4.35 4.05 3.87 a.63 3.48 3.36 3.19 8.05 2.95 2.84 2.58 2.33 2.25 2.15 2.13 2.05 2.00 1.93 1.83 1.77 1.72 1.67 1.60 1.57 1.52
A.
4. Ca(PO$
... ...
0.26 0.09 0.05 1.00 0.92 0.11 0.23 0.37 0.07 0.33 0.19 0. I9 0.19 0.08 0.04 0.13 0.15 0.02 0.07 0.03 0.04 0.03 0.03 0.04 0.03 0.05 0.05 0.02
Position,
b Intensity not measured. A.S.T.M. gives this and 3.69 A. lines intensit values of 0.75 with the 3.88 k. line as 1.00. TEis sample of dicalc+n phosphate probably contained a very small percentage of anhydrous dicalcium phosphate, which accounts for the line a t 3.35 A. with a relative intensity of 0.10. d,The 2.74 A. line was resolved,into 2 weaker lines a t 2.76 and 2.72 A. having approximately equal intensities.
(Table continued on next pane)
ANALYTICAL EDITION
August, 1945 Table
II (Contd.).
493
Interplanar Distance and Relative Intensities of Lines in the X-Ray Diffraction Patterns of Eleven Calcium Phosphates (Index linea indicated in each ease)
Position,
A.
Relative Intensity
Position,
Relative Intensity
A.
Position,
Relative Intensity
A.
Position.
A.
Relative Intensity
7. B-CaIPIO1 d . 38 3.23 3.10 3.02 2.92 2.80 2.76 2.66 2.57 2.40 2.33 2.23 2.10 2.00 1.95 1.85 1.77 I . 73 1.68 1.62 I 57 1.53 I .49 1.46 1.42 1.39 1.36 1.33 1.30
Index Lines
0.48 0.29 0.31 1.00
0.26 0.50 0.50 0.13 0.43 0.13 0.21 0 39 0.36 0.40 0.40 0.43 0.26 0 13 0.19 0.36 0.26 0.31 0.17 0.21 0.10 0.10 0.04 0.09 0 17
2.90 2.61 3.92 3.70
1.00 0.67 0.60 0.19
1.433 1.310 1.270 1.250 1.230 1.148 I . 110
0.08 0.05 0.05
11
Index Lines'
3.02 2.80 2.76 3.38
0.08
0.08 0.07 0.12
Index Linea
1.00 0.50 0.50 0.48
2.79 2.72 2.63 1.84
1.00 0.62 0.31 0.31
Hanawalt 222. L.S.T.M.. i454j
8. a-Ca*P*O, 7.14 4.94 4.30 3.80 3.55 3.34 3.24 3.10 2.79 2.68 2.61
...
2.12 2.06 2.00
0.13 0.08 0.24 0.35 1.00 0.71 0.62 0.24 0.34 0.32
0.37 0.24 0.77 Index Lines
3.336 2.000 3.242 3.100
1.00
Hanawalt 226 pattern (A.S.T.M. 16421) corieaponds to a mixture of 80% O-CazPz01 and 20% o-CazPzO7 (6).
0.77 0.71 0.52
9. 8.3 6.5 5.4 4.05 3.40 3.18 2.89 2.78 2.70 2.60 2.52 2.40 2.26 2.18 2.06 2.02 1.93 1.88 1,82 1.78 1,72 1.68 1.63 1.59 1.55 1.51 1.46 1.44 1.41 1.38
.. .
I
.
0.32 0.32 0.43 0.65 1.00 0.43 0.25 0.82 0.29 0.21 0.21 0.18 0.18 0.18 0.32 0.32 0.18 0.18 0.36 0.14 0.14 0.14 0.25 0.11 0.14 0.11 0.14 0.14
INTERPLANAR DISTANCE 54
325
2
I.7 15
13 I ?
Index Lines
2.89 2.60 3.18 3~ 4 0
1.00 0.82 0.65 0.43
(Hanawalt, 221' A.s.T.M., 2301;
The microdensitometer tracings (Figure 1) were obtained from powder diffraction photographs made in a General Electric Type XRD diffraction apparatus. The sample w m held in an oscillating wedge. Films were exposed for 12 hours at 30 kvp. and 25 ma. with Cu K - a radiation. The transition and melting points of calcium pyrophosphate were determined in a silicon carbide resistance furnace equipped with a Pt, P e R h thermocouple. As molten calcium phosphates can often supercool several hundred degrees, freezing curves were not used. Instead, samples were heated to a redetermined ! x evidence of temperature for several hours and then examined t fusion. An Arsem vacuum furnace (1) was used for the study of tricalcium phosphates. Temperature of the sample was observed
CaRPOd .2H2O CaHP 0 1 B-CazPaOi a-CarP&
Fi ure 1. Microdensitometer Tracings from Powder 8iffraction Photographs of 11 Calcium Phosphates Interplanar distances In A n s h a m unih en &en a i abrcir~as. O d i ~ t e s a18 in o w l t i e r , sa that v d l c d incnrws mean hisher x-rav intendties
INDUSTRIAL A N D ENGINEERING CHEMISTRY
494
through a mica window with an opt,ical pyrometer. Several intermediate mixtures of di- and tricalcium phosphates were trmtedin each furnace. A smallsilicon carbide resistance furnace with a chromel-alumel thermocouple WM used in the study of hydroxylapatite under various conditions. PRIMARY C A L C I U M PHOSPHATES
Four primary calcium phosphates are described: (1) the hydrate, (2)the anhydrous, (3)the acid pyrophosphate, and (4)the metaphosphate. The analyses of the phosphates used are as follows: CaO
h
1. Ca HsPOSrHiO
2. Ca HrP03: 8. Ca BPI% 4. Ca(P0dr
Found
Theoretical CaO PIO,
PI08
%
%
%
%
22.40 25.0 25.2 29.4
66.35 69.0 62.8 68.0
22.2 23.9 25.9 28.3
66.4 60.7 65.8 71.7
Boull6 haa reported (3)2 crystalline modifications of calciummetaphosphate; A transformed slowly to B at 400' C. and rapidly at higher temperatures. He cited positions but not inhnsities of x-ray diffraction maxima. The authors are unable to tell what, if an , CORndence exists between these diffraction patterns and tKe one authors obtained by heating monocaldum phosphate hydrate to 500' U. The authors are grateful to L. K. Frevel for calling this paper to their attention.
tc
The anhydrous phosphate used was a commercial product which contains some di- and tricalcium phosphates. This material was used because the crystals were well shaped and only occasionally twinned. The indices reported have been checked on crystals prepared in the laboratory by crystalliiing from liquors at high temperatures; the indices were the same. For the hydrate, Larson (11) reported N, = 1.4932; this value is lower than those in Table I. For the metaphosphate, Schneiderhoehn (13) gave the following data: probably rhombic system, b ~ a lnegative; , N , is 1.588,N , = 1.595. Calcium metaphosphate melts a t approximately 1000' C., giving an amorphous glass with an N of 1.542. Of the x-ray diffraction data, pattern 1 is comparable to Hanawait pattern 220, pattern 2 to 219; patterns 3 and 4 are not listed in the Hanawalt key. SECONDARY C A L C I U M PHOSPHATES
Four secondary calcium phosphates are described: (5)the dihydrate, (6)the anhydrous, (7)the p-pyrophosphate, and (8)the a-pyrophosphate, The source of the samples were described previously (2, 9). The analyses of the secondary phosphates used are as follows: CaO
6. 6. 7. 8.
CaHP01.2Hn0 CaHPOa CarP107 (sample 1) CarPaO7 (sample 2)
Found
Theoretical CaO PI08
Paor
%
%
%
%
32.8 40.9 44.2
41.4 53.0 57.0 55.1
32.5 41.4 44.1
41.3 62.2 55.9
13.4
..
..
The indices of the dihydrate are in good agreement with the data for the mineral brushite as repotted in the International Critical Tables (1.551,1.545, and 1.539) and by Larsen and Berman (10). The indices for the anhydrous form agree with those reported by Larsen and Berman for the mineral monetite; however, considerably different indices have also been given for this substance: Winohell (14) International Critical Tables Gaubert (6) Laraen and Berman
No
Nn.
1.63 1.525 1.623 1.631
1.518
1
1 :6i4
Nv 1.61 1.515 1.604 1.600
The agglomerates of very small crystals in the a-pyrophosphate samples make the determination of optical properties difficult.
Vol. 17, No. 8
Schneiderhoehn on Troemel's material (19) gives the following data: @-pyrophosphate, tetragonal, uniaxial, positive, na is 1.624, n, is 1.628; a-pyrophosphate, biaxial, positive, nol is 1.585, n7 is 1.604. Of the x-ray diffraction data, pattern 5 is comparable to HanaWalt pattern 218, pattern 6 to 217, pattern 7 to 226;.there are minor differences in the relative intensities ascribed to the index lines. TERTIARY C A L C I U M PHOSPHATES
Two tertiary phosphates are described: (9) the @-tricalcium phosphate and (10)the a-tricalcium phosphate. The tricalcium phosphate used contained 38.8% cslcium and 20.0% phosphorus; the theoretical values for Ca,P*Oe are 38.7% calcium and 20.0y0 phosphorus. The a-tricalcium phosphate was prepared by h e a t ing a sample of this tricalcium phosphate to a temperature over 1700' C. and cooling rapidly. Because only microscopic crystals of tricalcium phosphatea have ever been obtained, the determina. tion of the optical properties is very difficult. Schneiderhoehn gives the following data: 8-tricalcium phosphate, na is 1.620, n, is 1.622; a-tricalcium phosphate, biaxial, positive, na is 1.588,n, is 1.691. Of the x-ray diffraction data, pattern 9 is comparable to HanaWalt pattern 221 and pattern 10 is not listed in the Hanawalt key. HYDROXYLAPATITE
Some of the hydroxylapatite samples used in this work were comrhercial tricalcium phosphates, others were samples prepared in this laboratory (9). The hydroxylapatite used for optical measurements had the following composition: 36.7% calcium and 17.4% phosphorus; theoretical, 39.8% calcium and 18.5% phosphorus. The calcium-to-phosphorus ratio was 2.11 as eompared to 2.15,the theoretical value. The low calcium and phosphorus percentages are due to water content. Unignited samples appear to be amorphous with N = 1.610. After ignition (below 1200' C.) which has been shown to have no effect on the crystal lattice of precipitated calcium phosphates having this caicium-tophosphorus ratio, the index was 1.636, which agrees fairly well with the intermediate index reported by Winchell for tetracalcium phosphate (Ca4P20a)found in Thomas slags. The x-ray diffraction pattern, No. 11 (Table 11), is comparable to Hanawalt pattern 222, which is there listed as Ca,(P0J2H20. The fact that most commercial tricalcium phosphates are hydroxylapatite was established in 1938. The relation between lattice, calcium-to-phosphorus ratio, and mode of precipitation was also shown clearly a t that time. Only one basic calcium phosphate is known to be precipitated from water; it has the hydroxylapatite lattice more or less evidently developed. Ignition of such a precipitate, if the calcium-to-phosphorus ratio is near the theoretical for tricalcium phosphate, gives either 8- or u-tricalcium phosphate, depending on the conditions of ignition. On the other hand, if the calcium-to-phosphorus ratio is near the theoretical for hydroxylapatite, this lattice is stable on ignition below 1450" C. ACKNOWLEDGMENl
The authors acknowledge the opportunity to use the facilities of the Saranac Laboratories. The Department of Metallurgy of Rensselaer Polytechnic Institute permitted them to use certain equipment. The microdensitometer tracings were made by The Eastman Kodak Company. LITERATURE CITED
(1) Arsem, W.C.,J . Am. Chem. SOC.,28,921 (1906). (2) Bonner, John, thesis, University of Rochester, 1940. (3) Boull6, A.,A d . Sn'., Paris, 202, 1434 (1936). (4) Bredig, Franck, and Fuldner. 2.Elektrochsm.. 38, 158 (1932).
ANALYTICAL EDITION
August, 1945
(6) Frevel. L. K., personal communication. (6) Gaubert, P., Bull, aoc. fraw. mint?ral., 50, 504 (1927).
(7) Hanawalt, J. D., Rinn, H. W., and Frevel, L. K., IND.ENG. CHEM.,ANAL.ED., 10, 457 (1938). (8) Hill and Hendricks, IND. ENQ.CHIM., 28, 440 (1936). (9) Hodge, H. c . , L ~ F M,~ L,, ~ and ~ me, , w. F?, CHEM.,ANAL.ED.,10, 156 (1938). Survey, Bull. 848 H.3 u. s. (10) Larsen, E. and s . 8
(1934).
495
(11) Larson, H. W. E., IND.ENO.CHEM.,ANAL.ED.,7,401 (1935). (12) MacIntire, Winterberg, Marshall, Palmer, and Hatcher, IND. ENO.CHEM.,36,547 (1944); extensive bibliography. (13) Troemel, G., Mitteil. Kaiser-Wilhelm Imt. Eismfmsch. DiLsrrG dwf, 14, 198 (1932). (14) Winchell, A. N., “Microacopic Characters of Artificial Minerds”,New York, John Wiley & Sons, 1931. THISwork wan supported in part by of New York.
Quantitative Absorption
OF
a
grant from the Carnegie Corporation
Oxygen
Critical Factors in the Application of Acid-Chromous Solutions HOSMER W. STONE
AND
EDWIN R. SKAVINSKP
University of California, Lor Angeles, Calif.
Acetic acid solutions of chromous chloride provide an excellent reagent for the quantitative absorption of gaseous oxygen. This paper reporta a study undertaken to clear up controversial literature rtatementq it shows the effect of the concentration of acid hydrogen in the reagent on the evolution of hydrogen gas and sets forth methods of preparation and use that avoid or eliminate factors that have heretofore caused difficulties.
THE
reaction of acidified chromous solutions with molecular oxygen, represented by the equation 4CR++
+ 4H+ + O2 = 2H10 + 4Cr+++
was proposed for oxygen absorption by Pfordten (17) as early as 1885. This reagent interested him particularly because he wished to separate oxygen from hydrogen sulfide and other acid gases. Jannasch and Meyer (16) promptly adopted the reagent for removing oxygen from the nitrogen which they were using in organic combustions and thus the chromous solution took its place as one of the acceptled oxygen absorbents for gas analysis. Difficulties soon developed, with Bertholet ( 4 ) reporting that chromous solutions liberated hydrogen gas when acid hydrogen was present. This reaction is represented by 2Cr++
+ 2H+ = 2Cr+++ + Ht
The evolution of hydrogen gas by this unwelcome side reaction was confirmed in the publications of Doring ( I S ) , Peters (16)) and Dernstadt and Hassler (12). The report of Anderson and Riffe (1) in 1916, that hydrogen was evolved in acid chromous solution even in the presence of an excess of chromous acetate and that the absorption of the oxygen was only 9774 complete, appears to have been particularly discouraging to prospective users of the reagent. Such an array of evidence might have completely disqualified chromous solutions as an analytical reagent, but the report of Asmanow (2) in 1927 that chromous sulfate in sulfuric acid even up to 10 N acid yielded no hydrogen gas revived interest in the matter. A number of papers were published about this time by Zintl (26-28)and co-workers and by Brintzinger ( 7 , 8) and co-workers in which use was made of chromous solutions for potentiometric analytical titrations. Later, Thornton and Sadusk (93) reported that 0.067 N chromous sulfate in’ 0.18 N sulfuric acid
-1
Present address, Southern California Gee Co., Loa Angelae, Calif.
showed no change in titer over aperiod of two months. The United States Steel Corporation (24,using a hydrochloric acid solution of chromous chloride for oxygen in gas analysis, stated “if the excess acid is not great and the temperature is kept below 20” C., the amount of hydrogen liberated is not measurable with an ordinary buret”. A commercial oxygen absorbent for gas analysis which contains chromous chloride in acid solution has been available on the market for a number of years. Stone and Dixon (20) investigated and re orted favorably on the application of a chromous sulfate-suyfuric acid reagent for the absorption of oxygen in gas analysis, prepared by assing a sulfuric acid-chrome alum solution over zinc amalgam. %ranham (6)caIled the attention of these workers to the fact that traces of hydrogen sulfide me liberated by the action of the chromous ion on the sulfate. He indicated that the volume of hydrogen sulfide was not sufficient to be detected in the gas volume measurement but that sulfide formation on the surface of the mercury in the buret made reading of the mercury meniscus difficult. Branham made other important contributions to the chromous situation in pointing out that, under the conditions he used, the error due to the evolution of hydrogen was of the same order as that due to carbon monoxide formation with pyrogallate, and that the heat resultin from the oxygen reaction caused trouble by lowenng the solubiity of the nitrogen carrier gas, remaining 89 residue after each analysis. A consideration of these contradictory or a t least controversial statements found in the literature with regard to the suitability of this reagent for accurate work suggested to the authors that a clear understanding of the important factors might eliminate the difficulties and make available a much more satisfactory reagent for oxygen absorption than had been previously known. This has been accomplished by showing the effect of the concentration of acid hydrogen in the reagent on the evolution of hydrogen gas, and by setting forth methods of preparation and use which avoid or eliminate factors that have given rise to the various objections appearing in the literature. The determination of the apparent percentage of oxygen in the atmospheric air with the acid-chromous reagent in highly precise apparatus provides data which show the reagent to be satisfactory even where high precision is required. APPARATUS
The apparatus used in the investigation is illustrated in Figure 1. The two-bulb type of buret shonm was designed especially for the analysis of air, being similar to the one described by Carpenter (10, 11). This buret allowed the estimation of volumes to 0.002 ml. on a 50-mi. sample, the readings being made to approximately 0.004%. The compensating tube, the buret, and the pipet were immersed in water circulating rapidly from a thermostat, so that the temperature did not vary more than 0.05’. It was found unnecessary to use the thermoregulator for analysee if the order of accuracy desired did not exceed 0.01%. Even
