Determination of Active Oxygen in the Presence of Barium and Lead MICHAEL FLEISCHER, Geological Survey, U. S. Department of the Interior, Washington, D. C.
D
URING the past two years, the Chemical Laboratory of the Geological Survey has made a number of complete analyses of manganese minerals, in connection with studies of ores of manganese, a strategic metal. One of the determinations required is the percentage of "active" or "available" oxygen. This is usually easily made by adding to a weighed sample an excess of standard oxalic acid or sodium oxalate solution plus a little sulfuric acid, heating until solution of the mineral is complete, and back-titrating the excess oxalate with potassium permanganate. This method, however, is inapplicable to the analysis of the barium manganate minerals psilomelane and hollandite and the lead manganate mineral coronadite. Psilomelane is of very common occurrence, and the other two minerals are occasionally encountered. When these minerals are heated with oxalic and sulfuric acids, the precipitated barium or lead sulfate coats undissolved particles of the mineral and prevents them from reacting, with consequent low results for active oxygen. For the same reason, this method cannot be used for the determination of active oxygen in lead dioxide (PbOz), in red lead (Pb304),or in barium or strontium peroxides. Several methods have been proposed for this determination. Four of these were studied in 1937 by Mrgudich and Clark ( I ) , whose results may be summarized briefly:
the titration may be made with permanganate. There is no loss in accuracy or in speed, and no special technique is required.
1. The Lux method, involving solution in oxalic acid and nitric acid and the back-titration with permanganate, was found to be inarcur ite because of a side reaction between nitric acid and oxalic acid. 2. The Schaeffer method, involving solution in hydrogen pproxide and nitric acid and back-titration with permanganate, was found t o be inaccurate. d. The modified Bunsen method, involving treatment with hot hydrochloric acid, absorption of the evolved chlorine in potassium iodide solution, and titration of the liberated iodine with thiosulfate, was found to be slow and cumbersome and to require special precautions to obtain complete absorption of chlorine. 4. The modified Iliehl-Topf method, involving solution in acetic acid buffered with sodium acetate in the presence of excess potassium iodide, and titration of the liberated iodine with thiosulfate, was found to be accurate and fairly rapid.
Experimental
TABLE I. BLANKRUNS 0.0603,5 N
NO.
Oxalic Acid
MI. 15 18 13 25 22 4
50 50 50 50 50
.
50
HISOP Concn. voz. 7 0 5 5
20 30 30 30
Treatment
Time
Min. 800 Boiled Boiled 800
Boiled Boiled
0.0797N
KMnOd MI.
0a
37.84 37.85 37.82 37.88 36.92 36.76
45 45 45 0" 45 45
37.83 37.86 37.80 37.87 37.82 37.48 37.83 37.84
45 45 0' 45 45
HClOi Conon. 11 9 12 21 14 17 26 27
50 50 50 50 50 50 50 50
5 20 25 30 30 30 50 50
Boiled Boiled BoilEd 80 Boiled Boiled Steam bath Steam bath
60
45
aHeated t o 80' and immediately titrated.
A series of blanks was run with 50 ml. of 0.06035 N oxalic acid and varying amounts of sulfuric and perchloric acid in order to determine whether any side reaction occurred which consumed oxalic acid. Typical results, given in Table I, show that in sulfuric acid solution of moderate concentration there is no side reaction, but that some reaction which consumes oxalic acid occurs when 30 per cent sulfuric acid is boiled with oxalic acid for 45 minutes. This suggests the precaution of not allowing solutions to concentrate too far while the sample is being dissolved. I n perchloric acid solution, no appreciable side reaction occurred in acid of moderate concentration. When 30 per cent solutions of perchloric acid vere boiled with oxalic acid, the titer was usually unaffected, but sometimes, as indicated, it was appreciably lower than the normal titer, as though some side reaction had occurred. It will be noted that 50 per cent solutions, heated a t about 95' in the steam bath, showed normal titer. This is in agreement with the results of Mrgudich and Clark. The small amount of side reaction which perhaps occurred in 30 per cent acid may be due to the higher temperature during the heating, about 110" compared to 95" for the 50 per cent solution. Table I1 shows some analyses by different procedures and the time required for complete solution. It is evident that neither speed nor accuracy is lost by the modifications in procedure suggested here; in fact, the 50 per cent acid seemed to require a little longer for complete solution of the mineral, perhaps because the solutions heated in the steam bath were relatively quiescent and less well mixed than the boiling solutions of lower acid concentration. Mrgudich and Clark made comparative analyses of the same lead dioxide sample by different methods, but, as the purity of lead dioxide cannot be determined, they presented no data to show that the values obtained are the true figures.
The first three methods were accordingly discarded from consideration. The Diehl-Topf method, though apparently excellent for red lead, is unsuitable for the determination of active oxygen in manganese minerals, because these usually contain ferric iron which reacts with potassium iodide, liberating iodine. Not only do many manganese ores contain admixed ferric oxides, but the manganese minerals themselves often contain appreciable amounts (up to 11 per cent) of ferric oxide. Mrgudich and Clark suggested a fifth method, involving solution in oxalic acid plus concentrated perchloric acid (50 per cent), dilution, and back-titration electrometrically with ceric sulfate. They obtained reproducible and apparently accurate results in this way. It seemed worth while, however, to attempt to modify their method, particularly with respect to two features: (1) The amount of acid used by Mrgudich and Clark (25 ml. of 72 per cent perchloric acid) seemed to be excessive; (2) electrometric titration with ceric sulfate happened to be inconvenient. As shown below, it has been found that the amount of perchloric acid used may be diminished to less than one third the amount recommended by Mrgudich and Clark, and that 31
INDUSTRIAL AND ENGINEERING CHEMISTRY
32
perchloric acid. Titration by potassium permanganate may be substituted for electrometric titration with ceric sulfate.
TABLE 11. ANALYSES Material
Acid Used
Gram
%
Time for Solution
Active Oxygen
Literature Cited
%
Min. 55 50 48 65 12
6.18 6.16 6.17 6.05 2.06 2.05 2.07 15.31 15.26 15.29 15.33
10 12 30 Overnight 25 40
G. L., Im. ENG.CHEM.,ANAL. ED., 9,256 (1937). (2) Richmond, W. E., and Fleisoher, M.,Am. Mineral., 27, 607 (1) Mrgudich, J. N., and Clark,
(1942). PUaLrsHmn by permission of the Director, Geological Survey, U . S. Department of the Interior.
TABLE 111. DUPLICATE ANALYSES
New Reagents for
Active Oxygen
5% HrSO4 Cryptomelane (a) Urucum, Brazil Philipsburg, Mont. Deming N. Mex Sugar Sbck, Ark.' Pyrolusite, Lake Valley, N. Mex. Pyrolusite. Cuba Rancieite, Cuba
Vol. 15, No. I
5% HC104
%
%
14.52 16.07 15.92 14.97 17.89 16.32 13.80
14.56 15.99 15.94 15.12 17.86 16.30 13.82
The last of the three samples in Table I1 contained no lead and only 0.13 per cent barium oxide, so that the determination could be made using sulfuric acid. The results with sulfuric acid and perchloric acid are in excellent agreement. A number of similar samples have been run in duplicate, using sulfuric acid for one and perchloric acid for the other, with the results shown in Table 111. It is evident that the perchloric acid method gives correct answers for these samples and it is to be presumed that the values obtained on material containing lead or barium are also correct. It seemed probable that phosphoric acid could also be substituted for sulfuric acid in this determination. Experiments showed, however, that the c. P. phosphoric acid available gave fading and rather uncertain end points, presumably caused by the presence of some impurity. A few determinations made with 5 per cent phosphoric acid gave results in approximate agreement with those obtained using perchloric acid. KOattempt rTas made to purify the phosphoric acid.
Procedure Recommended To an appropriate weight of sample (0.15 gram of barium or lead manganate, 0.3 gram of lead dioxide or 0.7 gram of red lead), which is preferably ground to 200-mesh, add from a pipet, which need not be calibrated, 50 ml. of approximately 0.05 N oxalic acid solution, 8 ml. of 60 per cent perchloric acid, and 42 ml. of water. Place a short-stemmed funnel in the mouth of the flask to act as a condenser, and boil gently until solution is complete (usually about 20 minutes, but as long as 90 minutes may be required for resistant minerals such as hollandite). Add water from time to time if necessary to prevent the volume from diminishing to less than 50 ml. Dilute to 100 ml. and titrate directly a t 80' with approximately 0.07 N potassium permanganate. The blank experiment to determine the permanganate equivalent of the oxalic acid needs only to be heated to 80' and need not be boiled as long as the actual determination, since there is no loss of titer. Standard sodium oxalate solution or a weighed amount of sodium oxalate may be substituted for the oxalic acid solution, but is less convenient to use. Some samples may contain dark insoluble material. In such cases, the completeness of solution of the mineral containing active oxygen may be judged by noting whether bubbles of oxygen come off the undissolved particles. I t is convenient a t times to place the flask covered with a watch glass on the steam bath overnight, Solution is complete and the results obtained are accurate.
Summary The method of Mrgudich and Clark is modified by substituting 5 per cent (by volume) perchloric acid for 50 per cent
Sodium EARLE R. CALEY' AND LOCKHART B. ROGERS2 Princeton University, Princeton, N. J.
S
ODIUM reagents consisting of uranyl acetate and certain divalent metal acetates in dilute acetic acid solution have come into extensive use for the detection and determination of sodium. Unfortunately, all such reagents now in use are more or less sensitive toward lithium, so that they are not satisfactory for the detection or determination of sodium in the presence of appreciable quantities of lithium. An indication was found, however, in an investigation by Caley and Baker (1) that a sodium reagent containing uranyl acetate and cupric acetate is less sensitive toward lithium than other reagents of this general type. The value of such a reagent for the detection and determination of sodium in the presence of lithium was the principal subject of the present investigati on.
Aqueous Cupric Acetate-Uranyl Acetate Reagent By means of tests with a series of trial reagents in which the concentrations of the components were systematically varied, it mas found that a reagent of the following composition gave the most satisfactory results: Uranyl acetate dihydrate Cuprio acetate monohydrate Glacial acetic acid Water
88 grama 88 grams 60 ml. To 1000 ml.
The salts are dissolved in the acetic acid and nearly all the necessary water at a temperature of 50" to 60" C., after which the solution is cooled to room temperature, adjusted to final volume with water, and allowed t o stand a day. The solution is then maintained at 20' C. while being stirred vigorously for about 2 hours with a mechanical stirrer, and is next filtered through a dry filter to remove the small amounts of precipitated salts. The reagent prepared in this way is stable. The sensitivity of this reagent toward sodium and lithium
is indicated in Table I. Though it is evidently insensitive toward lithium, it is unfortunately also not very sensitive toward sodium. Experiments with the trial reagents showed that the sensitivity toward lithium can be further reduced by decreasing the concentration of uranyl acetate in the reagent, but unfortunately this is paralleled by a decrease in sensitivity toward sodium. An aqueous reagent of this type is obviously not satisfactory for the accurate quantitative determination of sodium, however usefuI it may be for the qualitative detection of sodium in the presence of considerable lithium. 1
2
Present address, Wallace Laboratories, New Brunswick, N. J. Present address, Department of Chemistry, Stanford University. Calif.
