The Tin-Phosphorus Precipitate in Bronze Analysis OWEN GATES AND LOUIS SILVERRIAN, 1919 Murray Ave., Pittsburgh, Penna. and add 6 CC. in excess. Add 2 CC. of concentrated hydrochloric acid, heat to solution, and precipitate the phosphorus with molybdate (6).
T HAS long been known (3, 5 ) that an insoluble precipitate of the oxides of tin and phosphorus forms in nitric acid
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solution. This paper reports a study of the tin-phosphorus precipitate and its use in bronze analysis.
While phosphorus completely removed the tin from solution as phosphate, the reverse is not true. For example, Bureau of Nature of Precipitate Standards No. 63, which contains 9.91 per cent of tin, removed 0.58 per cent of phosphorus from solution, but left The nature of the precipitate was first determined by the 0.04 per cent in the filtrate (No. 14). The analytical method procedure given below. Table I, Nos. 1 to 8, shows results (1) of the American Society for Testing Materials for gun using tin oxide obtained from an alloy of tin, copper, and zinc metal suggests the empirical factor of "two thirds" as the amount of the phosphorus which will probably be found in the tin precipitate, if TABLEI. PHOSPHORUS IN PRECIPITATE Tin Approximate Weight of P2Os in phos horus the bronze originally contained less than 0.2 X o . Oxide Phosphorus Added Oxides Oxides in 8xides per cent of phosphorus. The authors' results Gram Gram Gram Gram Gram Gram Gram Gram show that about 90 per cent of the phosphorus None None 1 0.0123 None 0.0123 2 0.0123 0.0054 0.0173 o.0170 o.oo50 0.0047 o.0022 0.0021 will be found in the precipitate (Nos. 9 to 14, 0.0108 0.0180 0.0181 0.0058 0.0058 0.0025 0.0025 3 0.0123 0.0058 0.0058 0.0025 0.0025 'I) when the tin is per cent Or more* 4 0.0123 0.0162 0.0180 0.0181 5 0,0250 0.0162 0.0361 0.0366 0.0101 0.0115 0.0044 0.0050 When a bronze containing up to 2 per cent of 6 0.0250 0.0216 0.0345 0.0362 0.0095 0.0112 0.0041 0.0049 tin and phosphorus is to be determined 0.0260 0,0364 . . . . 0.0114 . . . . 0.0050 . . . . 7 0,0250 8 0,0250 0.0314 0,0364 . . . . 0,0114 . . . . 0.0050 .... gravimetrically, the analyst has the choice of using an empirical factor for the phosphorus; of determining- the phosphorus in the precipitate by the nitric acid method of attack. A series of samples was exactly, as by peroxide fusion; of determining the tin, usually run in which known amounts of phosphoric acid were added. volumetrically; or of precipitating stannyl phosphate, factor From 1 gram of bronze 0.0123 gram of tin oxide was obtained. 0.536. Precipitation as stannyl phosphate has the advantage of speed and accuracy in routine analysis. Addition of phosphoric acid to the samples (Nos. 2 to 4) increased the weight of precipitates to a constant value, 0.0181 gram. Even though as much as 0.0162 gram of phosphorus was present, not more than 0.0181 gram of precipitate, conTABLE11. PHOSPHORUS IN PRECIPITATE taining only 0.0025 gram of phosphorus, could be obtained. Weight of Phosphorus in Phos horus No. Total Oxides Precipitste in &rate With a pure tin oxide weighing about 0.0250 gram (Nos. 5 Gram Qram % Gram to 8) samples treated as above gave increasing weights until 9 0.1535 0.0015 94 0.0001 0.0364 gram of oxide was obtained. Though as much as 10 0.0017 90 .... 0.0002 11 0.0011 0.1273 92 0.0001 0.0314 gram of phosphorus was present, not more than 0.0050 12 0,0037 0 1074 90 0,0004 0.0050 0.1147 13 93 0.0004 gram could be found in the precipitate. 14 0.1491 0.0058 94 0.0004 Thus a precipitate of tin oxide and phosphoric oxide is easily obtained in constant proportions. This compound is TABLE111. TIN stannyl phosphate, 2(Sn02).PzOb or (SnO)zPzO7, containing Phosphorus 53.6 per cent of tin. No. Tin Taken4 Present Tin Foundb Table I explains statements in the literature that the tinGram Gram Gram phosphorus precipitate is of variable composition. For ex15 0.0200 0.0125 0.0198 16 0.0200 0.0250 0.0210, 0,0210 ample, in Nos. 5 and 6, although more than the necessary 17 0,0200 0.0370 0.0204, 0.0205 18 0.0200 0.0600 0.0217 0.0050 gram of phosphoric acid was present, the weight of a Identical Sam les, 2% Sn, 6.6% Pb. precipitate varied from 0.0351 to 0.0364 gram, and contained b As (SnO)a.P&. 0.0044 to 0.0050 gram of phosphorus. This means that a precipitate of constant proportions will not be found (under the conditions of this procedure) unless there is present a Discussion minimum of 10 mg. of phosphorus for each 10 mg. of tin, and It is known that iron prevents the quantitative separation 20 mg. of phosphorus for 20 mg. of tin in the bronze to be of tin as oxide in the routine nitric acid method for bronze. analyzed. Accordingly, a bronze containing 3 per cent of iron and 8 per Procedure cent of aluminum was mixed with weighed amounts of tin metal. After the usual treatment with nitric and phosphoric Place 1 ram of bronze in a 50-cc. beaker (containing the phosphoric acij) and add 7 cc. of concentrated nitric acid. Allow the acids, it was found that tin could be quantitatively recovered, mixture to digest on a steam bath for about 20 minutes, then not as stannyl phosphate, but as the mixed oxides. On the dilute to about 50 cc. with hot water. After about 1.5 hours filter other hand, a sample such as Bureau of Standards No. 62 off the tin-phosphorus oxide on a 9-cm. No. 40 Whatman paper, (0.82 per cent tin and 1.13 per cent iron) gave recoveries containing paper pulp, wash with a hot (1 99 cc.) nitric acid solution, place the aper in a crucible, and dry on a hot plate. which were 0.05 to 0.10 per cent high in tin if calculated as Completely burn the carbon at about 600" C. and ignite at stannyl phosphate. Difficulty was also encountered with a about 800* C. sample high in lead (Nos. 15 to 18, Table 111). It is believed To determine phosphorus in the tin oxide precipitate, fuse with that such errors can be rectified. Work must also be done on sodium peroxide, leach out with water, acidify with nitric acid, ~
+
OH
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ANALYTICAL EDITION
JULY 15, 1939
the separation of tin and antimony (6, 6) on the possible interference of other metals with tin and on the effect of phosphate in the subsequent analysis of the other elements present. Lastly, bronzes of higher tin content seem to settle so rapidly after nitric-phosphoric acid treatment that sufficient phosphoric oxide is not withdrawn from solution.
Summary A study of the tin-phosphorus precipitate in the analysis of bronze has shown that the amount of phosphorus present must equal in weight the tin present t o get a precipitate of constant stannic oxide-phosphorus pentoxide ratio. Whereas
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phosphorus will remove all the tin from solution, the reverse is not true.
Literature Cited Testing Materials, B28-36T (1936). (2) Bornemann, Z. angew. Chem., 12, 635 (1899). (3) Friend, J. A. N., “Textbook of Inorganic Chemistry”, Vol. 5, p. 360, London, Griffon and Co., 1917. (4) Lundell, Hoffman, and Bright, “Chemical Analysis of Iron and Steel”, p. 212, New York, John Wiley & Sons, 1931. (5) Mellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry”, Vol. VII, p. 482, New York, Longmans, Green & Co., 1927. (6) Prescott, A. B., and Johnson, 0. C., “Qualitative Chemical Analysis”, New York, D. Van Nostrand Co., 1917. (1) Am. Soc.
A Photometric Method for the Determination of Carbon Dioxide RICHARD J. WINZLER’
AND
J. PERCY BAUMBERGER, Stanford University, Calif.
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X MANY chemical and biological studies it is convenient to make very frequent determination of the carbon dioxide content of gases. This paper describes a simple photometric method for the rapid determination of carbon dioxide by measuring the change in light transmission of a solution of pH indicator through which the gas is bubbled. Only one gas of known carbon dioxide content is reauired in order to be able to apply a simple equation to the determination of the carbon dioxide content of gases of unknown composition. Several workers-e. g., Parker (7) and Fegler and Modze(@-have used p H indicators to determine carbon ,dioxide. Most carbon dioxide methods make use of basic 1
Present address, Sterling Hall of Medicine, S e w Haven, Conn.
FIGURE 1. DIAGRAM OF APPARATUS
solutions and the consequent accumulation of carbonate in the solution. The present method depends, as did that of Higgins and Marriott (4), upon the equilibrium established between the indicator solution and the gas to be measured. The use of this method in closed and open circuit indirect calorimetry will be described in other papers.
Apparatus and Method Methyl red is the pH indicator used in this method. Its pKa is 5.1, being red on the acid and yellow on the alkaline side of this pH. A stock solution is prepared by neutralizing the acid indicator with sodium hydroxide, and dilutions are made from this stock solution to final concentrations ranging from 0.22 X to 1.5 X gram of methyl red per liter. To these are added a few drops of dilute sodium or potassium hydroxide to bring the excess alkali concentration to something in the neighborhood of to 10-3 molar. These dilutions are made up in liter flasks and calibrated as described below. They were found to remain unaltered over several months. Soft-glass cuvettes with depths ranging from 10 to 25 mm. are used in the apparatus. The arrangement of apparatus is shown in Figure 1. The photometric colorimeter consists of a wooden box with light-tight hinged lid. Light from a flashlight bulb, L, operated at 4 volts from a storage battery illuminates the entire sensitive area of a General Electric blocking layer photoelectric cell, A . Between the light source and the photoelectric cell are suitable light filters (Wratten No. 74 and Corning No. 397) and a rack bearing two cuvettes which may be moved into the light path. One cuvette, S , is a reference standard and contains methyl red in alkaline solution. The other, T , contains methyl red in very dilute bicarbonate solution and is equipped with a bubbling arrangement to permit passage of the unknown gas through it. The solutions are moved into position before the filters by means of the slider, H . The practice is to adjust the position of the standard cuvette by means of set screws, ss, so that the photoelectric current developed by the light passing through either of the two vessels is the same when the test solution is brought into equilibrium with gas containing no carbon dioxide. The ratio Io/I when no carbon dioxide is present, then, is unity.
