Procedure for Measurement of P” and P32in Plant Material A. J. MAGKENZIE AND L. A. DEAN Bureau of Plant Industry, Soils and Agricultural Engineering, United States D e p a r t m e n t of Agriculture, Beltsville, ‘Md. A routine quantitative procedure for determining the specific activity of plant material containing radioactive phosphorus has been successfully applied to vegetative samples from greenhouse and field experiments. By comparison of the specific activity of the fertilizer preparation with that of the plants grown on soil receiving this fertilizer, the proportion of the phosphorus contained in the plants derived from the fertilizer may be estimated.
T
H I S paper considers in detail a routine quantitative procedure for determining the specific activity [Paz/(P31 Ps*)] of plant material containing radioactive phosphorus. I t has been successfully applied to a large number of vegetative samples from greenhouse and field experiments designed to study the utilization of phosphorus from fertilizers labeled with P32, A comparison of the specific activity of the fertilizer preparation with that of the plants grown on a soil receiving this fertilizer enables estimation of the proportion of the phosphorus contained in the plants derived from this fertilizer. A considerable degree of precision (about 2 or 3%) in the measurement of the specific activity is a highly desirable prerequisite for experiments of this type. I n initial studies, the specific activities were calculated from direct measurements of the 8-ray activity, with Geiger-Muller counters, of solutions prepared from plant material digested with nitric and perchloric acids. However, the sensitivity and accuracy of such a procedure did not prove satisfactory. Consequently, a method was worked out for precipitation of the phosphorus and its collection in a uniform manner prior to the radioactive measurement. The methods previously suggested for determining S 3 5 ( 3 ) and 90-day arsenic ( 4 ) were helpful guides.
+
PROCEDURE
Samples of plant material containing from 5 to 30 mg. of P 3 I are digested with nitric and perchloric acids to destroy the organic matter and dehydrate the silica. After the silica is filtered off the phosphorus is precipitated as ammonium phosphomolybdate and then reprecipitated as magnesium ammonium phosphate. This precipitate is collected as a thin uniform layer on a filter ring under carefully standardized conditions, dried by washing with alcohol and ether, and finally stored in a desiccator a t 50% relative humidity. The weight of this magnesium ammonium phosphate hexahydrate precipitate affords a n estimation of the P31 and the radioactive measurement gives a value for the P32 FILTRATION APPARATUS
The filter rings (Figure 1)are’prepared by cementing \J7hatman 42 filter paper to a n aluminum ring. (The 3-hf trim cement manufactured by Minnesota Mining & Manufacturing Co., Adhesive Division, 411 Piquette Ave., Detroit 2, Mich., was found very satisfactory for this purpose.) These rings are 3/le-inch sections of seamless aluminum tubing 1.5 inches in outside diameter and with 1 / 1 ~ inch walls. Prior to use the filters are brought to equilibrium a t 50y0relative humidity (by placing in a desiccator containing sulfuric acid, specific gravity 1.33) and weighed. The assembly of the filtration apparatus is shown in Figure 1. The filter ring is centered on a relatively h e sintered-glass suction plate and the glass cylinder is fitted in place and held tightly against the filter paper. Three rubber bands attached to a wire collar around the neck of the filter flask suffice to hold the cylinder in place. The inside edge of the glass cylinder pressing against the filter paper is beveled and fire-polished. The cylinder can then be removed without serious disturbance to the layer of the precipitate. In handling the filter rings it is important to prevent wrinkling of the filter paper due to its expansion when wetted with water. If the filter paper is moistened with alcohol prior to placing the rings on the sintered-glass filter plate and if reduced pressure is maintained on the paper throughout the filtering process, no wrinkling will occur. FILTRATIOh
It is necessary to exercise some care during the filtration of the magnesium ammonium phosphate to obtain the desired uniform layer of precipitate. Satisfactory results are obtained by the following procedure:
RUBBER BAND
A filter ring is saturated with alcohol and centered on the sintered-glass filter plate and gentle suction is applied by adjusting the stopcock attached to the filter flask. The assembly of the apparatus is then completed and the supernatant liquid decanted into the filter with a minimum of disturbance to the precipitate. The walls of the beaker are washed with about 10 ml. of K ammonium hydroxide and the precipitate is rapidly transferred to the filter in one operation. This almost fills the cylinder with a suspension of the precipitate. The stopcock is then closed, thus retarding the filtration rate. The beaker is rapidly policed and washed. These washings are added to the filter cylinder before all the previous washings have had an opportunity to drain from the filter. Next, the stopcock is again opened and the filter allowed to drain. The walls of the filter cylinder are then policed and washed with 15 ml. of 507, alcohol by directing a fine stream around the top of the filter cylinder. This is followed by three washings with 95% alcohol. Finally, the filter is washed with 15 ml. of ether and air is drawn through the layer of precipitate for about 10 minutes or until drv. During the rrashing of the filter and precipitate care should be taken not to disturb the precipitate.
FILTER FLASK
Figure 1. Filtration Apparatus for Collecting a Uniform Layer of Precipitate for Measurement of P31and P32 559
ANALYTICAL CHEMISTRY
560 After the filtration is completed the rubber bands holding the cylinder are disconnected and the cylinder is carefully raised. Any particles adhering to the lower edges are recovered with a fine brush. The suction is then cut off, and the filter ring is removed and stored in a desiccator at 50% relative humidity for a t least 12 hours before weighing. Table I. Recovery of Phosphorus Weighed on Filter Rings as Magnesium Ammonium Phosphate Hexahydrate Phosphorus
Taken 1Mg.
hlg”1PO1.6H20 Recovered MQ.
Phosphorus Recovered
MQ.
TYPICAL MEASUREMEVTY
The weighing of magnesium ammonium phosphate hexahydrate
as a procedure for determining phosphorus is not a commonly accepted practice. However, it has been recommended by Fales (8) and Treadwell and Hall (5). The results given in Table I show the recovery of phosphorus when weighed on filter rings as magnesium ammonium phosphate hexahydrate to be approximately equal to the phosphorus taken and to vary in direct proportion with the amounts taken. The radioactivity measurements of the filter rings containing a Layer of magnesium ammonium phosphate hexahydrate with a Geiger-Xiiller counter should not present difficulties if an arrangement for adequately reproducing the geometry is provided. A shield and sample holder similar to that described by Dauben et ul. (1) has been in use a t this laboratory. Srlf-absorp-
Table 11. Radioactivity Measurements of Magnesium Ammonium Phosphate Precipitates Containing Varying Amounts of P31and Paa Relative P * z Concentration Taken 1
Psi
Taken,
ActivityQ, Counts/Sec. 10 8.01 20 8.44 30 8.21 2 , 10 15.70 20 15.95 30 16.01 3 10 23.80 20 23.76 30 24.02 a Activity corrected for background, radioactive decay, and recwery timr of counter. Mg.
tion was tested on amounts of precipitate up to 300 mg. and, aE was expected, no correction appears necessary. Table I1 gives the radioactivity measurements of nine precipitates prepared to contain three amounts of P32in combination with three amounts of P31. The data show that the radioactivity of given amounts of P 3 2 can be determined with accuracy in the presence of varying quantities of Pal. These data are representative of the degree of reproduction that has been obtained. LITERATURE CITED
(1) Dauben,
\v.
G., et d., IVD.EXG. CHEM.,A N A L . ED., 19, 82s
(1947).
Fales, H. A., “Inorganic Quantitative bnalysis,” p. 222, New York, Centuiy Co., 1926. (3) Henriques, F. C., et al., ISD. ESG. CHEM.,A N ~ LED., . 18, 349 (2)
(1946). (4) (5)
Henriques, F. C . , and Margnetti, C., Ibid., 18, 415 (1946). Treadwell, I‘.P., and Hall, W.T., “Analytical Chemistry,” Vol 11,9th ed , p. 267, New Tork, John Wiley & Sons, 1945.
RECEIVED October 1. 1947.
Precipitation of Oxalates from Homogeneous Solution Application to Separation and Volumetric Determination of Magnesium LOUIS GORDON AND EARLE R . CALEY, T h e Ohio State University, Columbus, Ohio
THE
separation or determination of certain metals by precipitation with oxalate ion is an old but useful technique. A difficulty frequently encountered in such separations or determinations is the formation of finely divided precipitates that require retentive filter media. As a consequence filtration and washing mav become SIOR- and tedious operations. Though this manipulative difficulty may usually be overcome to a large extent by the familiar technique of adding the preripitant slowly and digesting the precipitated solution for a sufficiently long time, this does not always resolve the difficulty. lJ7hen an oxalate is precipitated by such technique in a medium that is essentially nonaqueous, as in the method for the determination of magnesium described by Elving and Caley (I), the precipitate is still too Bnely divided and causes difficulty in filtration. What is needed therefore is a fundamentallj different technique which will always result in the formation of an oxalate pfecipitate of large particle size that may be filtered and washed with ease and rapidity. For the special case of the precipitation of calcium as oxalate, Willard and Chan ( 2 ) devised a procedure that produces a coarse precipitate that is easily filtered and washed. In this procedure part of the necessary oxalate is added to a calcium solution of such acidity that little or no initial precipitation results. Urea is then added and the solution is boiled gently to cause slow hy-
drolysis of the urea. This brings about gradual decrease in the hydrogen-ion concentration and a corresponding gradual increase in the oxalate-ion concentration, which in turn causes the slow precipitation of calcium oxalate in coarse crystals. The remainder of the necessary oxalate is then added to ensure complete precipitation. For the separation of thorium and most of the rare earths from a solution of a monazite sand, Willard and Gordon ( 3 ) found that the hydrolysis of methyl oxalate was a means of producing a slow increase in the concentration of oxalate ion, and that this technique yielded a precipitate of the mixed oxalates which was of better physical form than that obtained by the usual direct precipitation with oxalic acid. Neither of these techniques for precipitation from homogeneous solution can be applied directly to the quantitative precipitation of certain other metal osalates, such as the precipitation of magnesium oxalate in 85% acetic acid solution for example, though the principle involved in the second one appears to be generally applicable. I n actual experiments on the use of methyl oxalate for the precipitation of magnesium oxalate in, 85% acetic acid solution it was found that this ester decomposed too rapidly, especially at the elevated temperature desirable in this precipitation for the purpose of obtaining the best possible separation from lithium and sodium. However, the resulting precipitate was
