ANALYTICAL CHEMISTRY
1060 volumetric rate of liquid feed against the orifice prmure also gives a straight line, aa it should (Figure 3). When only a small quantity of sample is available, or it is desired to operate for a short length of time, the by-paea valve is opened and the mercury height in the feed tube raised until the liquid enters the mixing chamber. At the end of a run, the valve to the atmosphere is opened, relesaing the pressure in the system so that liquid from the “feed supply” reservoir may be fed into the “liquid feed tube,” preparatory for the next run. VAPORIZATION AND MIXING
If it is necessary to vaporize the liquid steadily and mix it with a gas, this can be accomplished with the mixing chamber shown in Figure 4. The glass nozzle extends just a fraction above the */,-inch hple, so that no aapirator action is obtained which would ve oscillations if present. The rapid passage .of air over the fquid dro lets, formed a t the tip of the nozzle, gwes a very fine spray. &e chamber and associated tubing are all stainless steel exce t for the lass tube from the liquid control system; thus any Tiquid can %e used which does not attack glass or stainless steel. Because the tube enters the chamber at a position where only air is present in the writer’s setup, the glass-metal seal is made with de Khotinsky cement. In a later setup hypodermic stainless steel tubin was substituted for the glass nozzle. By means of the controf valve on the gas inlet side, a pressure of approximately 10 to 15 pounds per square inch gage is maintained across the small annular air opening around the nozzle; some of the air is by-passed into the top of the chamber by the contrd valve. The heat of vaporization is furnished by heating the mixing chamber and the by-pass inlet gas line with an electric furnace.
Of the several types of mixing and vaporization methods tried at these very small flow rates, this is the only one which has performed satisfactorily under routine operation without oscillations. ACKNOWLEDGMENT
The author wishes to acknowledge the assistance of Moreland R. Irby, Jr., and Charles M. Barnett, who calibrated the system.
CONTROL FROM LIO!?STEM I
Figure. 4.
Vaporization and Mixing Chamber
The work described in thia paper was done in connection with Contract NOrd 9750 for the Naval Bureau of Ordnance, U. 5. Navy, as part of Project Bumblebee. LITERATURE CITED
(1) Anderson, J. W., and Friedman, R., Res. Sci. Instruments, 20,81 (1949). (2) Hogg, H., Verheua, J., and Zuiderweg, F. J., Trans. Faraday SOC., 35,999 (1939). REWEIV~D December 10, 1949.
f recipitation of Oxalates from Homogeneous Solution Separation and Volumetric Estimation of Zinc EARLE R. CALEY, LOUIS GORDON’, AND GEORGE A. SIMMONS, JR. The Ohio State University, Columbw, Ohio
an earlier paper ( 8 ) it was shown that magnesium could be INprecipitated in an easily filtrable form from 85% acetic acid
solution by the slow decomposition of ethyl oxalate, thus avoiding the experimental difficulties formerly encountered in the separsr tion of magnesium oxalate for the indirect determination of magnesium with permanganate. This paper summarizes the results of experiments on the precipitation of zinc by this =me technique. With slight modification, the procedure for the precipitation of magnesium is also suitable for zinc. If wed without modification, the ainc oxalate is precipitated in very large crystals that are not only inconvenient to filter but apperently impure. Such crystals are formed when the precipitation from homogeneous solution occurs a t too slow a rate. By using more ethyl oxalate Preaent a d d r w , Department of Chemistry, Byraou~eUnivenity, Byracum. N. Y. 1
initially the rate is increased, crystals of desirable physical properties and purity are obtained, and the time of precipitation is shortened. PROCEDURE
Concentrate the neutral zinc solution in a 25Gml. beaker to a volume of 10 to 12 ml. or dissolve the reeidue of dried salts containing the zinc in 11 ml. of water. Add 85 ml. of glacial acetic acid in which 1 gram of ammonium acetate has been dissolved. Then add 4 ml. of ethyl oxalate, stir well, and heat ra idly to a proximately 100’ C. Cover the beaker with a watch gram and pram on a hot plate eo regulated that the eolution ie maintained at approximatel this same temperature. Allow 1.6 hours for precipitation. there is any doubt as to maintenance of the recommended temperature durine; this riod, add 5 minutes before filtration, 5 d.of 85% acetic a c i f h a t .has been saturated wath ammouum oxalate at mom temperature. E’llter and wash
6
V O L U M E 22, NO. 8, A U G U S T 1 9 5 0 Table I. Determination0 of Zinc by Recommended Procedure. Zn Taken
Zn Found
&am
&am
0.0010
Difference, Error &am
0.0009 0.0010 0.0010 0.0049 0.0049 0.0051 0.0100 0.0102 0.0102 0.0499 0.0601 0.0503
0.0049 0.0100 0,0500
-0.0001 O.oo00 0.moo o.Ooo0 o.Ooo0 +0.0002 o.Ooo0 +0.0002 +0.0002 -0.0001 +O.Oool +0.0003
Table 11. Precipitation in 70% Acetic Acid Solution Anion Present
Zn Taken
Zn Found
Qram
Qram
Chloride
0.0010
0.0002 0.0003 0.0003
Sulfate
0.0010
0.0006
Chloride
0.0049
Sulfate
0.0100
Chloride
0.
Sulfate
0.0260
oam
0.0007 0.0007 0.0050 0.ooso 0.0101 0.0102 0.0248 0.0251 0.0250 0.0261
Differenoe, Error Qram
--0.0007 0.0008 -0.0007 -0.0004 - 0.0003 -0.0003 +O.OOol +O.Oool +0.0001 +0.0002 -0.0002 +0.0001 0.0000 +o. 0001
the precipitate, dissolve it, and titrate the solution as in the procedure for magnesium. This procedure is suitable for about 0.5 to 50 mg. of zinc. Larger amounts require a longer time for precipitation and the results may not be satisfactory. Care should be taken not to use acetic acid that has been in contact with paraffin on closures of bottles, as the paraffin may separate when the acid is added to the aqueous zinc solution. The use of partly hydrolyzed ethyl oxalate must be avoided ( 8 ) . TEST DETERMINATlONS
For these determinations, and for certain other experiments described below, standard ainc solutions were prepared by dissolving accurately weighed amounts of highly pure zinc in a slight excess of hydrochloric acid, and diluting to volume in calibrated flasks. Suitable aliquot portions were taken and evaporated to dryness to remove free acid, and the zinc content was determined by the above procedure. As shown in Table I, satisfactory results were obtained.
1061
cawed by insufficient insolubility of the precipitate in 70% acetic acid. T h e fact that satisfactory results are obtained with larger quantities of zinc by both methods may be due to retention of extra oxalate by the precipitate, so that possibly the results in 70% solution are satisfactory only because of a compensating effect. The need for having the concentration of acetic acid as high as 85% in order to obtain satisfactory results in the precipitation and determination of small quantities of zinc is further shown by Table 111. I n these determinations the zinc was initially present as chloride. Table I1 indieates that when zinc is initially present as sulfate in precipitations made in 70% acetic acid, the results tend to be higher than when present as chloride, especially with a small amount of zinc. This same effect was found for precipitations made in 85% acid, except that the effect was greater (Table IV). These high results are apparently caused by the retention of extra oxalate by the precipitate. On the other hand, low results may be obtained when an attempt is made to determine much larger amounts of zinc-Le., 100 mg. or more-in the form of sulfate, because the amount of zinc sulfate present considerably exceeds the solubility of this salt in the total volume of solution. In general, therefore, sulfate interferes with the determination of zinc when precipitated as oxalate in 85% acetic acid solution. In 70% acetic acid, according to Elving and Lamkin and the experiments of the present investigation, satisfactory results may be obtained when sulfate is present, but only for a limited range in the amount of zinc. Because of this interference from sulfate, and because sulfate is often present in solutions in which zinc is to be determined, the possibility of removing or destroying the sulfate before applying this procedure for zinc was tested. Removal with barium ion or lead ion is unsuitable, because any excess of these precipitating ions interferes in the precipitation of zinc oxalate. Though re moval of the excess is poasible if lead is used, this ia objectionable as involving still another step. Actual destruction of the sulfate appears to be better than its removal by precipitation. By treating zinc sulfate residues with a sufficient exceea of pure concentrated hydriodic acid and evaporating to dryness, sulfate is campletely destroyed and the zinc is quantitatively converted to the iodide. The easily available commercial acid containing hypophosphorous acid as a preservative unfortunately cannot be used for this purpose, because the residue after evaporation then contains zinc salts of acids of phosphorus that are insoluble in water
Table 111. Effect of Concentration of Acetic Acid on Results Obtained with a Small Quantity of Zinc Acetio a d d concentration, % I n taken, gram Zn found, gram
VAklOIJS EXPERIMENTS
Because Elving and Lamkin ( 1 ) state that “zinc can be completely precipitated as oxalate in 70% acetic acid medium,” the question arose as to whether the we of a medium aa concentrated aa 85% waa really neceaeary. A series of test determinations was run in which the zinc was precipitated homogeneously as oxalate from 70% acetic acid solution, by the method given above, except for the concentration of acetic acid (Table 11). As Elving and Lamkin ran their test determinations with sulfate solutions, the test determinations were made on sulfate aa well as chloride solutions. With the smallest quantity of zinc all the results are low, and when the zinc is in the form of sulfate they check closely with the corresponding results given by Elving and Lamkin (1, Table 11). This is important, aa showing that the low results on small quentities of zinc cannot be ascribed to the difference in the two methods of precipitation. It is likely that these low results are
Av. Zn found, gram Av. difference error, Oram
70 0.0019 0.0015 0.0016 0.0017 0.0017 0.0018 0.0018 0.0017 -0.0002
75 0.0019 0.001s 0.0016 O.OOl6 0.0017 0.0018 0.0019 0.0017 -0.0002
80
0.0019 0.0019 0.0017 0.0017 0.0017 0.0018 0.0021 0.0018
85 0,0019 0,0018 0,0018 0.0018 0.0018 0.0020 0 * 0020 0.0019
-0.0001
0.0000
Table IV. High Results in 85% Acetic Acid Solution Caused by Sulfate Anion Present
Zn Taken
Zn Found
Qram
Qram
Chloride
0.0019
Sulfate
0.0019
Chloride
0.0100
Sulfrta
0.0100
0.0018 0.0020 0,0024 0.0029 0.0100 0.0102 0.0104 0.010s
Difference, Error Qram
- 0.0001
+o * 0001 +0.0005 +0.0010 0.0000
+o , oooa
+O .0004 +0.000s
1062
ANALYTICAL CHEMISTRY
and in acetic acid solutions. For the experiments the pure acid waa made by the interaction of iodine and hydrogen sulfide. It waa found that 8 ml. of the constant boilingmid for each 100 mg. of zinc sulfate were sufficient for the complete destruction of the sulfate on a single evaporation. The results of determinations on ~ i n ciodide residues obtained by such treatment were, however, not 80 good aa those on chloride solutions; they generally were a little high, in part at least because of slight retention of iodide in the precipitate of zinc oxalate. Because of the general unavailability of pure hydriodic acid commercially, thismeana of destroying sulfate is of no great practical value in thie procedure for zinc. It is, however, of interest aa a widely applicable general method for destroying sulfate, and it may prove useful in other procedures involving nonaqueous solvents, where the presence of sulfates is undasirable because of their general low solubility in such solvents. Elving and Lsmkin ( 1 ) found that 10 mg. of ferric iron in 10 ml. of 70% acetic acid gave no precipitate with oxalate, and that by their procedure satisfactory results were obtained for the zinc content of alloys that contained low percentages of iron. They give no information on the behavior of larger quantities of iron, nor do they appear to have made systematic experiments on the determination of zinc in the preaence of various amounta of iron. When the present procedure is used, aa much aa 10 mg. of either ferrous or ferric iron as chloride may be present alone without giving any precipitate, but when zinc is also present, aa is shown by Table V, even 5 mg. will cause high results for zinc by reaaon of coprecipitation of the iron as oxalate. The occurrence of coprecipitation waa deduced not only from these quantitative results but from the visible discoloration of the precipitates of zinc oxalate obtained in the experiments. The results in Table V also show that the error from coprecipitation increases with increase in quantity of either zinc or iron and that ferrous iron interferes more than ferric. This interference from iron may obvioualy cause large errors in the determination of zinc by this procedure, and therefore all except minute amounta of iron must be removed beforehand.
Table V. Oxidation State of Added Fe
I1
I1 I11 111
Interference from Iron in-ffiq' Acetic Acid Solution Apparent Amount .of Zn Found
Preoent
Fe
Zn Taken
Cham
Cham
Qram
0.0101 0.0101 0.0101 0.0052 0.0100 0.0628 0.0098 0.0098 0.0088 O.OQ61 0.0098 0.0500
o.Ooo0 0.0019 0.0500 0.0100 0.0100 0.0100 o.oo00 0.0019 0.0600 0.0100 0.0100 0.0100
0.0000 0.0027 0.0529 0.0108 0.0114 0.0397 0.0002 0.0022 0.0516 0.0107 0.0111 0.0114
Differenoe,
Error
Qram
0.0000
f0.0008
+0.0029 +0.0008 +0.0014 -I-0.0297 +0.0002 +0.0003 +0.0015 +0.0007 +0.0011 +0.0014
LIMITATION OF APPLICATION
The practical application of this procedure is limited because of interference from various ions, some of them commonly associated with zinc. Sulfate interferes to some extent and iron interferes seriously. Cadmium, copper, lead, calcium, and magnesium, and any other metals that Precipitate as oxala,tes :n 85% acetic acid solution, must be absent. In practice this procedure appears to be most useful for the accurate volumetric determination of zinc in certain mixtures or preparations in which zinc is the only metal present, and in solutions that contain no interfering ions or from which interfering ions may be readily removed. LlTERATURE CITED
(1) Elving, P.J., and Lamkin, J. C., IND. ENQ.C H ~ MANAL. ., ED., 16, 19443 (1944). (2) Gordon, L., and Chley, E. R., ANAL.CHHIM., 20, 660-3 (1948). R B C ~ V BJuly D 12, 1949.
Photometric Determination of Available Phosphorus Pentoxide in Fertilizers ERNEST A. EPPS, JR. Louieianu Agricultural Experiment Station, Baton Rouge,
CCORDING to the method of the Association of Official Agri-
A cultural Chemists ( I ) , the available phosphorus pentoxide in acidulated samples, dicalcium phosphate, precipitated bone
phosphate, and precipitated bone is obtained by subtracting the aitrate-insoluble phosphorus pentoxide from the total phosphorus pentoxide, Thus, two analyses are necessary to obtain a single reeult. Obviously, a more direct method would provide a saving in time and a reduction of the poaeibility of error. The method proposed in 1938 by MacIntire, Shaw, and Hardin (6)waa baaed on a steam digeation but, though promising, has apparently not found general acceptance. In the official method (1) for determining citrahinaoluble phosphorus pentoxide the sample is washed with water and then digested with neutral ammonium citrate, and phosphorus pentoxide ie determined in the residue. Analysis of the combined water washings and filtrate from the citrate digestion would give the available phosphorus pentoxide directly, but this cannot be done by the volumetric molybdate method because citrate interferes with precipitation of the phosphorus as the phosphomolybdate. h e n t e l y Barton (.a) has ahown that the photometric phosphovanadomolybdate method is suitable for analysis of phosphate
La.
rock. Kitson and Mellon ( 4 ) in a thorough study have shown that the ions likely to be p w e n t in the analysis of fertilizer would not cause interference. Othere (6,7, 8) have demonstrated the application of the method for analysis of phosphorus in a variety of materials. This study waa undertaken to determine whether the m e method could be adapted to analysis of combined water washings and citrate filtrate. REAGENTS
Prepare vanadomolybdate according to the directions of Barton for the mixed r Dissolve 40 grama of "acid molybdic 85%' in 400 ml.?f%er, disaolve 1.0 gram of ammonium vanadate in 300 ml. of water and add 200 ml. of concentrated nitrio acid, Allow the two soiutione to cool and mix by pourin the molybdate solution into the vanadate solubon. Dilute to lqiter. Prepare neutral ammomum citrate according to directions of the h c i a t i o n of Official Aqicultura! Chemists ( I ) . Dieeolve 370 a m of crystallued citno acid m 1500 ml. of water and neargneutralize by adding "5 ml. of ammonium hydroxide (28 to 29% NHs). Careful1 adjust to pH 7 and dilute if necessary to a specific gravity of I.& to 200 C. Pre are standard phoephove pentoxide solution by dissolvin 0.479fgram of potassium dihydrogen phosphate in 1 liter
01
