V O L U M E 21, NO. 4, A P R I L 1 9 4 9
525
Table 111. Copper in Aluminum Alloys Sample Alcoa 3 2 4 Blcoa 355 Alcoa 1420 Alcoa 3-Sd Alcoa 2-9 85a 86b
Cu Present, %
Cu Found, %
Si Presenta. %
1 . 0 6b 1.35b 3.94b 0.16b 0 . 0 9b 2.488 7.878
1.04 1.34 3.98 0.15 0.11 2.48 7.89
12 5 0.2 0.2 0.1 0.114 0.47
*
Nominal values except 85a a n d 86b.
d
Contajns 2% nickel. Contains 1.2% manganese. National Bureau of Standards certified value.
h Determined by umpire method of Aluminum Co. of Smerica ( d ) . 6
by boiling with water. Insoluble precipitates are sometimes produced when aluminum alloys containing tin, antimony, or bismuth are dissolved in an acid mixture of phosphoric, sulfuric, and nitric acids. Silicides, silica, and elemental silicon are completely dissloved by a mixture of phosphoric and sulfuric acids (4, 6 ) , and the solution remains clear indefinitely even when diluted with water. For the determination of copper in aluminum alloys that COIItain less than 2% silicon the proposed method is neither more nor less accurate than the methods of Sloviter (IO) and \Veinberg(22). LITERATURE CITED
diameter) and a platinum spiral anode. During the electrolysis stir the solution. Immerse the cathode in water and then in alcohol, dry it in an oven a t 105” C. for about 3 minutes, cool, and weigh the deposit as metallic copper. The results obtained by the author on several representative aluminum alloys are shown in Table 111. The proposed method is not recommended for the analysis of aluminum alloys containing tin, antimony, bismuth, or silver, as these elements were found to contaminate the copper deposit. Possibly because of the presence of the sulfuric acid (which is not used in the brass and bronze or tin-base alloy procedures) the copper was contaminated by tin and antimony even when the pyrophosphoric acid had been converted to phoqhoric acid
(1) Am. SOC.Testing Materials, “Methods of Chemical Analysis of M e t a l s , ” ~249, . Philadelphia, Pa., 1946. (2) Churchill, H. V., and Bridges, R. W., “Chemical Analysis of Aluminum,” p. 36, New Kensington, Pa., Aluminum Co. of America, 1941. (3) Hoffman, J. I., and Lundell, G. E. F., J. Research Natl. But. Standards. 22,465 (1939). (4) Lisan. P., and Katz, H. L., ANAL.CHEM.,19, 252-3 (1947). ( 5 ) McKay, L. W., J . Am. Chem. SOC.,36,2375-81 (1914). (6) Norwitz, G., ANAL.CHEM.,20, 182 (1948). (7) Prescott, A. B., and Johnson, 0. C . , ”Qualitative Chemical Aiialysis,” p. 493, New York, D. Van Nostrand Go., 1933. (8) Kavrier, H., IND.EXG.CHEM.,ANAL.ED.,17, 41-3 (1945). (9) Scott, W.W., “Standard lMethods of Chemical Analysis,” Vol. I, p. 367, New York, D. Van Nostrand Co., 1939. (10) Sloviter, H . A., IND. ENG.CHEM.,ANAL.ED.,13, 235-6 (1941). (11) Weinberg, S., Ibid., 17, 197 (1945). R E C E I V EDecember D 8, 1947.
Urea Hydrolysis for Precipitating Calcium Oxalate R. S. ISGOLS AND P. E. 3IURRAY State Engineering Experiment S t a t i o n , Georgia School of Technology, Atlanta, G a .
T
HE determination of the calcium ion as calcium oxalate is a well established technique. Willard and Furman ( 3 ) give a method first described by Chan ( 2 ) for precipitating calcium oxalate, and, on a recent lecture tour, Willard presented some of the basic material indicating its advantages. By the older method ( I , 3),the precipitate is formed during the addition of ammonium oxalate and the pH is then raised with ammonium hydroxide. In the new method, acid is added to the sample to produce pH 1.0; then ammonium oxalate and urea are added. The solution must remain clear until the sample is boiled to hydrolyze the urea arid raise the p H t o the point of calcium oxalate precipitation, this may take 10 to 15 minutes. It is evident from Figure 1 that tlie crgrstals precipitated from the hot solution by the hydrolysis of urea are enough larger than those by the old technique to he ready for filtration shortly after formation. This eliminates much of the digestion period needed by the older method and permits a technician to speed up the trst for calcium. hIETHOD
;Inaliquot (100 ml.) of the sample is
laced in an Erlenmeyer flabk. A few drops of methyl red are adxed, then 2.4 ml. of a 1 to I hydrochloric acid solution. this develops a p H of 1.0 which is uccided to prevent preci itaAon of calcium oxalate until desired. Following this, 15 ml. o f a saturated ammonium oxalate solution and 10 grams of urea are added and mixed. The solution must remain clear. The urea must be added dry or as a fresh solution, because even a 24-hour solution contains sufficient ammonium hydroxide to cause precipitation. The solution is heated on a hot plate (generally for 15 minutes) until the methyl red changes color at pH 5.0, and is then ready for filtration. A coarse qualitative paper is used aa the a t e r medium, and, after adequate wash-
ing, the paper plus the precipitate is returned to the original flask or the precipitate may be filtered with vacuum on a small fine filter paper supported by a Gooch crucible. This is somewhat more rapid than the gravity filtration and reduces the amount of paper present in the titrption beaker. The Gooch crucible is returned to the original beaker with the filter paper and precipitate The calcium oxalate precipitate is dissolved with hot sulfuric acid solution and titrated immediately with a standard permanganate solution. A large quantity (5 gallons) of a solution of calcium chloride of known concentration \%asmade up and used as the test solution. S‘arious amounts of magnesium and aluminum sulfates and magnesium chloride were added t o portions of this stock test solution, and a t least ten determinations were made on every portion by the standard and the new techniques. The calcium sulfate solution studies were made on solutions of several batches of chemically pure calcium sulfat,, crystals. RESULTS
-1veruges of ten determinations on each solution by each technique were calculated and the probable error was computed (Table I). There is no significant difference in the concentration of calcium ion from calcium chloride when no other cations or anions are present originally. When a knoxm amount of calcium iulfate is iTeighed out and the calcium ion values are determined, the results by either method do not agree with the theoretical value, but the error is slightly smaller with the urea hydrolysis method than nith the standard procedure. 4 double precipitntion of tlie calcium oxalate from the sulfate solution shows that the sccond precipitation with the standard method fails to yield theoretical values TThile the double precipitation with the urea method does yield theoretical values. Table I1 indicates that
526
ANALYTICAL CHEMISTRY
only the urea hydrolysis technique will give a lower oaleium ion value with a double precipitation when the natural waters slso contain sulfate. This agrees with the calcium sulfate results. From the results with magnesium salts, it is evident that even 150 p.p.m. of magnesium ion do not tend to coprecipitate with the calcium oxalate, if no sulfate ion is present. I n the presence of the sulfate ion, higher apparent calcium values are again obtained as for calcium sulfate. Again, the urea hydrolysis procedure gives results that are in closer agreement with the theoretical values. In the presence of 7.0 p.p.m. of aluminum sulfeto, the calcium ion concentration by the standard method of precipitation is higher each time by an amount equivalent to the extra aluminum ion prescnl . T h c n thr urea method in,s uscd on 30 sxmplcs can-
. /
1
\
.A
Figwe 1. Crystal Sizes of Calcium Oxalate .A.
Precipitated by PIX adjusrmenr with ammonia Urea hydrolysis Supplied by Willard
1.
Precipitating Calcium Oxalate for Volumetrio D e t e r m i n a t i o n of Calcium
11.
3
Ion ~ l , ~ ~ ~Average ~ t iof ~Ten ~ Caloium l value, Vhluer witlr Probable Error Caloivm concontration of IO", Salts PreJent P.P.M. CaCb .2&0 1st precipitation 100 2nd preoipitation CaSOi .2Hr0 1st PreciDithtion 2nd preoisitation CaCIa 1300 I L P . ~ . MgClz. 6 I h O 100 CaCh.ZH*O 88 CaCk 400 P . P . ~ . iilpS0.. 7 H10 88 ChCb 800 P . P . ~ . hlLIS0, .7 1110 88 caci? 7 B.P.m. Ah(SO,)a. 18HzO 88 CSCI? 7 p.p.m. Ah(S03,. 18HzO 88 CSCIZ 7 p.p.m. AIdSO4a. 181310 88 CaCl* 7 p.IJ.m. AI,(SO,)r.18HxO 70 P . P . ~ tsr. trate 88
+ + + + +
+ ++
Stkndard methods, lJ.p.m. 99.3 99.5
103.3
+ .16 * 00.36
*r 0o ., 44 +
100.3 0.34 8 7 . 9 zk 0.05
92.0
'33.3 90.9
+ 0.14
* 0.G5
* 0.50
'30.7
+ 0.50 + 0.20
91.3
+ 0.3;
81.5
Urea hydrolysis, P.P.m.
99.7 i 0.10 98.5 + 0 . ~ 6 103,4 100.6
99.9 88.1
*+ o0 ., 5? + 0.35
* 0.10
8 8 . 3 i 0.15
88.1
+ 0.16 + 0.50 + 0.30 + 0.30
88.4
+ 0.20
89.1 '21.6
88.1
taining some aluminum ion, two groups of ten were as low in calcium ion vducs as the control, while the third group of ten was higher than the control by a n amount equivalent to the extra duminum ion. When tartrate ion was added to the sample and analyzed by standard methods, the results were high. When the newer urea method was used, there was no apparent calcium increase if 30 minutes were allowed for the aluminum and tartrate reaction
The results obtained indicate that the urees. hydrolysis method for calcium can shorten the time of a calcium ion determination while improving the accuracy of the calcium oxalate precipitrttion. During the formation of the precipitate, conditions that are h a s t favorable to its formation should he maintained. Thus, calcium oxalate is formed in the standard procedure from an acid medium instead of an alkaline medium where i t is less soluble; the urea hydrolysis method permits the formation of the precipitate from a solution n.hose pH is changing slowly from a point below to one somewhat beyond the value of precipitation. The precipitating solution should generally be added slowly with stirring to a. hot or boiling sample: The precipitate forms in the urea hydrolysis method from a uniform boiling sample. A digcstion period should he provided to inorease the particle size of the precipitate: This is eliminated in the urea hydrolysis method because of thc large size of the crystals a t their formation (Figure 1). The urea hydrolysis method adheres to well established analytical principles while reducing the time and attention needed far observing them. The formation of a precipitate from two ions in solution is primarily a funotion of the two ions, but the chemical purity of the compound in the precipitate is a function of the other ions present a t the time of the formation of the precipitate. Thus, a small amount of magnesium ion might react along with calcium ions a t the time of the calcium oxdate formation and increase the amount of oxalate in the precipitate. Results indicate that the magnesium ion does not tend to precipitate with the oxalate ion during calcium oxalate formation in the concentrations studied, even though the concentrations of the magnesium and oxalate ions are in excess of their solubility product. The presence of the sulfate ion, however. caues an increase in t,he amount of oxalate ion which'enters the calcium oxalate prccipitate. If the quantity of the precipitate had been determined gravimetrically, the identity of the extra ion would not be so readily knon-n. The high apparent calcium ion values from known amounts of calcium sulfate were determined by oxidizing the oxalate in the precipitate vith a standard permanganate solu1tion. To determine whether extra washing of the precipitate 'i5ould reduce the error of tho sulfate ion, known volumes of wash water were used upon aliquots of the same calcium sulfate solution n duplieato.
I
VOl. of "ad, rater, m1. Calcium, 7001 llreoretioal
50
100 106.5
l0Y.O
200 104.2
400 103.9
Table 11. Calcium in Natural Waters
Sample
:aSO?ZH,O
Caloium IO" Stkndard Method Urea H y d rolysis 1st pptn., 2nd m t n . , 1st mtn., Zndpptn., P.P.m. D.0.m. p.p.m. -t 0 . 3 2 47.5 + 0 . 3 3 4 3 . 0 i 0 . 4 0 ~~
102.0
.....
98.0
......
101.0
...... ......
36.6+0.33
'
8.0
......
......
92.5 8.2
* 0.00
Mg++,
Fe.
p.IJ.m. 20.0 20.0
0.lJ.m.
0.1 0.1
Other Ions SO+-; NO*-, CI; p.p.m. P.P.rn. D.P.'". 43.0 2.8 80 43.0 2.8 SO
20.0
0.1
160.0
2.8
80
5.0
... ... ...
.. ....
...
..
1.0
......
15.0
......
15.0 1.5
......
15.0
o:i
...
61.0 61.0
...
176.0 4
0.1
...
3
SO,, p.p.m.
... ... ... ... ...
...
i:i,
V O L U M E 21, NO. 4, A P R I L 1 9 4 9
527
The results indicate that douhling the volume of wash water used after washing out all the excess ammonium oxalate does not . remove the excess oxalate in the precipit,ate.
To determine whether dilution of the calcium sulfate solution at the time of the calcium oxalate precipitation would reduce the sulfate ion error, the original calcium sulfate solution was divided into a number of 50-ml. portions. Ten were run by adding the reagents directly, using the urea hydrolysis techni ue. Then 350 ml. of water were added to give an eightfold Iilution, and a large number of determinations were made. The averages of the first and second groups were identical. T n ~ ointermediate dilutions were tried with the same results. I t is concluded that reasonable dilution of the calcium sulfate will not decrease the sulfate error. The urea hydrolysis technique for calcium oxalate formation in the presence of the sulfate ion has given a slightly smaller error than the standard technique on the first precipitation. On the second precipitation the oxalate error is still present with the standard technique, but thcoretical values can be obtained by the urea hydrolysis method. If different methods of crystal formation give different oxalate valucs from the same solution, the extra oxalate caused by the sulfate must be a part of the crystal. When the calcium ion is determined by the standard technique in the presence of a small amount of aluminum ion, t’here is always a precipitation of the oxalate ion equivalent to the sum of the calcium and aluminum ions. When the urea hydrolysis method was used, two groups of results were obtained. A possible explanation may be that the aluminum ion precipitates mmetimes with the oralatc ion but gcncrally with the hydroxyl ion.
In the presence of the tartrate ion, the calcium ion may be precipitated viith oxalate without any aluminum oxalate interference if 30 minutes are given for the aluminum tartrate formation. CONCLUSION s
The use of the hydrolysis of urea for raising the pH of a solution containing both the calcium and oxalate ions permits the formation of large, readily filtered crystals of calcium oxalate. These are less contaminated by magnesium or aluminum oxalate or excess oxalate due to the presence of the sulfate ion than the small crystals formed by the standard method, while the over-all time for a set of duplicate determinations for calcium is reducrd by the use of urea froin 90 minutes to 40 to 50 minutcs. ACKNOWLEDGMENT
The work of this paper is a part, of a project supported by a granbin-aid from the Sational Institute of Health of the United States Public Health Service for research upon rapid analytical procedures for n-atcr and sewage. LITERATURE CITED
(1) Am. Public Health Assoc., “Standard Methods for the Bnalysis of Water and Sewage,” 9th ed., New York, 1946. (2) Chan, F. L., dissertation, University of Michigan, 1932. (3) Willard and Furman, “Elementary Quantitative Analysis,” 3rd ed., p. 397, N e w York, D. V a n Nostrand Co., 1040. RECEIVED December 29, 1947.
All-Glass Laboratory Long Tube Evaporator WILLIAM H. BARTHOLOMEW, Werck & Co., Innc., Rahwuy, .V.J . 011 evaporating heat-sensitive iolutioiis in the laboratory, F the usual batchwise procedurc for Concentration in flasks often ii unsatisfactory. The retention of unstable materials a t -29 m m . 0 . D. TUBING 35/25
-ABOUT
20mm.
7
elevated temperatures for considerable periods of time can lead to undesirable decomposition. Long-tube or film type evaporators, which provide short contact time for the evaporating liquid a t moderate temperaturcs and high evaporation rates, have been used successfully in pilot plant and large scale operation to solve this problem (1, 3 ) . An all-glass corrosion-resistant film type evaporator has been designed for laboratory application.
ABOUT 3 8 m m .
APPARATUS
The assembled apparatus is shown in Figure 1. ..1 detailed sketch of the small glass separator i3 shown in Figure 2 .
E ~5lmm.O.D.TUBING
r 6 . 5 cm
LIT^
TUBING
2 m m . BORE STOPCOCKS
Figure 1.
Laboratory Long-Tube Evaporator
The evaporator consists of a &liter graduated feed funnel, A , which has a constant head air inlet tube. This is joined with flexible tubing to the bottom of the steam-jacketed heating tube, B. A small rotameter in the feed line allows rapid sett’ing of the feed rate, but this is not absolutely necessary for satisfactory operation. The vapor head of the heating tube has an opening for insertion of a thermometer. This head turns a t a sharp angle and connects with the tangential inlet of the separator, C. Prcvision is made for pressure measurement with a mercury manometer. The bottom outlet of the separator connects with the concentrat,e cooler, D, which joins the 2-liter concentrate receiver, E. The vapor outlet from the separator is connected through the right-angle adapter, F , to the vapor condenser, G , which is joined in turn t,o the 3-liter distillate receiver, H . A stopcock-controlled pressure-equalizing line connects the top of the right-angle adapter to the top of the concentrate receiver. The line connecting yith the vacuum source is attached a t the top of the distillate receiver. The ground-glass seiniball joints used to connect the various sections of the apparatus provide a measure of structural flexibility in the unit. Yo breakage has occurred during several months of operation.
