Figure 2.
Reversal of elution order of acetylene and propylene with increasing activity of alumina I, II, and Ill recorded, respectively, after 5 , 6, and 9 days of reactivation 5. 6.
Acetylene (X1) Propylene (X1)
propylene; on the thirteenth day the acetylene peak appeared between propane and propylene (Figure 1). The first part of Table I not only illustrates the effect of alumina’s deactivation on the relative retention times of these hydrocarbons but also emphasizes the extreme variation of their absolute retention times. A comparison of Figure 1 with the chromatogram of Pollak and Uus (3) shows the advantages of isothermal operation a t room temperature of a deactivated alumina column for the separation of the seven hydrocarbons listed in Table I : adequate resolution, more symmetrical alkene peaks, and completion of the analysis is less than 25 minutes instead of some 75 minutes. The copper hydrates system was then replaced by drierite and aliquots of the gas mixture were chromatographed
each morning. As soon as the retention times were recorded, the temperature of the column was raised to 45’ C. for the rest of the day to speed u p the reactivation process. The heat supply was cut off at night and the column equilibrated overnight at room temperature. The progress of reactivation is given in the second part of Table I and in Figure 2 where the reversal in the elution times of acetylene and propylene is clearly shown. The column conditions used in our previous work ( b ) , when the wateralumina adsorption equilibrium is established, with the system CuS04.5H20-CuS04.3Hz0 at room temperature in the He stream, were close to those recorded in this study after 2 days of reactivation (Table I). Obviously the alumina column used by Pollak
and Uus (3) never reached the degree of deactivation which gave us the elution order quoted in our previous publication ( 2 ) . LITERATURE CITED
( I ) Burrell Corporation, ”Instruments for
Gas and Vapor Chromatography,” Catalog 84, p. 49, 1959; Alumina, activated, Cat. S o . 341-35. ( 2 ) Philippe, R. J., Moore, H., Honeycutt, R. G., Ruth, J. M., ANAL.CHEM. 36, 859 (1964). ( 3 ) Pollak, P., Uus, O., Ibrd., 37, 167 (1965). ROGERJ. PHILIPPE~ ROBERTG . HONEYCUTT Research Department Liggett and Myers Tobacco Co. Durham, Pj. C. 27702 Present address, Tabacofina, S A , , Service de Recherches, Merxem-Antwerp, Belgium.
Precise Assay of Copper Using Small Samples SIR: The usual methods for the precise assay of copper require relatively large samples, the size varying from 2 to 10 grams of the metal. For example, the ASTM method uses a 5-gram sample of copper to obtain agreement between duplicate determinations to within 0.01501, (1). During the course of the mass spectrometric redetermination of the atomic weight of copper a t the National Bureau of Standards (3), it was necessary to determine the concentration of solutions which contained about 1 gram of copper with a precision of 0.01% (expressed in terms of standard deviation), Because only portions of each solution could be used for this determination, it was necessary to develop a procedure for the precise and accurate determination of from 0.1 to 0.25 gram of copper.
EXPERIMENTAL
A study was made of the precision and accuracy which could be obtained by “scaling down” the macro method for the precise assay of copper ( 4 ) . Pure spectrographic grade copper rods were obtained commercially. Quantitative spectrographic analysis for the elements which are known to electrodeposit with copper or which are common impurities in copper showed that this material was about 99.99801, pure. Samples ranging from 0.1 to 0.25 gram were cut from the rods. After pickling in dilute hydrochloric acid and in dilute nitric acid to remove surface contamination due to fabrication and cutting, each sample was washed with distilled water, rinsed with ethanol, and weighed on a microbalance to within & l pg. Each sample was dissolved in dilute nitric acid in a 6-dram vial, the resulting solution was
evaporated to dryness to remove nitric oxide and excess nitric acid, and the residue was dissolved in 10 ml. of a solution which contained 0.1 ml. of HNO, and 0.1 ml. of H2S04. The copper in the solution was slowly deposited overnight from the unstirred solution into a platinum gauze electrode a t a current density of about 0.1 ampere per square dm., using the apparatus shown in Figure 1. The 6-dram vial which served as the electrolysis cell was stoppered with an inverted rubber septum. A small glass tube was inserted through the middle of the septum to serve as a vent to allow the gases produced during the electrolysis to escape and as an entrance port for the lead of the platinum gauze cathode. The gauze cylinder of this electrode was 1.5 cm. high and 2 cm. in diameter, and had an effective area of 0.4 sq. dm.; the electrode weighed about 1 gram. VOL. 37, NO. 7, JUNE 1965
929
A platinum wire which served as the anode was inserted at the edge of the septum. The electrodes were connected t o a d.c. source by "alligator" clips. When electrolysis was nearly complete, the vial was unstoppered, without interrupting the current, and the septum, tube, and sides of the vial were washed with a few milliliters of distilled water. The stopper was replaced and the electrolysis was continued for another 4 hours. The vial was again unstoppered without interrupting the current, and the electrodes were withdrawn from the solution, while rinsing with water t o prevent resolution of copper. The cathode was dipped into ethanol and dried for 1 minute at 100" C., allowed t o stand for 2 hours &t room temperature t o attain thermal equilibrium, and finally weighed on the microbalance. The copper was then stripped from the electrode with dilute HNOS and the electrode was reweighed after proper washing and drying. The weight of copper deposited was determined as the difference of these two weights. The reverse procedure of weighing the electrode before electrolysis gave consistently low results, due to the mechanical removal of platinum from the lead wire by the alligator clip.
Table I.
Sample NO.
1
2 3 4 5 6 7 8
The copper remaining in the electrolyte and wastiings was determined by the sodium diethyldithiocarbamate method (2) in the following manner. T h e solution was transferred t o a separatory funnel, and was mdesliKhtly ammoniacal (pH = 9), after which sodium diethyldithiocarbamate solution was added. Five milliliters of carbon tetrachloride was then added and the solution was shaken for 2 minutes. The carbon tetrachloride layer, which contained copper diethyldithiocarhamate, was drained into a 1-cm. cell, and the transmittance of the carbon tetrachloride was measured st 440 mr. T h e copper present was determined by comparison of the value with a reference curve. The amount of residual copper found in the electrolyte by this method was in the range of 10 t o 60 pg. RESULTS
Table I shows the results obtained by this method. The column labeled "copper found" gives the sum of the copper obtained by the electrolytic method and the "carbamate" method. The standard deviation of the average of four determinations would be expected to be 0.013%/ &, or 0.007%,
Accuracy and Precision of Method
Cu added, gram
Cu found,
0.202160 0.140753 0.197196 0.229912 0.106743 0.114G99 0.0~3349 0.107!lR9
0.202108 0.140746 0. 1972Oll 0,229893 0.106723 0.114690
Cu assay,
7%
gram
99.974 99.995 100. 002 99.992 99.981 99,992 ion.01~4 100.003
n.m:i:ix 0.1079F2
Error, % -0.026 -0.005
+o.oin
-0.008 -0.019 -0.008 fo.004 f0.003
Figure I. Apparatus used for electro-
deposition of copper
The standard deviation of the individual determination is computed to be 0.013%.
Table
Solution
I
Sample A B
II.
Assay of Copper Solutions
Wt. s o h , grams
CU found, grams
Concn., grams of Cu per gram of solution
C D
10.41386 8.37104 11.85268
0,134008 0.153135 0.123120 0.174306
0.0147067 0,0147049 0.014i078 0.0147060 A V . ~ 0.0147064 Calcd. 0.0147065
I1
A B C D
11.01498 9.74982 8.07624 7.65503
0.260151 0.230319 0.190758 0,180832
0.0236179 0.02R6229 0.0236196 0.0236226 A V . ~o.oz302118 Cdcd. 0,0236195
111
A B C D
8.68315 8.40850 8.10731 7.60791
0.182952 0.177169 0.170833 0.160315
0.0210698 0.0210702 0.0210715 0.0210720 Av.- 0.0210709 Calcd. 0.0210700
9.11206
The standard deviation of the average is estimated to he 1.3 x 10-1 gram of copper per gram of solution.
930
.
ANALYTICAL CHEMISTRY
which is well within the precision of 0.01% required. The procedure was applied t o solutions of copper such as were used in atomic weight work. A sample of rod weighing between 1.0 and 1.7 prams was pickled successively in dilute HCI and in dilute HKOs, rinsed with distilled water, and finally rinsed with ethanol. The sample was dried, weighed on a microbalance to the nearest microgram, and transferred t o a 50-ml. Erlenmeyer flask which had been weighed along with a rubber septum on a semimicrobalance. The copper was dissolved in dilute nitric acid, diluted to 45 ml. with distilled water, a n d the solution was thoroughly mixed by swirling the flask. The flask was then stoppered with the septum, allowed to attain thermal equilibrium, and weighed on the semimicrobalance. From the weight of the copper and the weight of the solution, the concentration of copper was calculated as grams of copper per gram of solution. All weighings were corrected for air-buoyancy.
Samples of 8 to 10 ml. containing from 0.1 to 0.25 gram of copper were taken with a hypodermic syringe and needle. A second needle served as a vent. After transfer of the solution to a 6-dram vial, the syringe and needle were washed with distilled water, and the washings were caught in the vial which contained the bulk of the sample. The weight of the sample withdrawn was determined from the weight of the flask before and after removal of the sample. This system of aliquoting solutions was used to take advantage of the precision and sensitivity of a semimicrobalance. T o do this, it was necessary not to exceed the 100-gram capacity of the balance. A glass weight buret could not be used because of its excessive weight. On the other hand, the 50-ml. flask, stopper, and 45 ml. of solution had a total weight of about 75 grams. Ten grams of solution could be weighed to
better than 1 part in 100,000 by this procedure. Table I1 shows the results of these determinations. Three copper solutions were prepared and four samples were withdrawn from each solution. The copper present in each solution was determined by the method described. The averages of the four determinations for each solution are compared to the calculated concentrations. The agreement in each case is well within the required precision (and accuracy). The standard deviation of the average is gram of estimated to be 1.3 X copper per gram of solution, or in terms of percentage, about 0.007%. While it is possible to obtain precise assays with a standard deviation of less than 0.01% by this method for both copper metal and copper solutions, it is applicable only to relatively pure samples of copper. The copper should
be examined spectrographically or chemically for the elements which could electrodeposit with copper, and the weight of the electrodeposited copper should be corrected as required. LITERATURE CITED
(1) Am. Soc. Testing Mater., “ASTM Methods for Chemical Analysis of *Metals,” p. 422, Philadelphia, 1960. ( 2 ) Sandell, E. B., “Colorimetric Metal Analysis,” 3rd Ed., p. 443, Interscience, New York, 1959. ( 3 ) Shields, W. R., Murphy, T. J., Garner, E. L., J. Res. IVatl. Bur. Sld., 68A, 589 (1964). ( 4 ) Wilson, C. L., Wilson, D. W., “ComDrehensive Analvtical Chemistrv.” vel. i, p. 373, Elsevrer, London, 1962.
THOVAS J. MURPHY JOHN K. TAYLOR
Yational Bureau of Standards Washington, D. C.
Chelometric Determination of Rare Earths in Presence of Aluminum SIR: The rare earth elements have been titrated in weakly acidic solution with (ethylenedinitrilo)tetraacetic acid (EDTA) (2-4, 8, 9 ) and, more recently, (diethylenetrinitri1o)pentaacetic with acid (DTPA) (14). I n most of these investigations aluminum was absent. Fritz (4)titrated the rare earths potentiometrically a t a mercury metal electrode, reporting that acetylacetone or sulfosalicylic acid can be used to mask aluminum. Chernikov ( 3 ) titrated 10-mg. amounts of the rare earths visually, masking up to 3 mg. of aluminum with sulfosalicylic acid; larger amounts of aluminum obscured the end point, which was based on the red-to-yellow transition of 3,3’-bis-N,Ndi (carboxymethyl)-aminomethyl cresolsulfonaphthalein (Xylenol Orange). Jablonski and Johnson ( 7 ) titrated lead and zinc in the same way using acetylacetone to mask aluminum, which Kas present in somewhat less than equimolar concentrat ion; they mention that lanthanum and cerium are not masked. The present work discusses a method that can be used to determine rare earths in the presence of a large excess of aluminum. (Diethylenetrinitril0)pentaacetic acid (DTPA), which forms stronger chelates with rare earths-at least with cerium-than does E D T A ( I ) , is the titrant and Xylenol Orange is the indicator. Acetylacetone (2,4pentanedione) is used to mask the aluminum, which, under the conditions described, may be present in up to 500: 1 molar ratio with respect to the rare earths.
GENERAL METHOD
Standard 0.01M DTPA Solution. Dissolve 7.9 grams of t h e reagent in 500 ml. of water by adding strong sodium hydroxide solution until t h e p H is 10. Transfer the solution t o a 2-liter volumetric flask a n d dilute to volume. Standardize chelometrically against standard calcium chloride solution (prepared from primary standard calcium carbonate). I n t h e present work t h e titration was a t p H 12.5 with Calcon (Mordant Black 17, CI 15705) as an indicator (6). hlternatively, the solution may be standardized a t p H 5.0-5.5 against standard lead or zinc nitrate, using Xylenol Orange itself. Procedure. T o approximately 150 ml. of solution containing the rare earths a n d aluminum, add 50 to 100
Table I.
me. of ascorbic acid. Add 0.5 ml. of-acetylacetone for every 10 mg. of aluminum present, and stir until the solution is clear. Add hexamethylene tetramine in increments until the p H is between 5.0 and 5.5. (If the solution is strongly acidic, first neutralize with sodium hydroxide solution to about p H 2, then complete the adjustment with the hexamine.) Add 0.3 to 0.5 mg. of Xylenol Orange and titrate with standard DTPA solution. The color change a t the end point is from redpurple to yellow. TITRATION
Preliminary experiments with aluminum absent showed that Ce+3and La+3 could be titrated with DTPA to the Xylenol Orange end point over the pH range of 4.5 to 6.0; from the work of
Effect of Acetylacetone Concentration on Recovery of Cerium
Acetylacetone, X
A1 present,
mg. 40 50
75 a
Acetylacetone added, ml. 5 5 3 25 5 2.5 2.0 0.5 25
stoichiometric amount 11 11 7 45
9 5 4 1 30
Taken 20,00 25.00 25.00 5.00 5.00 5.00 5.00 5.00 25,oo
Cerium, mg. Found 19 !)2 24 92, 24 93 24 06 4 75, 4 74, 4 74 4 96, 4 98 5 01, 5 01, 5 02 5 00 5 03a 24 88
Fleeting end point.
VOL. 37, NO. 7, JUNE 1965
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931
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