constants, hardness, ductility, and strain aging of vanadium metal (5,6). ACKNOWLEDGMENT
The authors are indebted to Charles
C. Hill, James Hurd, and Lawrence L. Altpeter for performing some of the analyses reported in Table 11. LITERATURE CITED
(1) Albrecht, W. M., Mallett, M. W., ANAL. CHEM.26, 401 (1954).
(2) . . Beach, A. L., Guldner, W. G., Ibid., 31, 1722 (1959). (3) Bennett, S. J., Covington, L. C., Zbid., 30. 363 11958). _.(4) Booth, E., Bryant, F. J., Parker, A., Analyst 82, 50 (1957). (5) Bradford, S. A., Carlson, 0. N., Trans. Met. SOC.AZME, to be published. (6) Bradford, S.A., Carlson, 0. N., Trans. Am. SOC.Metals 55, 169 (1962). -.I
~
(7) Covington, L. C., Bennett, S. J., ANAL.CHEM.32, 1334 (1960). (8) Derge, G., J . Metab 1, 31 (1949). (9) Evens, F. M., Fassel, V. A., ANAL. CHEM.33, 1056 (1961). (10) Fassel, V. A., in “Gases in Metals,” Iron and Steel Inst. Spec. Rept. 68, 84101 (1960). (11) Faasel, V. A., Altpeter, L. L., Spectrochim. Acta 16, 443 (1960). (12) Fassel, V. A., Gordon, W. A., Tabeling, R. W., in “Symposium on Determhation of Gases- in Metals,” “ASTM Spec. Tech. Publ. 222, 41-60 (1957). (13) Guernsey, D. L., Franklin, R. H., Zbid., pp. 3-13. (14) Guldner, W. G., Beach, A. L., ANAL. CHEM.22, 366 (1950). (15) Hanin. M.. Rev. mRt. 1960, 1134. i l 6 j Hansen. W. R. , Mallett, M. W., ‘ Trzeciak, M. J., ANAL.CHEM.31, 1237 (1959) ,- - - - ,. (17) Horrigan, V. M., Fassel, V. A., Goetzinger, J. W., Zbid., 32, 787 (1960). (18) McDonald, R. S., Fagel, J. E., Balls, E. W., Ibid., 37, 1632 (1955).
(19) McKinley, T. D., private communication, “Procedure for Preparation of Absolute Oxveen Standard.’’ (20) Schutze, M ,: Ber. 77B,484 (1944). (21) Singer, L., IND.ENG.CHEM.,ANAL. ED. 12, 127 (1940). (22) Sloman, H. A,, Hal-vey, C. A., J. Znst. Metals 80, 391 (1951). (23) Smilev. W. G.. ANAL. CHEM.27. 1098 (19y5). (24) Smiley, W. G., Nucl. Sci. Abslr. 3, 391 (1949). (25) Smith, W. H., ANAL CHEM. 27, 1636 (1955). (26) Stanley, J. K., von Hoene, J., Wiener, G., Zbid., 23, 377 (1951). (27) Walter, D. I., Zbid., 22, 297 (1950). (28) Wilkms, D. H., Fleischer, J. F., Anal. Chim. Acta 15, 334 (1956). (29) Yeaton, R. A., Vacuum 2 , 1 1 5 (1952). >
-
RECEIVEDfor review June 4, 1962.. Accepted July 26, 1962. Contribution 1163. Work performed in the Ames Laboratory, U. S. Atomic Energy Commission.
Determination of Phosphate in Presence of Silicates by Molybdenum Blue Method C. Y. SHEN and D. R.
DYROFF
Inorganic Chemical Division, Monsanio Chemical Co., St. louis, Mo.
b Silicates present in built detergents interfere with the determination of phosphates by the molybdenum blue method. The rate of developing molybdenum blue color from silicate complexes, however, is slower than the development rate from phosphate complexes. This property, rate of color change, can be used as a measure of the amount of silicate present or used to estimate the actual quantity of phosphates. Correlation equations and response surfaces are used for simultaneous determination of silicate and phosphate contents based on the color intensity reading at a given time and the rate of color change.
T
blue color developed by reducing heteropoly compounds, such as ammonium molybdophosphate or molybdosilicate, has been extensively used for spectrophotometric determination of phosphates or silicates ( 1 , 2 , 5 - 9 ,IS). Because theabsorption spectrum of the phosphatemolybdenum complex is similar to that of the silicate-molybdenum complex ( d ) , special attention must be given to the interference of phosphate or silicate in determinations of either anion by the spectrophotometric method. Phosphates can be separated from silicates by a conventional precipitation HE
MOLYBDENUM
method, but this separation is time consuming. Short-cut methods are desired. I n silicate determinations, the phosphate interference can be avoided by proper p H control (6) or by destroying the molybdophosphoric acid complex with oxalic acid (9). There is no short-cut procedure for phosphate determinations in the presence of silicates. Absence of silicates in the test solution has been recommended for phosphate determinations (6); however, data from Woods and Mellon (IS) indicated that as much as 10 p.p.m. of silicon can be tolerated in determining phosphorus at the 1-p.p.m. level with about =t2yO accuracy. There is no known method which determines silicates and phosphates simultaneously by using the spectrophotometer. Heavy duty detergents produced today contain substantial amounts of phosphates and silicates. The phosphates in these detergents are generally ortho-, pyro-, and tripolyphosphatee. Quantitative determination of the various species of phosphates in a detergent product is important from the standpoint of process and quality control. This determination is usually done by using an ion exchange chromatographic procedure (7, 8, 11) to separate the phosphate species prior to the determination of phosphate in each fraction using the molybdenum blue spectrophotometric method. Any silicates
present owing to incomplete removal by a dialyzer (8) or reagent contaminations are concentrated in the blank and the orthophosphate fraction by the chromatographic separation, thus resulting in an erratically large orthophosphate content. When seeking a way to reduce the silicate interference, we noticed that the rate of molybdenum blue color development due to silicates is much slower than the rate due to phosphates. This difference can be utilized for simultaneously determining phosphate and silicate concentrations frorr, color intensities a t different times. EXPERIMENTAL
Reagents. Standard stock solutions-orthophosphate solution: 0.04620 gram of recrystallized KH2P04 and 10 ml. of p H 5.0 acetate buffer (78.51 grams of CH3COOK/liter adjusted to pH 5.0 with glacial CHsCOOH) were diluted to 1000 ml. with distilled water. Silicate solution: 0.9240 gram of Ka.SiO3.5HzO and 10 ml. of pH 5.0 buffer were diluted to 1000 ml. with distilled water. The resulting concentration is 0.2617 gram of SiOz/liter. Reducing agent, 0.15% I-amino-2 naphthol-4-sulfonic acid [commercial product was purified by recrystallization ( I d ) 1. Recrystallized 1-amino-2-naphthol-4-sulfonic acid (1.5000 grams) and 7.0 grams of NazSOs were dissolved in 75 ml. of water prior to mixing with a soluVOL. 34, NO. 1 1 , OCTOBER 1962
0
1367
Table 1.
Characteristic Values of Silicate Solutions
Silicate Absorbance concn. g. X 100 at SiOg/l. 40 min. 0.0104 2 . 6 . 2.9 5.21 5.6 0.0209 7.9, 7.6 0.0313 0.0418 10.0, 9.7 0.0522 11.6. 11.7 0.0627 12.9; 13.4
(&/AT) X 100 (rnin.-l)
0.043. 0.049
0.088; 0.082
0.129, 0.121 0.170, 0.160 0.190, 0.187 0.200; 0.205
tion consisting of 700 ml. of HzO and 90 grams of NaHSOs. The final solution was diluted to 1000 ml. and stored in a low-actinic glass flask. Direct light was avoided during this preparation. Sulfuric acid, 8N, and 225 ml. of concentrated HzS04, were diluted to 1000 ml. Ammonium molybdate solution: 100 grams of (NH4)6Mo7Ou.4Hz0 were dissolved and diluted to 1000 ml. Fresh solution should be prepared if insoluble material forms upon aging. Apparatus. Fisher Electrophotometer with a 650-mp filter and IO-mm. i.d. cuvettes was used. Distilled water was the reference. Laboratory glasswares were of borosilicate glass from which no detectable amounts of silica were picked up. PROCEDURE
Pure Phosphate or Silicate Solutions. The spectrophotometric pro-
cedure used fqr determining phosphates was given by Fiske and Subbarow (4). The absorbance of the molybdenum blue from phosphorus, using a 650-mp filter followed Beer's law for concentrations from 0 to 20.0 mg. of PzOs/liter. Appropriate amounts of the stock DhosDhate solution were thermally equilibrated a t room temperature (25' f: 1' C.). Three milliliters of 8N H 2 S 0 4 ,1 ml. of 10% ammonium molybdate, and 2 ml. of the reducing agent were added in sequence and the solutions were diluted to 100 ml. The absorbance of these solutions
Table II.
was constant for about 20 minutes after mixing and for a t least 4 hours thereafter, The phosphate concentration of an unknown solution can be estimated from the absorbance 40 minutes after mixing and a calibration curve of absorbance vs. concentration of phosphate. The precision in determining phosphate is about +2.001,. If a test solution is prepared in the above manner from the stock silicate solution, absorbance changes with time for a period of more than 10 hours after mixing. If absorbance is measured periodically and plotted us. time for the period between 5 minutes and 100 minutes after adding the reducing agent, a linear plot is obtained. The curve is actually logarithmic but the per cent reaction is sufficiently small in this time interval to permit a good straight line approximation as shown in Figure 1. Such a plot is completely described by the value of its slope AA/AT and the absorbance il a t a fixed time such as 40 minutes. These two values, measured for an unknown solution, permitted the determination of both the phosphate and silicate concentrations. Table I lists these values for various pure silicate solutions. The method of determining the unknown concentrations from A and AA/AT is described in the next section. Solutions Containing Both Phosphate and Silicate. The procedure
used for determining A and AA/AT for phosphate-silicate mixtures is the same as that for pure silicate solutions. These values were determined for a series of known mixtures. The presence of silicate causes the absorbance to increase with time, but AAIAT apparently decreases with increasing phosphate concentration for a given silicate concentration. Test results are shown (Table 11). These results together with the data from pure phosphate and silicate solutions were correlated by means of an IBM 704 computer, using a stepwise multiple regression program (3). The equations which give the minimum standard errors are as follows: Y I =1.168+190~1+4.88~~ 0.0141 3.43 51 - 0.427 2122 Yz
+
Characteristic Values of Solutions Containing Phosphates and Silicates
Absorbdnce
x '00
( A A I A T ) X 100
41.2, 40.0 28.4, 29.0 14.0, 13.9 23.5. 24.4 33.9; 33.6 16.6, 17.4 25.4, 24.0 25.6, 24.9 30.7, 31.0 12.5, 13.5 10.1, 8 . 9 17.1, 16.7
0.0710, 0.0676 0.0766, 0.0698 0.0660, 0.0671 0.1868, 0.1947 0.0314; 0.0328 0.0920, 0.0930 0.0634, 0.0683 0.0657, 0.0680 0.0166, 0.0171 0.104'3, 0.1028 0.0728, 0.0675 0.0341, 0.0337
at 40 min.
0.0627 0.0419 0,0209 0.0627 0.0209 0.0163 0.0209 0.0209 0.0052 0.0313 0.0209 0.0104
1368
5.79 3.85 1.93 1.93 5.79 1.93 3.85 3.85 5.79 0.96 0.96 2.89
ANALYTICAL CHEMISTRY
min. - 1
Estimated mg. Pz05/1. From From equations A alone 5.80 4.25 1.86 2.00 5.90 2.05 3.72 3.80 5.94 1.10 0.98 2.87
7.69 5.44 2.66 4.56 6.42 3.22 4.68 4.80 5.85 2.48 1.80 3.21
0 10
30
50 70 Time, min.
90
110
Figure 1 . Typical molybdenum blue color development from silicate-molybdenum complex (650 mp, 1-cm. cell)
where
Yl= absorbance X 100 at 40 min.
Yz = rate of change of absorbance X 100 ( A A I A T X 100 min.-l) z1 = concentration of silicates, g. Si02/l. z2 = concentration of phosphates, mg. P205/1.
These equations comply with the following desired restraints, although these restraints were not used in the original correlation: when z1 is 0, Y 1 is proportional to zz and Yzis a very small value; when xz is 0, Y1 and Y Zare proportional to zl. The standard errors of these equations are 1.113 and 0.0146 for Y1 and Yz, respectively. These values are very close to the constants in the correlation equations. In other words, the constants would approach zero as they should, if the errors in the determinations were negligible. A response surface plot of these equations was also made by the 704 comput2r as shown in Figure 2. This plot can be used to determine phosphate and silicate simultaneously when A and AA/AT are known. Yland Yzare obtained by multiplying A and AAIAT, respectively by 100. These two values determine a point on Figure 2. The ordinate and abscissa of the point so determined are the concentrations of phosphate and silicate, respectively. Alternatively the silicate and phosphate concentrations can be obtained by solving the equations given earlier, but the graphic method is more convenient. Illustrative Procedure for Detergent Analyses. It was mentioned before
that the standard ion exchange chromatographic-molybdenum blue spectrophotometric procedure (7) to determine the amounts of the various phosphate species in a sample cannot be used for detergents because of interference by silicate. The following modification of the standard procedure has been successfully applied and is given here as a particular example of the application of the methods described above. Dissolve about 4 grams of detergent in about 600 ml. of HzO in a I-liter volumetric flask. Add 10 ml. of pH 5 buffer solution and dilute to the mark. Shake well and store the solution a t 40' F. (4.44' C.) for about 2 hours. At this pH, sodium silicate is partially removed as insolubles, but there is little
degradation of the condensed phosphates. Transfer a 100-ml. aliquot of the top part of the solution to a separatory funnel, and wash three times with an equal volume of chilled chloroform. Use tt 10-ml. aliquot of the H20 phase for analysis. The separation of the phosphate species by ion exchange chromatography and the spectrophotometric analysis of all fractions except the orthophosphate and blank fractions are according to the standard procedure ( 7 ) . Residual silicate will collect in the blank and orthophosphate fractions which must, therefore, be analyzed by the following modified spectrophotometric procedure. Adjust the volume of each solution to 200 ml. Add 7.5 ml. of 8 5 HzS04, 2.5 ml. of 10% ammonium molybdate solution, and 5 ml. of reducing agent as rapidly as possible, adding the reducing solution last, and immediately dilute to 260 ml. Measure the absorbance 40 minutes, 70 minutes, and 100 minutes after adding the reducing solution. Plot absorbance us. time, and draw the best straight line. Determine the absorbance a t 40 minutes ( A ) and the time rate of change of absorbance (AA/ AT, min.-’) Y , = 100 A . Y z = 100 A A / A T . By interpolation, locate the unique point on Figure 2 corresponding to the measured values of Y 1 and YB. The ordinate of this point is the corrected phosphate concentration in the fraction. If a corrected absorbance is desired, locate the point having the same ordinate but with abscissa zero, and estimate the value of Yi a t this point on Figure 2. Division by 100 gives the corrected absorbance. The phosphate composition of the sample is calculated by the usual method (7) using the corrected values of absorbance or phosphate concentration for the ortho and blank fractions. This procedure mas applied to a detergent formulation typical of anionic heavy duty detergents which consisted of 50% sodium phosphates by weight, Myc actives. 0.5% sodium carboxylmethyl cellulose, 1.5y0 sodium toluene sulfonate, 6% sodium silicate, 16% sodium sulfate, and 8% HzO. The phosphates present were ortho-, pyro-, I
/
/
’
Table 111.
Determinations of Phosphate Species in Simulated Detergent Samples
Sample No.
B
A
Orthophosphate, yoof total PZOS Actual 0.2 ~~. . Analysis Unmodified (4.5) hlodified 0.3 Pyrophosphate, % ’ of total P z O ~ 3.6 Actual Analysis Unmodified (3.0) 3.2 Modified
C
F
7.8 (12.4) 7.9
0.4 (3.4) 0.5
4.0 (8.7) 4.0
10.9
1.5
(8.6)
( 1, O )
P.9 (6.8)
7.1
7.9 (6.5) 6.8
13.0 (11.4) 12.0
0.0 (0.0) 0.0
8.1 (9.0) 9.5
9.~ 1 . 7.
(90.1) 92.7
63.5 (60.5) 63.7
97.0 (93.4) 97.1
75.2 (71.8) 75.5
0.0 (0.0) 0.0
19.5 (19.3) 20.3
5.8
9.0
~
tripoly-, and trimetaphosphates. The proportions of each type of phosphate were varied in the six samples, analyzed as shown in Table 111, but the total phosphate present was constant,. Also shown in Table I11 are the analytical results using the standard colorimetric procedure (7) both with and without the modifications described above. DISCUSSION
Foj the orthophosphatesilicate solutions used in the correlation, the PZOS concentrations estimated by the modified procedure are given in Table I1 along with the values estimated directly from the absorbance, ignoring the presence of silicates. The latter values are about 1 mg. of PzO$liter too high. The average error of the corrected values is only 0.1 mg. of Pz05/liter. In Table I11 the average errors in the analysis of a typical detergent (in yo of total Pz05) using the unmodified procedure were 4.2, 1.4, 1.3, and 1.95 respectively, for ortho-, pyro-, tripoly-, and trimetaphosphate. Using the 1
E
1.5 (5.6) 1.8
5.7 (9.6)
Tripolyphosphate, yo of total P z O ~ 83.4 Actual 41.4 Analysis Unmodified (42.2) (81.8) 44.0 85.2 Modified Trimetaphosphate, % of total PZOS Actual 54.8 0.0 Analysis Unmodified (50.3) (0.0) Modified 52.5 0.0
I
D
I
I
1.1
modified procedure, the average errors were 0.1, 1.1, 1.1, and 0.7 in the same order. Thus, the accuracy was improved for all fractions but especially for the orthophosphate fraction as expected. In determining silicates in the presence of phosphates, the probable error is about +0.01 gram of Si02/liter. Although the accuracy is not good enough for some quantitative determinations, the proposed method is simple and may be used as a preliminary estimation. For pure silicate solutions a t a concentration below 0.04 gram/liter, however, the method is accurate enough for quantitative determinations. The particular data, equations, and plots in this paper are given only for purposes of illustration and may not apply to other apparatus. Also there are several ways in which the accuracy of the method could be improved if necessary. A spectrophotometer would, no doubt, give better results than the filter photometer which was used. Also greater care could be given to reproducibility in adding and mixing the color developing reagents. Under the conditians employed, silicates formed almost immediately a light green solution which converted slom-ly to the molybdenum blue color. The observed increase in absorbance with time is probably due to both the conversion of silicomolybdous acid to hyposilicomolybdous acid and a simultaneous conversion of 8-molybdosilicic acid to a-molybdosilicic acid (IO). These reactions are sensitive to many variables such as molybdate concentration, pH, and temperature.
LITERATURE CITED SiOe Concentration, 9/-?
Figure 2.
Determination of PzO6 and Si02 concentrations from absorbance 100 CYl)and rate of change of absorbance X 100 (Yz)
X
(1) Am. Pub. Health ASSOC.,Inc., New York, “Standard Methods for the Examination of Water, Sewage, and Industrial Wastes,” 10th ed. (1955). VOL. 34, NO. 1 1 , OCTOBER 1962
* 1369
(2) Boltz, D. F., Mellon, M. G., IND. ENG.CHEM.,ANAL.ED. 19,.873 (1947). (3) Efroymson, E. F., “Stepmse Multiple Regression Program,’’ available from SHARE-IBM program, Esso Research and Engineering Co., July 1, 1958. (4) Fiske, C. H., Subbarow, Y., J. Bid. Chem. 66, 375 (1925). (5) Kahler, H. L., IND.ENG. CHEM., ANAL.ED. 13, 536 (1941).
(6) Kitson, R. E., Mellon, M. G., IND. ENG.CHEM.,ANAL.ED. 16, 466 (1944). ( 7 ) Kolloff, R. H., A . S.T. M.Bull. 237, 74 (TP-94-TP-100) April 1959. (8) Lundgren, D. P., Loeb, N. P., ANAL. CHEM.33, 366 (1961). (9) Schwartz, M. D., IND.ENG.CHEM., ANAL.ED. 14,893 (1942). (10) Strickland, J. D. H., J . Am. Chem. SOC.74, 862, 868, 872 (1952).
(11) Weiser, H. J., J . Am. SOC.34, 124 (1957).
Oil Chemists’
(12) Welche2 F. J., “Organic Analytical Reagents Vol. I, p. 229, Van Nostrand, New York, 1947. (13) Woods, J. T., Mellon, M. G., IND. ENG.CHEM.,ANAL.ED. 13, 760 (1941).
RECEIVEDfor review April 16, 1962. Accepted August 3, 1962.
Optical Emission Spectrographic Method for Analysis of Microgram Deposits on Electron Tube Parts ALFRED M. LIEBMAN Radio Corp. of America, Elecfron Tube Division, Harrison, N . 1.
b This paper describes a rapid, precise, and accurate spectrographic method for the determination of seven elements (Bo, Sr, Ca, Mg, Mn, Si, Ni) found in small sublimed deposits on electron-tube parts. These deposits constitute a variable matrix because the above elements vary over unusually wide concentration ranges (from tenths to hundreds of micrograms). Matrix effects are suppressed by a relatively high concentration of cobalt in the solution used to remove the deposit. The cobalt serves as a buffer and an internal standard in a modified graphite spark technique. The applicability of this method for the quantitative analysis of deposits on other substrates is also discussed.
s
form on the internal parts of an electron tube during its processing and operating life. When these deposits form on the plate, they can cause secondary emission. When they form on the insulators, they can cause interelement leakage. The effects of films which form on grids depend on the tube type and the composition of the film. Until a short time ago, these tube deposits could only be qualitatively or semiquantitatively analyzed by rapid and convenient techniques. Although the results of these analyses were helpful in many cases, they Kere not accurate enough for many critical tube studies. Thus, it was necessary to develop a new quantitative method having much improved precision without sacrificing the speed necessary for the routine analysis of many samples. The elements which had to be determined in microgram quantities by this method-Ba, Ca, Sr, Mg, Mn, Si, and Ni-were chosen for several reasons. Barium, calcium, and strontium were selected because they are the major constituents of the cathode MALL DEPOSITS
1370
ANALYTICAL CHEMISTRY
coating which partially sublimes during tube operation. The sublimation rate of these three elements is in turn influenced by the reduction of their oxides by magnesium, silicon, and manganese which diffuse from the cathode itself. Because these reducing agents may also sublime, they were included in the analysis. The analysis of nickel was also required because direct sublimation of the nickel cathode base material could also occur. A literature search showed that many attempts had been made to analyze sublimates in receiving tubes. A wide variety of conventional and imtrumental methods has been developed. Only the barium in tube deposits could be determined by a wet microchemical method ( 3 ) . Mass spectrometric methods either reported qualitative data (15) or were calibrated to give ratio measurements (1) of Ba to BaO and Sr to SrO. Radioactive tracer techniques (6) giving qualitative data have been reported. Although a tracer technique (fl)which provided quantitative results for barium, calcium, and strontium has been developed, the sublimate analysis cannot be performed until 1 week after the tube has been subjected to the required treatment. This week delay is necessary to allow the Bal@ and its radioactive daughter, Lala, time to attain radioactive equilibrium. Seutron activation techniques (19) have been employed for sublimate analysis, but this method requires preliminary chemical separations which are time-consuming. Two methods based on electrical measurements have been reported. The first method (12) detects increased thermionic emission from a tungsten filament target; however, this method is applicable only to the analysis of barium sublimates. In the other technique ( I 6 ) , only the electrical resistance of the sublimed film can be determined. An opticalemission spectrographic ( I 7) method
has been developed for the determination of barium and strontium. Jaycox (9) later extended this method to a general semiquantitative technique which included all other chemical elements of interest. This latter method, however, involves several operations after the removal of the deposit from the tube, such as evaporating the solution in a weighed amount of buffer and firing a t 700” C., and is more time-consuming than the method described here. illso, there are mixing errors involved in this procedure which are discussed later in this paper. An x-ray emission spectrometric method (2) developed a t RCA can only be used in the analysis of Ba, Sr, Ni, and Mn in tube deposits. This concludes a representative survey of the literature on the analysis of tube sublimates. Although more papers have been published on this subject, no one as yet has reported an analytical method which is capable of determining microgram quantities of the seven elements in electron tube sublimates with the convenience, rapidity, and precision necessary for tube problem work. Accordingly, a new analytical method was needed for this purpose. An optical emission spectrographic method was chosen because of its excellent sensitivity and good precision a t low concentration ranges. EXPERIMENTAL
The tube component on which the sublimate is deposited is placed in a commercially available cylindrical plasinch (available comtic vial 1 X mercially from Spex Industries, Inc.). Structural members of most tubes (grids, plates, shields, and micas) can be placed inside this container by cutting or bending the parts. Then 0.50 ml. of a 2% by volume nitric acid solution containing 0.600 mg. per ml. of cobalt is pipetted into the vial. The vial is closed and shaken for 1 minute. This procedure dissolves the sublimate on the surface of the tube element without
