V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5 the total observed wave height for nitroglycerin. Since the amount of 2-nitrodiphenylamine in the ponder is small, there is only a very small correction, and sometimes a t the instrument sensitivity used there will be no interference from this material. Most nitro compounds, such as dinitrotoluene, will be reduced within the voltage range of nitroglycerin; however, o-phthalate esters are reduced a t about -1.90 volts (8). Thus, nitroglycerin and o-phthalate esters can be determined either alone or in admixture by the polarographic method of analysis. No interference from nitrocellulose was observed during this study. The amount of nitrocellulose extracted by 95y0 ethyl alcohol appears to be a negligible factor in the analysis. Because oxygen produces a wave in the same voltage range as nitroglycerin, it must be removed Rith pure nitrogen prior to the polarographic analysis. I n the determination of nitroglycerin, the aliquot of the sample should be adjusted so that the wave height for nitroglycerin is a t least half of the full scale division for the instrument. This will give greater precision in measuring wave heights. For samples of double-base powder containing about 30% nitroglycerin a 5-ml. aliquot of the ethyl alcohol-diluted extract will give about a 150-mm. wave height a t a sensitivity of 0.150 Fa. per mm., using the Sargent Model X X I polarograph and a capillary with a value for m2/3t1’60fabout 1.35 mg.2’3se~.-1/2
901 ACKNOW LEDGhlENT
The authors wish to evpress their thanks to the Solid Propellants Branch a t the U.s. Naval Ordnance Test Station, Inyokern, Calif., for samples of powder used in this investigation, and to Rlichael D. Castronova for samples of pure nitroglycerin. This paper is published nith the permission of W.B. RlcLean, technical director of the U.S. S a v a l Ordnance Test Station. LITERATURE CITED
(1) Becker, W.
W., ISD.ESG. CHEM.,AXAL.ED.,5 , 152-4 (1933). ( 2 ) Fisher, R. -I., “Statistical Methods for Research Workers,” 10th ed., Oliver and Boyd, Edinburgh, 1946. (3) Fuller, H. C., “Chemistryand Analysis of Drugs and Medicine,”
pp. 736-8, Wiley, Xew York, 1920. (4) Hirschhorn, I. S.,ANAL.CHnf., 19, 880-2 (1947). (5) J.AKAF Analytical Chemistry Panel for Solid Propellants, private communication. (6) Kolthoff, I. AI., and Lingane, J. J., “Polarography,” Rev. ed., p. 317, Interscience, Ken. York, 1946. (7) Ibid., pp. 60-9. (8) Whitnack, G. C., and Gants, E. S. C., ASAL. CHEM.,25, 553-6 (1953). (9) Whitnack, G. C., and Gants, E. S. C., J . Am. Chem. Soe., 76, 4711-4 (1954). (10) Yagoda, H., IND.ESG. CHEM.,d s a ~ED., . 1 5 , 27-9 (1943). RECEIVEDfor review September 14, 1954. Accepted February 5 , 195%
Polarographic Determination of Traces of Copper, Nickel, Cobalt, Zinc, and Cadmium in Rocks Using Rubeanic Acid and 1-Nitroso-2-naphthol LLOYD E. SMYTHE and BRYAN M. GATEHOUSEl Chemistry Department, University of Tasmania, Hobart, Australia
Trace quantities of copper, nickel, cobalt, zinc, and cadmium may be separated from interfering elements by precipitation as metal rubeanates, followed by separation of cobalt as the nitrosonaphtholate and estimated by subsequent polarographic analysis. The utility of the method is discussed and a comparison is made with colorimetric and spectrographic methods.
T
RACE element studies in rocks using spectrographic and colorimetric methods gave values which differed seriously enough to warrant checking by a suitable polarographic method. The most promising method appeared to be that of Malyuga (3) for the “polarographic determination of copper, cadmium, nickel, zinc, and cobalt in minerals, soils, natural maters, and organisms.” Following the 7%-orkof Rby and ROy (6)rubeanicacidhasbeenused in several analytical methods, mainly for the determination of copper, nickel, and cobalt in the presence of iron, aluminum, and other interfering ions. The more stable, insoluble, metal rubeanates (copper, nickel, cobalt, zinc, and cadmium) are selectively and quantitatively precipitated in the presence of many other ions under carefully controlled conditions. However, Malyuga’s method applied to rock samples and synthetic solutions gave very poor recoveries including ill-defined polarographic waves in the range 5 to 400 p.p.m. of the elements. INVESTIGATION OF METHOD
If Malyuga’s method was to be compared with spectrographic and colorimetric methods, i t became apparent that an investigation of the variables involved was desirable. This investigation
was carried out ( I ) , and in addition an investigation of the formation and properties of metal rubeanates was commenced (6) and will be published later. Maximum recoveries of the rubeanates concerned were obtained a t p H 8 ( I ) . Adjustment of the solution prior to precipitation by adding 4-V ammonium hydroxide until slightly alkaline (3) did not offer sufficient control for reproducibility and maximum recovery. Losses also occurred where normal filtration and ignition procedures were employed (3). Centrifuging of the precipitated rubeanates and digestion with perchloric acid were found preferable to destruction of the rubeanates and filter paper with sulfuric acid and subsequent ignition ( 3 ) . It is probable also that losses on ignition might have resulted during the thermal decomposition of the rubeanates, as some rubeanates produce a red liquid containing part of the metal on destructive distillation ( 5 , 6). KOsignificant differences in recoveries of precipitated rubeanates were observed using times of precipitation of 12 and 36 hours a t 18’ C. Variation of the p H of the final supporting electrolyte used by Malyuga (3) showed that the procedure of adding a few drops of ammonium hydroxide, to make the final supporting electrolyte slightly alkaline, did not give consistent diffusion currents, particularly for copper and nickel. For euample, diffusion currents for a given concentration of copper in the supporting electrolyte, with changing pH, mere: pH 6.0 = 0.10 pa.; pH 8.0 = 0.145 pa.; p H 10.0 = 0.25 pa. Obviously, reproducibility with trace-element estimations could not therefore be obtained without careful control of final supporting electrolyte conditions. A t p H 7 to 8 copper is likely to be 1 Present address, Department of Inorganic Chemistry, N.S.W. University of Technology, Sydney, Australia.
902
ANALYTICAL CHEMISTRY
present as the hydroxide and the diammine. When ammonia is added and the pH moves towards 10 or 14, the tetrammine is also certainly present. A better defined two-step wave for copper is observed a t the higher pH's, whereas the waves are often ill defined a t p H 8 with trace quantities of copper. A final supporting electrolyte 1N with respect to ammonium hydroxide was found to be satisfactory in all respecta.
Table I. Solution No. 1 2 5 10 cu Cd 400 300 Xi 150 200 Zn 40 100 200 Coa 300 a Cobalt in each case
Composition of Solutions
3 4 5 6 7 8 9 1 40 70 100 400 300 250 200 250 200 I50 5 10 10 70 250 400 300 100 70 40 10 70 10 5 150 250 300 200 250 10 150 40 100 70 400 was separated and separately deterniined.
Table 11. Element cu Cd Ni Zn CO
0 150 100 J
400 J
Half-Wave Potentials
E112 v s . H g Anode -0.45 -0.79 -1.10 -1.32 -1.27
Ei/z vs. N.C.E. -0.69 -0.99 -1.30 - 1 56 -1.51
APPARATUS AND REAGENTS
Polarographic measurements were made with a Tinsley ink recording polarograph (V722/1) employing direct current amplification of the current passing through the solution in the polarographic cell. The recorder unit was a moving coil direct current pen-type milliammeter, the standard speed being 1 inch per minute corresponding t o a voltage change of 0.5 volt. The capillary constant, K, vas 24 in the supporting electrolyte employed containing dissolved air; drop time, 3 seconds; temperature, 25" C. The radius of the capillary orifice, p , determined in 0.1.W potassium chloride a t 25" C. using an open circuit was 25 microns. -411 pH adjustments were made using a Leeds and Sorthrup p H meter (7663-A1), with glass electrode. The procedure of Hills and Ives ( 2 ) was used to prepare the calomel half cells for checking half-wave potentials. Potassium chloride, lN, was used in these cells, and agar bridges were prepared. The best quality analyzed C.P. reagents were used throughout. Double-distilled water was used in all aqueous solutions, and blank determinations were run with all estimations. Reagents were: perchloric acid, 60%; citric acid, 10%; sodium hydroxide for p H adjustment, 500 grams per liter; ammonium hydroxide solution, specific gravity, 0.88; alcoholic rubeanic acid solution, 0.5% in ethyl alcohol; 1-nitroso-2-naphthol, 3% solution (0.38 gram of 1-nitroso-2-naphthol in 10 ml. of glacial acetic acid plus 2.5 ml. of ethyl alcohol); final supporting electrolyte stock solution, 13.38 grams of ammonium chloride plus 13.65 ml. of ammonium hydroxide solution (specific gravity, 0.88) made up to 100 ml. with distilled water; 10 ml. of this solution plus 1.25 ml. of 0,2y0 w./v. gelatin solution, placed in 25-ml. standard flasks for receiving the solutions of trace elements. The gelatin solution is prepared as a 0.2% w./v. aqueous solution which may be preserved for some time with a crystal of thymol. For cobalt a 0.4% w./v. solution of gelatin is used in place of the 0.2% w./v. solution, RECOMMENDED PROCEDURE
Place 1 gram of the carefully sampled and finely powdered rock sample in a platinum basin and add 10 ml. of distilled water, 2 ml. of perchloric acid (all perchloric acid digestions were safe under the conditions in this procedure), and 5 ml. of hydrofluoric acid. Evaporate the solution to dryness leaving a residue of metal salts and silica. Repeat the addition and evaporation. To the residue, add 4 ml. of concentrated nitric acid and 20 ml. of distilled wi,ter. Mix with a glass rod, transfer the contents of the basin to a centrifuge tube, and centrifuge. Transfer the supernatant solution to a 100-ml. beaker. Wash the residual silica twice with 5 ml. of dilute nitric acid, and combine the washings Tvith the solution in the beaker. Add 10 ml. of citric acid solution to the solution in the beaker, and adjust the pH to 8 to 8.5 using the sodium hydroxide solution. Add 10 ml. of rubeanic acid solution to the solution in the beaker, cover with a watch glass, and leave overnight to precipitate a t room temperature. Transfer the precipitated rubeanates and solution to a centri-
fuge tube, separate the precipitate, and wash with 1% ammonium chloride solution. Discard supernatant solution and washings. Transfer the precipitate to a Vitreosil crucible with the minimum amount of distilled water, evaporate to dryness, and add 2 ml. of perchloric acid. Cover with a watch glass and leave until the precipitate has decomposed. Evaporate the solution to dryness on a hot plate and dissolve the residue in 3 drops of concentrated hydrochloric acid. Evaporate to dryness and take up residue in 2 ml. of distilled water. Transfer to a 50-ml. borosilicate glass beaker and to the solution in the beaker add 4 ml. of glacial acetic acid and 0.5 ml. of 1-nitroso-2-naphthol solution. Make up volume to 15 ml. with distilled water, cover the beaker with a watch glass, and warm on a water bath a t 90" to 100' C. for 0.5 hour. Cool, and add 1 ml. of 12% hydrochloric acid (to dissolve the naphtholates of copper, cadmium, nickel, and zinc which mav have precipitated). Separate the cobalt precipitate by centrifuging, and transfer the summatant solution to a 50-ml. borosilicate glass conical flask. 'Kash the precipitate in the centrifuge tube 'iwice with 3 ml. of warm 12% hydrochloric acid and three times with 3 ml. of distilled water. Transfer the combined washings to the 50-ml. flask and evaporate the solution to dryness on a hot plate. Add 2 ml. of perchloric acid to the residue in the flask to decompose excess 1-nitroso-2-naphthol. Evaporate to dryness again, take up the rwidue in 1 ml. of concentrated hydrochloric acid, and wash the flask down with 5 ml. of distilled water. Make the solution in the flask slightly alkaline with concentrated ammonium hydroxide solution (odor just detectable) and transfer to the previously prepared supporting electrolyte in a 25-ml. standard flask. Transfer the previously separated precipitate of cobalt to the original Vitreosil crucible and decompose with 1 ml. of perchloric acid. Cover the crucible with a watch glass during decomposition. Evaporate the solution to dryness and take up the residue in 4 drops of concentrated hydrochloric acid. Make this solution slightly alkaline rvith concentrated ammonium hydroxide solution and transfer to the previously prepared supporting electrolyte in another 25-ml. standard flask. Take the polarograms using the requisite volume of solution from each 25-ml. standard flask, plus a crystal of sodium sulfite. Bubble hydrogen through the cell for 5 minutes, seal off the cell, and record composite polarogram for copper, cadmium, nickel, and zinc and a separate polarogram for cobalt. Measure and compare with calibration curves. STANDARDS
Stock solutions of 1000 p.p.m. of the various elements were prepared in the first instance from the pure metals. Two-stage
Table 111. P.P.hl. 5 10 40 70 100 150 200 250 300 400
Diffusion Currents, in Microamperes
Cu 0.08 0.11 0.24 0.30 0.40 0.60 0.83 1.02 1.18 1.60
Cd 0.04 0.03 0.09 0.14 0,235 0.325 0.44 0.57 0.69 0.93
Element Ni 0 03 0 04 0 14 0 23 0 38 0 57 0 77 0 96 1 17 1.56
Zn 0.04 0.06 0.13 0.26 0.38 0.61 0.82 0.96 1.16 1.70
Co 0.04 0.06 0.21 0.34 0.50 0.72 0.95 1.14 1.40 1.72
Linearity of id u s . concentration is satisfactory for all elements. Standards u p t o 1000 p.p.m. were prepared in the case of Co and Ni t o encompass the results on miscellaneous rocks, b u t are not recorded in Table 111. 0.4% w./v. gelatin solution is used t o suppress the Co maxima. Some curves do not extrapolate t o zero owing t o errors inherent in t h e method below 50 p.p.m. of the elements.
Table IV. Element Present,
P.P.M. 5
10 40 70
100 I50
200 250 300 400
a
Cu 20 i 10 17 i 12 33 f 11 55 f 20 so f 20 113 f 32 160 i 30 250 f 28 310 f 20 415 i 20
Summary Synthetic Solutions Ni 7 1 4 10f 5 27 i 10 59 i 19 78 f 20 124 & 18 16B i 28 214 f 32 274 f 30 356 & 40
Founda for Co 31. 3 7f 5 3 4 i 9 54 f 21 93 f 19 137 19 194 i 20 232 + 22 262 29 366 i 42
+
*
Zn 0
12 f 5 35f 5 5 6 i 9 105 f 10 164 f 8 212 i 13 236 i 18 306 f 22 387 i 19
Results given as means i standard deviation of mean.
Cd 31: 3 7f 5 3 5 i 5 66f 7 85f 9 130 i 20 177 i 20 230 18 270 i 20 382 f 20
*
V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5
903
81'2 6
4 '&'4
Rocks. Duplicate analyses and blank were carried out on each rock and where necessary a third determination was made. For the polarographic method, results are given in Table V as the means plus or minus standard deviation of mean. Table V includes values obtained by other workers employing different methods.
. 50 ..
.. .. ..
., .. ..
DISCUSSION
