cases, This problem will be discussed a t length in a future report. Investigations on the feasibility of using conducting glass as well as other visible range transparent conducting materials for FMIR plate-electrodes is also in progIt would be of considerable ress, interest to obtain the charge transfer spectra of metal complexes as well as other species in the electrical double layer on electrolysis. The results of this work will be published at a later date. ACKNOWLEDGMENT
The authors are endebted to Paul A. Wilks, Jr., of the Wilks Scientific Corp., South Norwalk, Conn., for his interest in this project and for the donation of the Wilks Model 12A Double Beam Internal Reflectance attachment for this project. We also thank T. Kulyana, University of California at Riverside, and R. A. Osteryoung, North American Aviation Science Center, Thousand Oaks, Calif., for their helpful discussion about this work; and Jeffery L. Huntington for some preliminary research on the electrochemical behavior of germanium.
LITERATURE CITED
(1) Adams, R. X., Symposium on Electrode Reaction Mechanisms, Division of Analytical chemistry, 145th Meeting, ACS, New York, Sept. 1963. Abstract p. 7B. (2) DeFord, D. D., Division of Analytical Chemistry, 133rd Meeting, ACS, San Francisco, Calif., kpril 1958. (3) Delahay, P., New Instrumental Methods in Electrochemistry,” Interscience Publishers. New York. 1954. (’$) Fahrenfort, J., Spectrochim.’ Acta 17, 698 (1961). (,5 ) Galus, Z., Adams, A. N., J . Am. Chem. SOC.84, 2061 (1962): (6) Gerischer, H., in “Advances in Electrochemistry and Electrochemical Eneineering.” P. Delahav. ed.. Vol. I. pp.- 139-252, Interscience,’ New Yorkj 1961
( 7 ) Hansen, W. N., ANAL. CHEM. 35, 765 (1963). (8) Harrick, N. J., Ibid., 36, 188 (1964). (9) Harrick. N. J.. Ann. N . Y . Acad. h i . 101, 928 (1963). (10) Harrick, N. J., J . Phys. Chem. 64, 1110 (1960). (11) Holmes, P. S., “Electrochemistry of Semi-Conductors,” Academic Press, New York, 1962. (12) Kaflatsky, B., Keller, R. B., Inst. ATews, Perkin-Elmer Corp., Norwalk, Conn., 15, No. 2, l(1964). (13) Katritzky, A. R., Jones, R. A., J . Chem. SOC.1960, p. 2942.
(14) Kuwana, T., University of California, Riverside, private communication, 1965. (15) Kuwana, T., Darlington, R. K., Leedy, D. W., ANAL.CHEW 36, 2023 (1964). L. H., Geske, D. H., J . Chem.
vision of Analytical Chemistry, ’145th Meeting, ACS, New York, Sept. 1963. Abstract p. 8B. (19) Sharpe, L. H., Inst. News, PerkinElmer Corp., Korwalk, Conn., 15, No. 4, 1 (1965). (20) Sharpe, L. H., Proc. Chem. SOC. (London) 1961, p. 461. (21) Stock, J. T., J . Chem. SOC.1949, 586. (22) Wilks, P. A., Jr., S. Norwalk, Conn., private communication, 1964. HARRYB. MARK,JR. B. STANLEY PONS Department of Chemistry The University of Michigan Ann Arbor, Mich. RECEIVEDfor review October 18, 1965. Accepted November 9, 1965. Research supported by a grant from the U. S. Army Research Office, Durham, Contract No. DA-3 1-124-ARO-D-284.
~~
Determination of Molybdenum in Ferrous Alloys by Atomic Absorption Spectrometry SIR: Following upon the work of David (1, 2 ) , who examined various factors affecting the determination of molybdenum by atomic absorption spectrometry, the suggested analytical procedure was applied to the estimation of molybdenum in several types of ferrous alloy. Errors of more than the indicated figure of 10% were often obtained, with the majority of results tending to be low and it appeared that the recommended addition of 2000 p.p.m. aluminum and 4 ml. concentrated HNOa to all solutions to control interferences was not always effective. The differences in results could probably be ascribed to differences in optical arrangement and flame conditions between the Perkin-Elmer Model 303 equipment used in the present work and the original apparatus of David (1). I n addition, however, preliminary experiments indicated that the molybdenum absorption was extremely sensitive to small changes in solution composition and i t was probable that these solutions formed a complex interfering
ion system of the type described by Firman (3), in which the magnitude of an interference by one ion upon another, and also the further effects produced by the presence of additional ions could not safely be predicted for all concentration levels from trials at one level. Moreover, the method of successive additions, normally a reliable device for overcoming interferences due to variable composition, also failed in a number of the analyses. This determination has therefore been re-examined, and the use of ammonium chloride as a new and more general interference suppressing agent for molybdenum is suggested. EXPERIMENTAL
Apparatus and Operating Conditions. These are given in Table I. T h e experimental conditions are essentially those recommended previously (1) except that there is a case for adopting the line Mo 3798.3 A. instead of the more sensitive Mo 3132.6 A. The same beam energy can be achieved at a lower
gain control setting and hence the null meter noise is reduced. The sensitivity is still adequate for the analysis and there is a small improvement in precision of absorption readings; interference effects are similar at both wavelengths.
Table
1.
Experimental Conditions
Perkin-Elmer Model 303 spectrophotometer Wavelength, 3798.3 A. Range, ultraviolet Slit, 3 (0.3 mm.) Lamp current, 25 ma. (or nearest steady value) Air Supply, 25 p.s.i. Flowmeter. 5.5 Acetylene Supply, 8 p s i . Flowmeter, 9.0 Burner height adjusted for maximum absorption Sample uptake, 3 ml./min. Scale expansion, 1 Sensitivity, 1.3 p.p.m. Mo/lyo absorption
VOL. 38, NO. 1 , JANUARY 1966
121
Table II.
Added Fe, p.p.m.
Variable Extraneous Ion Effect (Fe)
Absorbance (Mo 3132.6 A,)" 100 p.p.m. Mo 100 100 p.p.m. p.p.m. 'Mn Mo 100 5 ml. p.p.m.Mn HN03/100ml.
Nil
+ +
+
0.270 0.187 0.162 0.167 0 . i75 0,225
250
500 750 1000
3000
0.081
0.304 0.457 0.434 0.428 0.354
a Absorbance for 100 p.p.m. Mo with no additions = 0.500.
Reagents. All reagents were anaminutes, dilute, filter, and make u p lytical grade. A stock solution concombined filtrate and washings to a taining 2000 p.p.m. Mo was prepared standard volume with distilled water. by dissolving 1.829 grams of ammoAdjust either this final volume or the nium molybdate (82.0y0MOOa) in 500 weight of alloy taken so that the molybdenum content is of the order 20 to 40 ml. of distilled water. Stock solutions of manganese (1000 p.p.m.), p.p.m. PROCEDURE A. Add 1.0 gram aluminum (20,000 p.p.m.), and iron (20,000 p.p.m.) were prepared by NHdCl to 40 ml. of sample solution and dissolving spectrographically pure dilute to 50 ml. With the experimental conditions shown in Table I, compare metals in a minimum volume of concentrated HC1 before dilution to volume t h e absorbance with the absorbances of a range of standard ammonium molybwith distilled water. Procedure. SAMPLEPREPARATION.date solutions containing 0, 10, 20, T o 1.000 gram of alloy in t h e form of 30, 40, and 50 p.p.m. Mo and 1.0 gram NH&1 per 50 ml. drillings or turnings, add 15 ml. aqua regia (2 parts concentrated HCl, 1 % Mo in sample = part concentrated " 0 , ) and warm gently t o commence reaction. When p.p.m. Mo found x V reaction has ceased, boil for 2 to 3 8000
Table 111.
Interference Suppression by NH4Cl Absorbance (3798.3 A,)" (a) 25 p.p.m. Mo 2000 (b) Soln. (c) 25 p.p.m. (d) Soln. p.p.m. A1 (a) 2 . Mo 1 ml. (c) 2 . Added Mn, Added Fe, 4 ml. HNOd NH4Clf "OS/ NHnClT 100 ml. p.p.m. p.p.m. 100 ml. 100 ml. 100 ml. 0.073 0.073 0.061 0.045 0 0 0.048 0.073 0.054 0,029 0 1000 0.040 0.072 0,057 0.031 0 2500 0.041 0.071 0.049 0.036 0 5000 0.010 0.073 0.056 Sn n 0.041 -0.073 0.046 0.051 1000 0.028 50 0.073 0.038 0.048 0.031 2500 50 0.071 0.039 0.044 0,037 5000 50 0.073 0.014 0.054 0.037 0 100 0,073 0.044 0.051 0.027 1000 100 0.073 0.036 0,046 0.030 2500 100 0,071 0,037 0.044 0.036 5000 100 Absorbance for 25 p.p.m. Mo as aqueous ammonium molybdate solution = 0.073.
+
Table IV.
BCS.246 No additions 2y0 NHpCl BCS.214 No additions 2% NHiCl BCS.251/1 No additions 2% NHpCl
+
+
+
+
Method of Additions. NH&I Effect
+
0 0232 0.0413
0.0693 0,0919
0,1238 0,1522
0,1720 0,2048
1.75 2.88
2.89
+
0.0211 0.0496
0.0437 0,1035
0.0708 0.1566
0.0953 0.2104
0.24 0.28
0.27
+
0.0383 0.0501
0.0914 0.1018
0,1406 0,1530
0,1925 0.2055
1.12 1.47
1.51
I
Table V.
Analysis of Standard Alloys yo Molybdenum Found Nominal Procedure A Procedure B
Alloy Low alloy steel BCS 251/1 255/1 256/1 258/1 Mild steel BCS 271 273 275 BCS 172/2 Cast iron 214/1 Mn-Mo steel 220/1 7% W, 5y0 Mo steel 246 Nb-Mo 18/12 steel
122
ANALYTICAL CHEMISTRY
1.51 0.30 0.53 0.83
1.54 0.30 0.85
1.47 0.32 0.52 0.83
0.095 0.43 0.27 5.20 2.89
0.086 0.43 0.25 5.12 2.85
0.44 0.28 5.31 2.88
0.50
where V is t h e volume corresponding t o 1 gram of sample. PROCEDURE B. Prepare four 50-ml. solutions, each containing 25 ml. of sample solution, 1.0 gram NH4Cl and additions of 0,15,30, and 45 p.p.m. Mo, respectively. Plot the mean absorbance of each solution against the added Mo concentration to obtain the M o concentration of the diluted sample "blank" solution.
yo Mo in sample
=
p.p.m.
k o found x
V
5000 RESULTS AND DISCUSSION
Interference Studies. The earlier published data ( I ) showed that iron and manganese were the principal interfering ions present in ferrous alloys to have a serious effect on molybdenum absorption. Tests were made on the various possible combinations of ions involving iron, manganese, aluminum, nitric acid, and molybdenum, the concentrations being varied in these solutions between the limits Mo 0 to 100 p.p.m., M n 0 to 200 p.p.m., A1 0 to 5000 p.p.m., Fe 0 to 15,000 p.p.m., and 0 to 5 ml. concentrated NOa- (as "0,) acid per 100 ml. Several interesting results were observed. Thus, although the effects of Mn, Fe, Al, and NOS- individually upon X o absorption were as reported previously ( I ) , the addition of two, three, and four extraneous ions produced more complex effects, and with the Perkin-Elmer Nodel 303 apparatus, A1 and NOa- in combination gave very large depressions of 130 absorptionabout 40% by 2000 p.p.m. A1 and 4 ml. concentrated HNO, per 100 ml. Table I1 illustrates a typical complex effect, where the addition of Fe may produce either depression or enhancement of Mo absorption in a Mo-Mn system, depending upon whether nitric acid is present. I n each case, also, there is an optimum addition. There is no indication that, with these experimental conditions, the addition of
A1 and/or HKO3 would necessarily stabilize the Mo absorption in systems where Ptln, Fe, and possibly other ions were varying in concentration. Effect of NH4+Addition. The significant feature of the above experiments was t h a t in no case was a n absorption reading obtained equal t o t h a t given by the same molybdenum concentration in an aqueous ammonium molybdate solution. Further experiments, in which the object was to achieve maximum Mo absorption from a given sample solution, showed that if the solution was just neutralized with ammonia the full Rlo absorption was obtained; the same result could be achieved more conveniently, and with no risk of hydroxide precipitation, by the addition of ammonium chloride. The experimental results given in Table I11 show that the addition of ,hH4Cl effectively destroys the interference by M n and Fe on Mo absorption
in solutions analogous to those produced by dissolution of a ferrous alloy sample. The addition is less effective in the presence of added A1 and NO,-, and it is also apparent that the latter combination does not overcome interference by Mn and Fe in these conditions. Method of Additions. Analytical results obtained by the method of additions were often seriously in error, despite the fact t h a t a straight line calibration graph was obtained. The errors may arise from the change in ratio of Mo:Mn, Mo:Fe, and Mo :acid as successive Mo additions are made. The effect of the NH4Cl addition is to produce a shift in the calibration line to give a different, usually higher, concentration intercept. Results illustrating this are presented in Table IV. Analysis of Standard Alloys. T h e results of trial analyses on a number of British Chemical Standard alloys (Bureau of Analysed Samples, Ltd.,
Middlesborough, England) are presented in Table V. The proposed method has proved to be simple and accurate, with a precision of the order 3 to 5%. The method of additions (Procedure B) appears slightly more accurate in these tests, while direct comparison with prepared standards (Procedure A) is slightly more rapid, There is no clear advantage in either technique and both appear satisfactory for routine metals analysis. LITERATURE CITED
(1) David, D. J., Analyst 86, 730 (1961). (2) David, D. J., Nature 187, 1109 (1960). (3) Firman, R. J., Spectrochim. Acta 21, 341 (1965).
R.A. MOSTYN A. F. CUNNINGHAM Chemical Inspectorate Royal Arsenal London, S.E. 18 England
Determination of Extractables Content of Nylon 6 by Differential Refractometry and Determination of Caprolactam Monomer Content in Nylon 6 Extractables by Infrared Spectrophotometry SIR: Methods previously reviewed in the literature for the estimation of cyclic oligomers or caprolactam or both in nylon 6 extractables have generally been based on the weight differences and often involve the application of special techniques. Anton ( 1 ) evaporated the aqueous extract of nylon 6, dissolved the residue in tetrafluoropropanol, and determined the oligomers content in the residue by measuring the absorbance of the N H deformation band a t 1550 cm.-' using the base line method. Our results, however, show that caprolactam interferes with this determination. Caprolactam can be partly separated from oligomers by carbon tetrachloride extraction and determined by the sharp absorbance band it exhibits in solution a t 3430 cm.-' The total caprolactam present in the extractables solution may then be calculated from the caprolactam CClJH20 distribution ratio. Oligomers content is found from the total nylon 6 aqueous extractables by difference. By differential refractometry the total aqueous extractables are determined to a precision of =!=lo%, and, by infrared spectrophotometry, the caprolactam content itself is determined to a precision of *2%. EXPERIMENTAL
Reagents and Instruments. T h e cyclic oligomers standard was obtained by concentrating the aqueous extract of nylon 6 to dryness, washing t h e residue several times with distilled water, and drying the residue a t 60"
to 70' C. under vacuum for 4 hours. The caprolactam standard, supplied in crystalline form by National Aniline Division, Allied Chemical Corp., contained less than 75 p.p.m. water. Carbon tetrachloride, supplied as Baker and Adamson reagent grade by General Chemical Division, Allied Chemical Corp., was used without further treatment. Tetrafluoropropanol, obtained from Organic Chemicals Department, E. I. D u Pont de Nemours and Go., was redistilled (b.p. 109' C.), and dried over molecular sieve to 500 p.p.m. water. Differential refractive indices were measured using a Waters Associates Model 1000 digital differential refractometer equipped with an external highsensitivity null meter and a sealed reference liquid cell of distilled water. Measurements were made a t 38" C., which is the temperature of the instrument's optical system. Infrared spectra were recorded using either a PerkinElmer Model 221 double-beam infrared spectrophotometer with sodium chloride optics, or a Beckman Model IR 8 double-beam grating infrared spectrophotometer. Differential spectra were recorded using matched 5 c m . solution cells with near infrared silica windows, and quantitative measurements were made a t the following instrument settings: Perkin-Elmer Model 221 : attenuator speed, 1100; slit program, 927; gain, 3; speed, 32 (60 cm.-l/ minute) ; suppression, 4; scale, 1 X ; and source, 0.36 amps; Beckman Model IR 8: gain, 4.5; and speed, slow (136 cm.-l/minute). Procedure. A calibration curve was made for the differential refractometer by using a series of solutions
ranging from 0.025 to 1.00 gram of dry caprolactam per 250 ml. of aqueous solution (equivalent to 0.5 to 20% extractables of a 5-gram sample), Five grams of polymer or finish-free yarn of unknown extractables content were extracted with distilled water in a Soxhlet extractor for 5 hours. The solution was filtered and diluted to volume (250 ml.). The differential refractive index of the solution was measured, and the extractables content calculated from the calibration curve. Normally, a 100-ml. aliquot of the aqueous extract solution remaining was concentrated without boiling to 15 to 20 ml. and diluted upon cooling to 25 ml. For samples of extractables content of 4 to 20%, however, a 25-ml. aliquot of the extract solution was used without concentration. This was placed in a 125-ml. jacketed separatory funnel maintained a t 30.0 i 0.1' C., and 7.0 f 0.1 grams of potassium chloride (for "salting out") were added. After 75 ml. of carbon tetrachloride were pipetted into the funnel, the mixture was shaken thoroughly and allowed to separate. The carbon tetrachloride layer was withdrawn and diluted to 100 ml. A calibration curve was made by using a series of solutions ranging from 0.2 to 10 mg. of caprolactam per 100 ml. of carbon tetrachloride solution. Each solution was filtered through glass wool into the solution cell, and the spectrum from 3800 to 3200 cm.-* was recorded using the solvent as reference. Using a base line drawn between 3500 and 3400 cm.-l, the absorbance a t 3430 cm.-1 was measured. Solutions of unknown concentrations were analyzed in the same way. The caprolactam content VOL. 38, NO. 1, JANUARY 1966
123
