to reduce the oxygen content. Thebe solutions were mixed and the reaction was allowed to take place in total darkness. The absorption spectra, the intensity of color, and the speed at vr-hich the maximum intensity was reached were compared with solutions saturated with oxygen and developed in direct light. There was no detectable difference in any of these samples and neither light nor air was required for the interaction of cyanamide 15 ith this particuiar reagent. Sodium nitroprusside, Naz[Fe(Cn’)~NO], will also react with cyanamide to produce a color (4) with an absorption maximum a t 530 mp. The xork of Raudisch (8, 3) and Fearon (6) suggests that this reaction may in fact be due to the photocatalyzed partial hydrolysis of nitroprusside to form pentacyanoaquoferrate, [Fe(CN)&Tz01-3, which in turn reacts with
cyanamide. This aquo analog of pentacyanoammineferrate differs only in the substitution of a molecule of mater for one of ammonia and has been reported (6) to have the same general reactivity as that of the ammino analog. There has appeared in the literature no report on the use of sodiuni nitroprusside for the quantitative analysis of cyanamide. In our own laboratory, various combinations of pH, buffers, concentration of nitroprusside, and illumination were employed, but the optimum sensitivity obtained when nitroprusside was used to determine cyanamide was only 25y0 of that obtained with pentacyanoammineferrate. ACKNOWLEDGMENT
The authors are indebted to E. A. Peets and Allan Shurr for technical
assistance and to E. H. Dearborn for encouragement and continued interest during the course of this work. LITERATURE CITED
(1) ASSOC.Offic. Agr. Chemists, “Methods of Analysis,” 7th ed., p. 15, 1950. (2) Baudisch, O., Ber. 62, 2706 (1929).
(3) Ibid., 68, 769 (1935). (4) Buchanan, G. H., Barsky, G., 2. angew. Chem. 44,383 (1931). (5) Capitani, C., Gambelli, G., Chinz. e ind. (Milan)35,890 (1953). ( 6 ) Fearon, W. R., Analyst 71, 562 (1916).
(7) Feigl, F., Gentil, V., Mikrochim.
Acta 1959, 44, (8) Inaba, H., Japan Analyst 3, 196 (19.54). (9) Milks, J. E., Janes, R. H., ANAL. CHEW28,846 (1956). (10) Sato, M.,Sato, J., Fujesaiva, T., J . Electrochem. SOC. Japan 22, 370 (1954). \ - - - - I
RECEIVEDfor review April 7 , 1960. Accepted July 25, 1960,
Chlorate I rn p w rities
n I EUGENE A. BURNS1 Jet Propulsion laboratory, California lnsfitute o f Technology, Pasadena, Calif.
A rapid, accurate spectrophotometric method for the determination of small amounts of chlorate impurities in ammonium perchlorate is based on the production of a colored haloquinone b y reaction of chlorate with benzidine in a hydrochloric acid medium. The effect of temperature and time of color development on the intensity of the colred habquinone has been investiuted. A method for the determination of trace amounts of reducing impurities [sulfite) or oxidizing impurities [chlorine) of reagent grade hydrochloric acid is reported.
applied to rocket-grade ammonium perchlorate, which requires better than 10% relative accuracy. Accordingly, the benzidine-chlorine test (1) has been investigated for applicability and adaptability to a spectrophotometric method which would be suitable for use by semitechnical personnel. The spectrophotometric procedure is based upon the color produced when chlorate ion is converted to chlorine: C108-
+ 5C1- + 6H+
+
3C12 -I- 3HzO
(1)
The chlorine subsequently oxidizes benzidine to 8, yellow haloquinone (1) many years the procurement and quality control of ammonium perchlorate for employment as a solidpropellant oxidizer have lacked a rapid, accurate, and sensitive analytical method for the determination of chlorate impurities. Because of the low chlorate content, conventional “wetchemistry” techniques (3) are inadequate. A colorimetric method sensitive to low chlorate levels is currently in use (g), but is unsatisfactory when OR
e
ANALYTICAL CHEMISTRY
However, because of favorable reaction rates, the experimental parameters can be adjusted so that a desirable quantitative method is obtained. This method is not selective for chlorate; corrections must be made for the presence of oxidizing agents capable of oxidizing chloride to chlorine-e.g., bromate. APPARATUS
A Beckman Model DK-2 ratiorecording spectrophotometer was employed. If the room temperature is subject to variations greater than &t5O C., a constant temperature bath is necessary. REAGENTS
NH2 +C1-
The method is complicated by decomposition of the haloquinone:
NHnfC1-
-+
decay product^
Bemidine Solution, 0.4yG. (y./v.). Dissolve 2.0 grams of benzidine in 2.5 ml. of reagent grade hydrochloric acid and 100 ml. of distilled water, heating 1 Present address, Propulsion Department, Poulter Laboratories, Stanford Research Institute, Menlo Park, Calif.
Figure 2.
Effect of temperature and color-development time on absorbance
solution and 3.00 ml. of benzidine solution to a 100-ml. volumetric flask and filling to the mark with 1 t o 1 hydrochloric acid solution. WVE
LENQTH, mp
Figure 1. Spectra of haloquinone formed by reaction of a large excess of benzidine in 19% (w./w.) hydrochloric acid with varying amounts of potassium chlorate
if necessary; cool, and dilute to 500 ml. with distilled water. Potassium Chlorate Solution, 1,OQ X 10-4M. Transfer e weighed portion of 0.122 gram of reagent grade potassium chlorate t o a 1-liter volumetric flask, dissolve in water, and dilute to volume. Transfer a 25.00-ml. aliquot of this solution to a 250-ml. volumetric flask and dilute to volume. Calculate the exact molarity of the potassium chlorate solution as follows :
w
co
-&ere W chlorate.
=
=
1220
weight of potassium PROCEDURE
Weigh approximately 6.0 grams of the ammonium perchlorate sample to the nearest 0.01 gram and dissolve in water in a 100-ml. volumetric flask. Dilute to volume. Transfer a 5.00-ml. aliquot of the benzidine solution to the flask, and fill the flask t o the mark with 1 to 1 hydrochloric acid solution. Mix well, transfer to a clean 1.00-em. Corex sample cell, and determine the absorbance of the solution, A,, 12 to 15 minutes after mixing, a t a wave length of 438 mp against a reference solution contained in a similar sample cell. Prepare the reference solution by mixing 3.00 ml. of the benzidine sohtion and 97 ml. of 1 to 1 (v,/v,) hydrochloric acid solution, To evaluate the impurities present in the hydrochloric acid solution, concurrently obtain the absorbance of a solution, A,, 12 to 15 minutes after mixing, prepared by transferring 5.00 ml. of stock chlorate
CALCULATION
Calculate the per cent ammonium chlorate in ammonium perchlorate as follon~s: Per cent ammonium chlorate = 0.1730
1-(
+ 1015 C, FV
- 0.69B
(4)
where
A,
A,
= absorbance of sample solution =
1 = W = C, = B =
after developnient of color for 12 to 15 minutes absorbance of solution containing stock potassium chlorate after development of color for 12 to 1.5 minutes path length, em. weight of sample, grams concentration of potassium chlorate solution used per cent ammonium bromate in sample is 0, increase the weight of
If A , sample. If A , > 0.7, decrease the weight of sample. RESULTS
The absorption spectra obtained by adding varying amounts of potassium chlorate to benzidine solution using this procedure are shown in Figure 1. The maximum of the absorption curve a t 435 to 440 mp varies as a function of color development time but is reproducible for a specified period of color development. A study of the absorbance a t 438 mp as a function of color development time is shown in Figure 2 . At room temperature, the maximum of this curve is broad and is essentially constant for 11 t o 15 minutes of color development. It was observed that temperature
Figure 3. Absorbance at 436 mp after color development a t room temperature for 12 minutes as a function of potassium chlorate concentration
strikingly affected the absorbance or A color-development time curve. “semiquantitative” technique was established to study thia effect. The experiments were not performed at truly constant temperatures; although volumetric flasks containing the solutions were maintained a t constant temperatures, the sampling and absorbance measurements mere made a t room temperatures. Therefore, the solutions in the absorption cells were either warming up or cooling down to room temperature during measurement (approximately 20 to 30 seconds). The effect of temperature, as shown in Figure 2, is readily seen. Although the maximum value for the absorption is the same, the rate of build-up and decay is markedly affected by temperature. It may be surmised from these data that within a temperature range of 20” to 25” C., the absorbance should be read 12 t o 15 minutes after the reagents are mixed. Because the cell compartment in many spectrophotometers is heated by heat transferred from other spectrophotometer components, the cell and cell holder should not be placed YOL. 32, NO. 13, DECEMBER 1960
1801
I.
Table
Analysis of Synthetic Ammonium Perchlorate-Chlorate Samples
Moles XClOa Taken X 106
NH&lO,, Grams
Total Chlorate Taken as KHaClOa, %
KH&IOa5 Error, Found, % A, % 1 .ooo 0.00 A , = 0.406b 1.000 6 00 0.0172 0.416' 0.0172 0,400 6.00 0.0070 0.077 0.0074 +O. 0004 0 .GOO 6.00 0.0104 0.184 0.0105 +o.ooos 0.800 6.00 0.0138 0,288 0.0135 -0.0003 1.400 6.00 0.0240 0 637 0.0236 -0.0004 0.800 2.40 0.0315 0.261 0.0318 $0.0003 1 000 2.40 0.0426 0.430 0.0426 O.oo00 0.800 1.20 0.0679 0.295 0.0686 +O ,0007 I 000 1.20 0,0849 0.402 0.0840 -0.0009 1.400 1.20 0.1187 0.642 0.1170 -0.0017 a Calculated by Equation 4, with AO= 0.406, (I, = 1.00 x 10-4M. * Values used to calculate per cent ammonium chlorate in reagent ammonium perchlorate. I
I
I
Table I!.
Determination of Reducing Impurities (Sulfite) in Reagent Grade Hydrochloric Acid
Moles Chlorate Consumed x 107 1.59 2.08 0.93 1.30 Table 111.
HC1 Used Batch I, 1:l Batch I, 2 : s Batch 111, 1:1 Batch 111, 2 : l
6.00
7.00
10.00
38% (W./W.) HCl Used, G. 54.6 72.9 54.6 72.9
Redudng Impurities (Sulfite),
%
x 104 7.0 6.9 4.1
4.3
Determination of Reducing Impurity (Sulfite) of Reagent Hydrochloric Acid, Batch I
Chlorate Concn., Moles/L. X 106 2.00 3.00 4.00 5.00
HC1 Used, Mi. 92 92 92 92
1:1 HC1
Zsed, *%1.
92 92 92 92 92 92 89
38% HCl
Used, G. 54.6 54.6 54.6 54.6 54.6 54.6 52.9
Grade
Absorb-
Reducing Impurity (SuIfite),
ance 0.050
6.9
76
0.158
x
10'
7.3
0.287 0.406 0.519 0,634
6.9
6.8 7.0
7.0
7.2
0.987
Average sulfite impurities = 7 . 0 x 1O-s%, Standard deviation = 0.17 x 10-6 yo.
in the compartment until immediately before measurement. The absorbance after a 12-minute reaction time a t 25' C. is plotted in Figure 3 us. concentration of chlorate in the solution. The curve follows the least-squares line ( d ) , A = 0.117~ 0.186, where c is expressed in moles per liter X lo4. The concentration intermole per liter. The cept is 1.59 X absorbance index of the haloquinone, as obtained from the slope of this curve, i s 1.17 X lo5 liters per molecm. and compares excellently with that calculated from the data of Boltz and Holland, 1.18 x IO6 liters per moleem. (1). The results of analyses of synthetic chlorate-ammonium perchlorate samples are reported in Table I. From the difference in absorbance of solutions containing a known amount of potassium chlorate in the presence and absence of ammonium perchlorate, the ammonium chlorate content of the 1802
e
ANALYTICAL CHEMISTRY
reagent grade amnionium perchlorate used in these experiments was calculated to be 0.00028%. The determination may easily be completed in 20 minut,es; when more than one sample is to be analyzed, the time of analysis per sample ratio is greatly reduced. DISCUSSION
I n the procedure described above, advantage has been taken of the rates of reactions shown in Equations 1 to 3, so that color intensity is maximum at 11 to 15 minutes after the reagents have been mixed a t room temperature. Boltz and Holland (1) have studied the effect of acidity upon the formation of chlorine by oxidation with chlorate. They observed that in solutions containing a t least 21% hydrogen chloride the reaction is complete in 8 to 10 minutes, in 24 minutes for 16.8%,, and there is no reaction for 10 minutes a t
a 12.6% level. To obviate long analysis time, all solutions used in this work contained at least 19% hydrogen chloride. If the chlorate is converted to chlorine prior t o addition of colorforming agent, the color formation is almost instantaneous and the fading of the color (Equation 3) begins immediately. These facts indicate that the reaction rate of Equation 2 is very rapid in comparison with the reactions given in Equations 1 and 3. However, if the reaction given in Equation 1 is not complete prior to addition of the color-forming agent, the rate of evolution of chlorine closely parallels the rate of fading and a steady-state condition results 11 minutes after mixing, i n which the intensity of coloration is constant. The color intensity fades a t 15 minutes after mixing, because all the chlorate has been consumed. Thus, it is possible to obtain quantitative measurements which are nearly independent of color-development time. The absorbance-concentration curve (Figure 3) does not appear to follow the Lambert-Beer-Bouger law, because i t does not go through the origin, but rather has a concentration intercept. Consumption of chlorate and/or chloTine by impurities in the benzidine was Tuled out because the absorbance was invariant as a function of benzidine concentration. However, a similar experiment revealed that impurities present in hydrochloric acid were Tesponsible for the intercept. The absorbance-concentration curve obtained using 2 to 1 hydrochloric acid solution was shifted to the right with a concentration intercept equal to 2.08 X mole per liter. The trace amounts of sulfite salts in reagent grade hydrochloric acid (Mallinckrodt, 0.0001%; Baker, 0.0001%; Du Pont, 0.00008%) are sufficient to cause this effect:
503-2
-
+ Clz $ H20 2C1- +
" 3 4 -
+ H+
(6)
Conversely, it is possible to use this observation to advantage to determine the amount of sulfite (or reducing impurities) present in hydrochloric acid. Sulfite impurities in two different batches of reagent grade hydrochloric acid were determined using the following expression:
% sulfite
= 24000
a/W
where a = moles of chlorate consumed TP = %,eightof 3870 (w./w.) hydrochloric acid used
These data are shown in Table 11.
It is inconvenient and unnecessary to set up a calibration curve for each batch of acid used to evaluate the moles of chlorate consumed. Instead,
the sulfite impurities of reagent grade hydrochloric acid may be calculated from only one point of the calibration curve by w e of the following equation:
% suEte
=
(C
- 8.528)(0.00240)
PV
(7)
where c
=
analytical concentration of chlorate, moles per liter X 106
A = absorbance of solution W = weight of 38% (w./.cv.) hydrochloric acid used, grams
The sulfite impurity of reagent grade hydrochloric acid, Batch I, for each of the calibration curve points shown in Figure 3 has been calculated and is listed in Table 111. The average of these results is 0.000070%.
Similarly, hydrochloric acid may also contain small amounts of free chlorine. Because this oxidizing impurity and the reducing impurity would not be compatible, only one impurity would exist in the solution. If the term c - 8.528 in Equation 7 is negative, chlorine is in excess and its content may be calculated by :
yo chlorine
=
(8.52A - ~ ) ( 0 . 0 0 2 1 3 )
w
(8)
There were no oxidizing impurities in three batches of hydrochloric acid examined during the course of this investigat’ion. ACKNOWLEDGMENT
The author thanks Frances D. Chang and Lois L. Taylor, who performed
some of the determinations required for this paper. LITERATURE CITED
(1) Boltz, D. F., Hollpd, W. J., “Colorimetric Determnation of Nonmetale,” D. F. Bolta, ed., pp. 172-3, Interscience, Kew York, 1958. (2) Joint .Army-Navy Specification for
Ammonium Perchlorate, JAN-A-192 (March 1, 1945). (3) “Scott’s Standard Methods of Chemical Analysis,” N. H. Furman, ed., 5th ed., Vol. I, pp 274-6, Van Nostrand, New York, 1939. (4) Youden,. W.,, J., “Statistical Methods for Chemsts, pp, 40-2, Wiley, New York, 1951.
RECEIVED for review November 20, 1959.
Accepted August 5, 1960. One phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract NASw-6, sponsored by the National Aeronautics and Space Admhktration.
A N e w Spectrophotometric Procedure for the Microdetermination of Methyl Chloride MARGARET REDFORD-ELLIS and JAMES E. KENCHI Departmenf o f Pathology, Manchester University, Manchesfer, England
b A procedure, based on the Fujiwara reaction for chloroform, is described whereby methyl chloride is converted into an unknown material absorbing at 365 mp b y treatment with pyridine and alkali. Quantities of methyl chloride in the range 2 to 80 pg. either in gaseous form or in aqueous solution can be measured accurately. The procedure is nonspecific and all alkyl the molar halides tested reacted j absorptivity of the chromogens produced by different alkyl halides differed greatly. The nature of the reaction has been investigated b y using pyridine homologs. Although it was not possible io isolate the reaction product, the evidence suggests that N-methylpyridinium chloride combines with one or more molecules of pyridine a t the a-position of the latter.
Methods employed for the determination of halogenated hydrocarbons in air and in biological tissues have been various and relatively nonspecific. For high concentrations of methyl chloride in air (40% v./v. or more), Allison and Meighan (1) absorbed the methyl chloride in acetic acid and measured the change in volume. Smaller quantities in the range 0.5 to 10 mg. have been
measured by conversion of the organic halide to inorganic halide by combustion, hydrolysis, or reduction with hydrogen, followed by determination of inorganic halide by titration, nephelometry, or colorimetry. Greater sensitivity appeared possible using the Fujiwara (3) reaction, which takes place when one of many halogenated hydrocarbons or derivatives is
t
to manometer
to pump c
T
PRESEST INVESTIGATION was undertaken because a sensitive met,hod was needed for the determination of methyl chloride in order to study its metabolism. The determination of methyl chloride is complicated by the fact that it is a gas at ordinary laboratory temperatures and is toxic to man (9). HE
1 Present address Department of Chemical Pathology, $he Medical School, University of Cape Town, South Africa.
Figure 1. Diagram of vacuum-line assembly used for handling methyl chloride (not to scale) VOL. 32, NO. 13, DECEMBER 8960
*
