Quantitative Infrared Analysis of Solids in Potassium Bromide Using an Internal Standard STEPHEN E. WIBERLEY Deparfmenf o f Chemistry, Rensselaer Polyfechnic lnsfifufe, Troy,
N. Y
JAMES W. SPRAGUE and JOHN E. CAMPBELL Behr-Manning Corp., Troy, N. Y.
In previous methods for the quantitative infrared analysis of solids in potassium bromide i t has been necessary to obtain pellets of reproducible or known thicknesses. By using an internal standard in potassium bromide the necessity for determining the sample thickness i s eliminated, thereby simplifying the preparation of calibration curves and the actual analysis. Potassium thiocyanate was the most suitable internal standard of all the materials investigated. The effect of grinding the sample and the internal standard i s the most critical factor involved in the quantitative analysis of solids in potassium bromide. Using potassium thiocyanate as an internal standard, quantitative methods have been developed for the analysis of poly(viny1 chloride)-poly(viny1 acetate) copolymers and of free fatty acid in aluminum soaps. This general method employing potassium thiocyanate as an internal standard should have wide application for the quantitative analysis of solids b y infrared absorption.
I
N PREVIOUS METHODS for the quanti-
tative infrared analysis of solids in potassium bromide it has been necessary to obtain pellets of reproducible or known thicknesses (4, 8). Barnes and associates ( 3 ) developed a quantitative infrared method, using a n internal standard in Kujol mulls, with which it is not necessary to determine the sample thickness. This assumption is readily justified by consideration of Beer's law (7). The absorbance of the known material to be assayed a t X k I d 1 be given by
Ak
=
ak
bCk
and the absorbance of the internal standard at wave length A, by A,
=
asbcs
Xow, dividing the first equation by
the second, bck u s bc,
d_ k = -U k
.a8
The 6's cancel and because ak and a. are both constants a t the w v e lengths 210 *
ANALYTICAL CHEMISTRY
a t which the measurements are made, and c,, the concentration of the internal standard, is constant, these constants can be accumulated in an overall constant, K , and
Hence, a plot of A r / A , us. ck will give a straight line. I n this method i t is not necessary to determine u k and a, or even to know c, exactly to obtain an empirical working curve. This paper is concerned with the study of suitable materials for internal standards for potassium bromide and the variables involved in obtaining reproducible quantitative results.
4CI
I
I
1
Figure 1. Effect of grinding time in vibrator on ratio of 5.75-micron band o f poly(viny1 acetate) to 4.7micron band of potassium ferricyanide
EQUIPMENT A N D MATERIAL
Infrared measurements were made 011 either of two Perkin-Elmer Model 21 recording spectrometers. Instrument settings were as follows: slit auto; resolution 950, gain 5 to 6, response 1 , source amperes 3.0 to 3.4, speed 3 to 5 , and suppression 0. The potassium bromide pellets n-ere pressed in a Hilger cylindrical, evacuable die. This die is available commercially along with 3 pellet holder from the Jarrel-drh Co., Boston, Mass., and was selected because of its simplicity and comparatively IOT eo+ The pellets pressed in this die are 13 nim. in diameter. All pellets were pressed with a hydraulic laboratory hand press of 10ton capacity. Following the recommendations of Kirkland ( 8 ) , the sample incorporated in potassium bromide was ground in a Wig-L-Bug amalgamator. The potassium bromide was powdcred infrared quality obtained from the Harshaw Chemical Co. (Lot 1342). The samples mere weighed on a semimicro Ainsworth balance with a sensitivity of 0.01 mg., while the potassium bromide \vas weighed on the usual hinsworth macrobalance. EXPERIMENTAL TECHNIQUE FOR PELLET PREPARATION
All pellets were pressed in the'same manner. A known amount of potassium bromide sample mixture was placed in the partially assembled Hilger die and leveled with a small spatula. The
plunger was then inserted and rotated in both directions and a t the same time a slight pressure xas applied. The plunger mas then carefully removed and the steel disk inserted into the chamber. The steel plunger was reinserted and the remainder of the die assembled. The die was evacuated before the pressure was applied. A pressure of 15,000 pounds on the plunger was maintained for 5 minutes while the system was under T-acuum. SELECTION OF INTERNAL STANDARD
For universal application in quantitative infrared work a n internal standard should possess the following important qualities. -1 simple spectrum with a fev sharp bands Stability to heat and to excessive moisture pickup Ease of reduction to a particle size smaller than the wave length of the incident radiation with a reasonable amount of grinding Read)- availability, not contaminated v i t h varying amounts of impurities Preferably a comDound that produces clear i\-indoks Preferably a nontoxic compound
Substances recommended for internal standards in Kujol mulls have been n-alanine ($), calcium carbonate (9), lead thiocyanate ( I I ) , and naphthalene ( 5 ) . Of these compounds only the lead thiocyanate fulfills the first require-
be seen from Figure 3, the ratio reaches a constant value after 5 minutes' grinding. Of all the materials investigated potassium thiocyanate fulfilled best the requirements for a good internal standard, so it mas adopted for all subsequent work.
nient. &Alanine and naphthalene have too complex spectra and calcium carbonate has several broad bands which might overlap available analytical absorption bands. Lead thiocyanate was briefly investigated but abandoned because it changes from white to yellow on heating a t 120" C., it blackens the windows of potassium bromide on standing, and it is toxic. Using the work of Miller and Wilkins (10) as a guide, other substances were investigated. Sodium and potassium cyanide were found to contain large amounts of carbonates, in agreement with Miller and JVilkins, paper. The toxic nature of these compounds was a second limitation. Potassium ferricyanide was considered a logical choice and yielded clear windowg. It was used extensively in initial work, until it was observed that the band intensities a t 4.7 and 4.9 microns were not consistent from sample to sample. As a check on this change of band intensities, 100-nig. portions of a mixture containing 0.5y0 vinyl acetate and 0.5% potassium ferricyanide by weight in potassium bromide were ground in the vibrator for increasing lengths of time. The ratio of the absorbance of the 4.7micron band of the potassium ferricyanide to the 5.75-micron band of the vinyl acetate was plotted against the grinding time. As shown in Figure 1, the ratio does not level off even after 20 minutes. I n this same grinding series, it rvas found that the 4.7- and 4.9micron bands changed radically from their initial intensities. This effect is illustrated in Figure 2. Finally sodium and potassium thiocyanates were investigated and the potassium was finally selected because the sodium thiocyanate picks up moisture too rapidly for accurate weighings. The same technique used t o test the potassium ferricyanide was applied to a potassium thiocyanate-vinyl acetate mixture in potassium ;bromide. As can
a
APPLICATION T O TYPICAL SAMPLES
I n analyzing solids by infrared absorption, two weighing techniques may be used, depending upon the amount of material available. If a large amount of material is available samples may be weighed on the usual analytical balance to a n accuracy of 0.1 mg. and diluted and intimately mixed with potassium bromide. A suitable aliquot may then be weighed and added to more potassium bromide to form the pellet for infrared analysis. If a small amount of material is available, the sample must be weighed on a semimicrobalance and then added to the potassium bromide for direct pellet preparation. I n order to test the general application of an internal standard in quantitative analysis, a poly(viny1 acetatechloride) polymer was analyzed for acetate content and an aluminum soap was analyzed for free fatty acid. Analysis of Poly(Viny1 AcetateChloride) Polymers. Poly(viny1 acetate-chloride) polymers are of industrial importance, and it is difficult t o determine the acetate content of such polymers by chemical methods. The spectra of poly(viny1 chloride) and poly(viny1 acetate) are shown in Figure 4. Poly(viny1 acetate) has a strong carbonyl band a t 5.S microns 11-hich is absent in poly(viny1 chloride). This band was selected for the quantit a t i r e measurements.
A base mixture of potassium bromide containing the internal standard, potassium thiocyanate, was prepared by adding 0.2% by weight of the potassium thiocyanate (preground for 10 minutes
x
c m
* * e.
u-
LO
" Y"
Y
4
4
I
5
I 10 Grindin9
Time
I
1
15
20
I
25
In Minutes
Figure 2. Effect of grinding time in vibrator on ratio of 4.7- to 4.9-micron bands of potassium ferricyanide
in the Wig-L-Bug) to 4 grams of the potassium bromide which had been dried a t 120' C. This mixture was ground in an agate mortar for 5 minutes and then divided into six equal portions. Each portion was then ground in the Wig-LBug for 2 minutes, recombined, and remixed again in the mortar. Again the mixture was divided into six portions, reground in the Wig-LBug, remixed, and then stored in a desiccator over phosphorus pentoxide. This rather elaborate mixing procedure was deemed necessary because it is obvious that the accuracy of this method is based on the constant concentration of the internal standard.
I5O
100
I
I -
0 50
i
0
I Gnndinq
Time
in
Minutes
Figure 3. Effect of grinding time in vibrator on ratio of 5.75 micron band of poly(viny1 acetate) to 4.8-micron band of potassium thiocyanate
A standard calibration curve was then obtained by adding 10.01 mg. of poly(vinyl acetate) (also preground for 10 minutes in the Wig-L-Bug) to 90 mg. of the potassium bromide-potassium thiocyanate mixture and grinding for 3 minutes in the Wig-L-Bug. Samples of this mixture weighing 0.39, 0.58, 1.02, 1.51, and 2.55 mg. mere diluted with the internal standard mixture to a final weight of 100 mg. These samples were then ground for 10 minutes in the Kig-LBug and pressed into pellets, and the absorbance values recorded a t 4.7 microns (the internal standard band) and a t 5.8 microns [the poly(viny1 acetate) band]. The ratios of these absorbance readings uncorrected and corrected for the base line absorbance are plotted in Figure 5 . I n order to check the reproducibility of any point on this curve, five pellets containing 1.02 mg. of poly(viny1 acetate) were pressed in the standard potassium bromide-potassium thiocyanate mixture. These pellets were measured on the Perkin-Elmer Model 21 spectrometer a t Rensselaer Polytechnic Institute and on the one a t Behr-Manning Corp. The data are tabulated in Table I. -4ppsrently better reproducibility was obtained on the Behr-Manning instrument than on the one at Rensselaer Polytechnic Institute. A known mixture containing 5.00% poly(viny1 acetate) and 5.0Oj, poly(viny1 chloride) was then analyzed, and 5.05% VOL. 29, NO. 2 , FEBRUARY 1957
21 1
poly(viny1 acetate) was found. Three commercial samples of poly(viny1 acetate)-poly(viny1 chloride) copolymer Tvere then analyzed, with the following results. The reported values were obtained from the Bakelite Co. (2). According to Atkinson ( I ) : The values for poly(viny1 chloride) and poly(viny1 acetate) (per cent by weight) given in the table are approximate values based on the weight of charged monomers together with average values obtained by spot checking certain batches of the final copolymer by chemical analysis for chlorine. T i t h the exception of VNCH and T’AGH, the chemical composition of a particular poly(viny1 chloride-acetate) sample can be accurately determined by analyzing for chlorine, calculating the chlorine found as vinyl chloride, and subtracting this value from 100 to give the vinyl acetate. TT’e determined the chlorine content of these vinyl chloridevinyl acetate copolymers by decomposition in a Parr sodium peroxide bomb Ivith subsequent determination of the inorganic chloride by any of the several classical gravimetric or volumetric procedures. We d o not have any satisfactory method for determining accurately the vinyl acetate content.
No evidence is given of the accuracy of these reported values. I n spite of this limitation: the agreement is very good. Free Fatty Acid Content of Aluminum Soaps. Aluminum soaps are of considerable importance in t h e field of lubrication and t h e determination of free fatty acid content is important. Free fatty acid content may be determined directly by extraction with cold iso-octane a t 0’ C. a n d subsequent titration of t h e acid, or indirectly by determination of the aluminum content (6). With an infrared internal standard method this analysis is readily accomplished. TKOmilligrams of the aluminum soap are weighed on the seminiicrobalance and diluted with a potassium bromide-potassium thiocyanate mixture containing 0.14y0 potassium thiocyanate. This mixture had been prepared like the one in the poly(viny1 acetate) determination. The mixture is ground in the Wig-L-Bug for 10 minutes and the pellet is pressed and measured in the spectrometer. The ratio of the absorbance of the internal standard band a t 4.7 microns to that of the free fatty acid band a t 5.8 microns is determined. From the standard calibration curve the per cent free fatty acid is determined. The spectrum of the potassium bromide-potassium thiocyanate mixture is shown in Figure 6 along with the spectrum of a n aluminum dilaurate soap containing free fatty acid. A calibration graph was obtained by measuring
212
ANALYTICAL CHEMISTRY
-
~~
Table I.
Statistics on Five Replicate Determinations
.4bsorbance Ratios, A S . S ~Behr-Manning /A~.W R. P. I. Uncorr. COll. Uncorr. Corr.
Poly( vinyl
Acetate), % 0.102 0.102 0.102 0 102 0.101
0 0 0 0
593 610 618 595 0 610 hIean value 0 605 2 u 0 021
0 595 0 580 0 591 0 594 0 590 0 590 0 012
0 527 0 550 0 553
0 536 0 544 0 542 0 021
0 538
0 0 0 0
529 533 540 531 0 534 0 009
POLY(VI NY L ACETATE ) I
I
I
I
I
1
4
6
8
IO
12
14
WAVELENGTH, M I C R O N S Figure 4. Spectra of poly(viny1 chloride) and poly(viny1 acetate) in potassium bromide containing 0.2y0potassium thiocyanate
I
---
I
U n c o r r e c t e d Absorbance Values Corrected Absorbance Values
I
vj,
0.1
0 %
02
0.3
POLY(VINYL ACETATE)
Figure 5. Calibration curve for determination of poly(viny1 acetate) in copolymer
have )vide application for the quantitative analysis of solids by infrared absorption.
W 0
z
POTASS IU M
2 c
ACKNOWLEDGMENT
T HI OCYANATE
One of the authors (S.E.W.) is indebted to the Board of Trustees Research Fund of Rensselaer Polytechnic Institute for a grant toward the purchase of the Model 21 Spectrometer, and to the Socony Mobil Oil Co., Inc., for partial support of this work.
il
.-ALUMINUM
SOAP
#
LITERATURE CITED
(1) Atkinson, J. V., Bakelite Co., private
communication to authors, Sept.
0
WAVELENGTH, M I C R O N S Figure 6. Spectra of potassium thiocyanate and an aluminum soap in potassium bromide
4, 1956. ( 2 ) Bakelite Co., technical data sheet on surface coating products, Oct. 19, 1953. ( 3 ) Barnes, R. B., Gore, R. C., TVilliams, E. F., Linsley, S. G., Peterson, E. I f . , IXD.ENG.CHEX., AXAL. ED.19, 620 (1947),. Browning, R. S., Riberley, S. E., Sachod, F. C., ANAL.CHEJI.27, 7 (1955). Childers, E., Struthers, G. IT.,Zbzd., 27, 737 (1955).
Harple,
Table II.
snnlplc
Designation
Results on Commercial Samples
7cPolv(vinl.1 hcctate)
Reported
VYChI VYDR VYHH-1
9 4 13
Found-
84, 8 5 35,35 13 0 , 13 1
the absoibance ratio of the 4.7- and 5.8micron bands on 2-nig. samples of four aluminum soaps, the free fatty acids of which had been determined by chemical methods (6).
In view of the results obtained on these typical materials, this general method employing potassium thiocyaiiate as an internal standard should
\J7.
W.,Wiberlev S. E.,
Railer. W. H . . Zbzd.," >4, 635 (1952 j. Hughes, H . K., associates, Ibid., 24, 1349 (1952). Kirkland, J. J., Ibid., 27,1537 (19%). Kuentzel, L. E., Ibid., 27, 301 (1955).
Miller, F. A , , Wilkins, C. H., Ibid., 24, 1253 (1952).
Kright, S . ,-4ppl. SDectroscopy 9, 105 (1955).
RECEIYED for review June 20, 1956. ilccepted November 23, 1956. Division of .4nalytical Chemistry, 130th Meeting, .4CS, A4tlantic City, S. J., September 1956.
Instrument for Controlled Potential Electrolysis and Precision Coulometric Integration GLENN L. BOOMAN Atomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho
b Control of electrolysis potential within 3 mv. and response to changes occurring as fast as 10 psec. are attained with a 600-ma. capacity potentiostat circuit. A standard deviation of less than 0.05% for integration of electrolysis currents is possible over the range from 10 pa. up through 100 ma. The instrument is easily constructed and is adaptable to many different types of electroanalytical methods.
I
study of the coulometric determination of uranium (VI) at controlled potential, the N BEGINXIKG a
need arose for an instrument which could accurately control the mercury cathode potential and precisely integrate the resulting electrolysis current. Controlled potential methods of analysis have not been used t o the extent merited by their versatility and selectivity, mainly because of the complex electronic circuitry needed and the previous unavailability of suitable commercial instruments. For precision work with a rapidly stirred mercury clectrode, a servocontrol potentiostat does not have the necessary speed of response to compensate for the large random fluctua-
tions in current due to the continually changing electrode area. The servocontrol instruments such as those made by Fisher Scientific Co., Pittsburgh, Pa., and Analytical Instruments, Inc., Bristol, Conn., and the various servocircuits reviewed by Lingane (6) are necessary when large scale controlledpotential reductions are required, as in organic and inorganic synthesis by electrolysis. These are also suitable for many coulometric applications. For practically all analytical work a maximum available electrolysis current of 0.5 to 1 ampere is sufficient. This assumes the accurate pipetting of about VOL. 29, NO. 2 , FEBRUARY 1957
213
