Calcium Carbonate as an Internal Standard for Quantitative Infrared Analysis L. E. KUENTZEL Wyandotte
Chemicals
Corp., Wyandotte,
Mich.
eral oil always concentrated a t the outer edge. Small quantities of mineral oil may b e added from time t o time t o keep the sample washed d o x n into t h e mortar. T h e total amount of oil used should be about 10 ml. After t h e ether has evaporated, t,he pestle may he lowered into the mortar and normal mulling continued. T h e effect of mulling time on the absorbance niensurenients was studied. This factor ~ n without s effect upon the allsorbances of the alkyl benzene sulfonate and c:tlcium cnrl)onste, as might he expected from the fact t h a t both exist in extremely small particle size before mulling, but the alisorhances of the sodium toluene sulfonate became progressively greater n-ith increased grinding time as a larger and larger proportion of the material was ground to sufficiently fine particle size. After 2 hours of mulling there was no :ipprecini)!e change. However, the correlation lwtn-ren mulling rime :tiid absorbance values obtained from measiirement:: on Podium toluene ~ulforiate1)nnds n-as constant, so that reproducible ahsor1)ance measurements could lw oiltained from anmplee mulled for any ar1;itrary time of 30 minutes or more. T h e calibration from known synthetic mixture5 and thc analysis of unkriowi mixtures werp parried out b y the method used b y Barnes er a / . ( 1 ) . A single-lieam, custom-huilt infrared spectrograph v a s used and the familiar cell-in, cell-out method of obtaining the incident and transmitted energy R-as employed. An 0.03-mm. platinum spacer \vas used in the infrared cell t o obtain approximately reproducible sample thicknesses. Because the mull could not be introduced into t h e cell in t h e normal manner, it was necessary t o disassemble it each time, place a drop or two of mull on the lower window inside t h e shim, and reassemble. T h e ratio of the absorbance measured at the 11.4-micron band of calcium carbonate t o t h a t measured a t a characteristic band for each of the other two materials v a s plot.ted against t h e concentration of t h e corresponding compound in the synthetic mixture and two straightline calibration curves resulted. Interest \vas in the analysis of approximately 50-50 mixtures of the tn-vo sulfonatcs, so the calib r a t i m covered concentrations of 40 to 60% of earh in the other. An average deviation from the mean of slightly less than 1% ~ m obtained s from repeated analyses of a given sample and the average deviation from the true value for repeatrd analyses of the synthetic mixtures was the same.
Calcium carbonate is a good internal standard for quantitatite anal) sis of solids by infrared absorption methods. It is stable and inert, has three widel) spaced sharp bands, and ma? be obtained as a pure powder with all particles less than 1 micron in diameter.
C
ALCIURl carbonate is a useful internal calibration standard
for the quaiititative analysis of solids by infrared absorption methods. It, is stable chemically and physically, is inert t,o the action of many other solids, and m a y be obtained in pure form with all particles less than 1 micron in diameter. T h e latter property eliminates the need for tedious mulling. AIoreover, its infrared spectrogram exhibits three widely spaced, relatively sharp and strong absorption hands with transparent regions in betwerii ( 2 , S). -lctu:dl>-, the quantitative analysis of mixture:: of solid m:iteriala t)j- infrared is most conveniently carried out in suitable solvents. If this is not possible, the solid materials must he handled as suspensions in mineral oil or potassium bromide pellets. K h e r c sample or cell thickness cannot be determined accuratel?; the use of internal standards becomes necessary. Khile interiial standards are often used in quantitative analytical procedures, a n inorganic material as such finds rather limited use in the ficld of infrared. It w t s neccssarj- t o calibrate for the quantitative analysis of mixtures of sodium toluene sulfonate and alkyl benzene sulfonate by infrared methods. T h e former is a dry, easily muiltti solid, while the latter is waxlike. ,Ifter a rather thorough se:irch failed to reveal a comnion solvent which was relatively transparent, in the analytical regions of the Fpectrum, i t \\-as d e c i d d t o use an internal standard and calibrate with mineral oil suspensions. Calcium carbonate proved to be sat,isfactor)-, b u t the alkyl benzene sulfonate could not he mulled into suspension in mineral oil. This difficulty \\-as overcome by dissolving the waxlike component in ethyl ether, mixing this solution with the mineral oil, and then precipitating the :llli>-l benzene snlfonate into the oil by evuporating the ether. PROCEDURE
Weigh exactly 1 gram of sample, transfer it t,o t h e mortar of Fisher motor-driven grinder, and add 0.0100 gram of Purecal L (Wyandotte Chemicals Corp.) calcium carbonatt. Wash down the sides of the mortar with about 5 ml. of ethyl ether, so as t o collect sample and standard in the bottom. Stir slowly with a spatula until all t h e alkyl benzene sulfonate has dissolved. T h e sodium toluene sulfonate and calcium carbonate do not dissolve. ridd about 5 ml. of mineral oil by pouring i t around the sides of t h e mortar and allon it to run into t h e ether solution. This prevents the evaporation of ether from the mortar valls, v i t h resultant deposition of a film of alkyl brnzenr sulfonate. Adjust the mortar \Val1 scraper t,o sweep the mineral oil and et,her toil-ard t h e center of t h e mortar as it rotates. Do not Ion-er the pestle into the mixture. Adjust a heat lamp about 12 inches from the mortar, so as to x a r m t h e mixture and drive o f f the ether. T h e rotation of the turntable and the gentle sweep of the scraper produce a gradual mixing of ether and mineral oil, with the min-
ACKNOWLEDGJIENT
The author wishes t o thank t h e Wyandotte Chemicals Corp., Wyandottr, lIich., for permission to puhlish this {vorli and to acknon-ledge the helpfiil apsistance of Dolores Piiillips.
LITERATURE CITED
(1) Barnes, 1%.13.. k r e , R. C., Williams, E. I;.. I.insley, S. G., and Petersen. E. AI... A N . ~ L . CHEM.,19, 620 (1947). ( 2 ) ~ I i l l c rF. . A , and Kilkins, C . H., I h i d . , 24, 1262 (1952). (3) Sadtler. S. P., “Catalogue of Infrared Spectra,” Xo. 1755, Samuel P . Sadtler and Son. Inc., Philadelphia, Pa., 1950.
RECEIVED for review Mal- 14, 1054. Accepted October 30, 1954. Presented before the Pittrburyh Confermce on Analytical Clie~r,irtry and Applied Spectroscopy. March 1 t o 5 , 1034. Pittsburgh, P a .
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