V O L U M E 2 8 , NO. 1, J A N U A R Y 1 9 5 6 Color Comparutor for Determination of Water in Cellophane Howard A. Scopp and Charles P. Evans, Film Research and Development Department, Olin Mathieson Chemical Corp., N e w Haven, Conn.
HE
speed irith which cellophane reaches equilibrium with
Tatmospheric humidity makes necessary an accurate and rapid method for determining water in production control and research. The Karl Fischer titration met,hod, which has been in use in cellophanr laboratories for some time, \\-as chosen over distillation methods because of the economy in time and sample size. Various methods for determining the end point of the Karl Fischer titration have been presented. Frediani ( 3 ) has devised a way to accomplish this amperometrically by the “dead stop” technique. -AImy, Griffin, and \Tilcos ( 1 ) have shown that the end point of the t,itration can be determined potentiometrically as well as colorimetrical1)-. Fischer ( 2 ) determined the end point using methylene blue indicator, and Whittum ( 4 ) used gold fluorescent’ lamps to niat.ch the yellow color of the solution of Karl Fischer reagent and water, thus giving a solution of colorless appearance until the end point was reached. The work reported in t8he present paper was undert.aken to find a simple, rapid, and inexpensive means of determining this end point with an accuracy eclual t.o that of one of the above-mentioned t,echniques.
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Table I.
Titration of Water with Karl Fischer Reagent
Sample Weight of Water, Gram 0.1356 0.1997 0.1512 0.1913 0.1501 0.0961
VOl. of Karl Fischer Reagent, MI. 30.10 44.28 33.90 42.30 32.40 20.40
Water Titer, Mg./bIl. 4.50 4.51 4.46 4.52 4.63 4.71 Mean 4 . 5 6
Deviation from Mean -0.06 -0.05 -0.10 -0.04 f0.07
+O.l5
f0.08
on these materials agree closely with those obtained using the distillation method. It appears possible to use the method on many other materials. One limitation is the tendency of certain substances, salts such as barium chloride and sodium tartrate, to cloud the solvent, thus masking the end point. With colored solutions the same trouble would be encountered.
n
APPARATUS
The color comparator consists of a box containing a light source from which two light beams emanate. One beam passes through the sample jar, containing solvent and indicator, and the other passes through a 670-mp filter. The operator simultaneously views both light beams on a ground-glass plate and matches sample color against the standard color while titrating with Karl Fischer reagent. The sample vessel is a 16-ounce screw--cap bottle xrhich rests on a magnetic stirrer. The bottle is fitted with a rubber stopper which contains three holes, one for the buret tip, a second for the Drierite tube, and the third and largest for inserting the test sample. The largest hole is in turn fitted with a rubber stopper which can be temporarily removed for placing the sample in the titration vessel. Subsequent samples can be placed in the titration vessel without removing previous samples or solvent until the vessel is filled or the magnetic stirrer bar become3 clogged. The apparatus is shon-n schematically in Figure 1.
E-
.lU RUGEN1 SOURCE F-
REAGENTS
Karl Fischer reagent is prepared by dissolving 84.7 grams (0.33 mole) of iodine in a mixture of 269 ml. (3.3 moles) of pyridine and 667 ml. of methanol. The solution is cooled in ice and 64 grams (1 mole) of gaseous sulfur dioxide are added slowly to prevent excessive warming. The reagent is allowed to stand a t least 1 day prior to use. Other methods for the preparation of the Karl Fischer reagent may be used. Methylene blue indicator, 0.1 % in pyridine.
Figure 1. A. B. C.
PROCEDURE
D.
Methanol is added t o the sample vessel until it is half full. Two drops of the methylene blue indicator solution are then added, the vessel is closed with the large stopper, and the magnetic stirrer is set to give a constant rate of agitation. Approximately 0.5-ml. increments of Karl Fischer reagent are added until the color of the sample solution matches the amber color of the standard. The instrument is now ready for a sample determination. Because of its instability, the Karl Fischer reagent was standardized everv day. The solution in the s a m d e vessel wm titrated to t6e end point and a 120- to 150-mg. sample of water was added. The water sample was then titrated and the water titer calculated. Cellophane samples were analyzed by weighing a film sample containing 20 to 30 mg. of water, stirring the sample for several minutes to extract the water, and then titrating to the end point.
E.
DISCUSSION
The color comparator method is more rapid than an electronic method. Ifaterials other than cellophane have been analyzed readily, including benzene, polyeth?-lene, and paper. Results
F.
Schematic drawing of color comparator
Buret Drierite tube Rubber s t o m e r Sample entiance Ringstand bar Standard viewing screen
G.
Sample-viewing screen
H. SamDle bottle
J.
Magnet for magnetic stirrer
K. Magnetic stirrer M.
Ringstand base
Because any water analysis employing the Karl Fischer reagent requires daily standardization of the reagent, weighed samples of distilled water were analyzed using the color comparator. This test also illustrates the reproducibility of measurement using the color comparator. The weight of water was determined by difference; usually 2 to 4 drops of water were used to keep the weight between 0.1 and 0.2 gram. The data in Table I show a maximum deviation from the mean of 0.15 mg. per ml. and an average deviation from the mean of 0.08 mg. per ml. This amounts to a variation of approximately 4=1.7’% about the mean, which was considered sufficiently accurate for the analyses for which the color comparator was intended. A further test of the reproducibility of measurement using this instrument was demonstrated by analyzing duplicate specimens of a number of cellophane samples (Table 11). The maximum
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ANALYTICAL CHEMISTRY Table 11. Water Content of Cellophane
Sample
Water,
NO.
%
1
2.70 2.76 2.51 2.71 2.68 2.53 2.53 2.45 7.17 7.12 7.11 6.97
2 3 4 3
6
.\Iean, 7% 2.73
Sample NO.
7
2.61
8
2.60
9
2.49
10
7.14
11
Water,
Mean,
6.04 5.94 7.35 7.47 5.90 6.02 6.83 6.90 6.29 6.35
5.99
75
% 7.41 5.96 G.86
6.32
7.04
Table 111. Comparison of Data Color Comparator Water, Sample No.
1 2 3 4
%
(av. of 4 values) 3.9 6.1
Av. deviation izO.17 0 10.03 0 4v. 1 0 . 0 5
2:;
Electronic Titrator R'ater,
%
av. deviation 14.0 0 11,4 0 11.3
%
(av.of 4 values) 4.0 5.7 5.1 5.1
Av. deviation io.11 10.04 10.08 +0.17 10.10
R
.
av deviation 14.4 12.8 14.1 18.6 15.0
deviation from the mean was 0.1% and in the majoiity of cases measurements within O.O6Q/, of the mean were easily obtainable. I n order to compare the accuracy of measurement possible n-ith the color comparator with that obtainable on a commercially available electronic titrator, specimens of the same cellophane were analyzed on each of these types of instrument. Four separate samples were tested, and four observations were made on each sample using both instruments. It is readily seen from Table I11 that the results when using the comparator were equivalent to those obtained using the electronic titrator. The maximum deviation from the mean for the color comparator of 1 0 . 1 7 % vater and the average deviation from the mean of *0.05'% water or about *1.3% compare favorably with a maximum deviation from the mean of 1 0 . 1 0 % water or &5.0yo for the electronic titrator. Samples 2 and 4 for the color comparator data show zero deviation. This is not usual, but may occur with a small number of samples. I n a majority of analyses, a reproducibility of =k2% can be expected. ACKNOWLEDGMENT
The authors express appreciation to P. 11. Iiampmeger for assistance in preparing this paper, and t o the Olin Mathieson Chemical Corp. for permission to publish. LITERATURE CITED
(1) rllmy, E. G., Griffin, W. C., and Wilcox. C. S., IND. ESG. CHEM., ANAL.ED. 12, 392 (1940). (2) Fischer, E., Angeu. Chem. 64, 592 (19,52). ( 3 ) Frediani, H. A , , ANAL.CHEM.24, 1126-8 (1952). (4) Whittum, J. B., Ibid., 23, 209 (1951). PRESENTED before the Pittsbiirgh Conference on .4nalytical Chemistry and Bpplied Spectroscopy, Pittsburgh, Pa.. 1955.
great value t o the authors. Hon-ever, in the course of the research a t this laborator>- it hrcanie necessary t o determine the free amino nitrogen in wool (B), potato granules (I), and other solid materials such as poultry meat, and feathers. Wool and other fibrous materials could not be introduced int,o the reaction chamber of the Van Si!-ke-Sei11 apparatus. There was great difficulty and loss of many samples in the introduction of potato granulrs and similar materials into the apparatus. There was need for a larger opening into the reaction chamber without alteration of the moothly o p e r a t i n g s t o p c o c k reservoir arrangement of the conventional apparatus. Therefore, the reaction chamber was modified by the use of a S 35/25 ball joint as shown in the diagram. The chamber was constructed t o have the same length, volume, stopcock-reservoir arrangement, and calibration as the conventional apparatus available commercially. The chamber can be mounted with split rubber stoppers in the conventional apparatus by omitting the jacket. Wool and other solid materials could be easily introduced into this modified reaction chamber. Of almost equal importance was the ease with which the apparatus could be cleaned during the analysis of the Eample.
w
OPERATION
The mercury is lowered below the ball joint and the upper section of the chamber is removed. The sample, 5 ml. of Figure 1 distilled n-ater, and 1 ml. of glacial acetic acid are introduced into the lon-er section of the chamber. The ball joint is carefully lubricated. the chamber is assembled to f0rm.a vacuum seal, and the hall joint is secured with a clamp (not shown in Figure 1). The standard reaction of the sample with nitrite and the transfer of the reaction gases to the Hempel pipet are carried out as described by Van Slyke (5'). While the nitrogen is in the Hempel pipet the mercury is lowered below the ball joint, the reaction chamber is opened, and the sample residue is removed from the loner section of the chamber by suction. Water is added, and scrubbing with a rubber policeman is applied as needed. The upper section of the chamber can be cleaned in any suitable manner. ;ifter cleaning, the hall joint is lubricated and the reaction chamber is assembled as before. The analysis is completed as descriheti h!- \'an Slyke (3). The discharge tube, A , is included in the modified apparatus, so that it can be used in the manner described by \-an Slyke ( 3 ) for the analysis of samples that do not require opening of the hall joint.
Van Slyke Manometric Apparatus Modified for Determination of Free Amino Nitrogen in Solid Samples Kenneth T. Williams and Marion C. Long, Western Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture, Albany 10, Calif.
apparatus described by Van Slyke and h-eill (4)and its use for the determination of amino nitrogen ( 3 )have been of HE
T
LITERATURE CITED
(1) Hendel, C. E., Burr, H. K., and Boggs, 11.l f . , G. S. Dept. -%gr., Western Regional Research Laboratory, Albany, Calif.,
AIC-303, 1951. (2) Jones, W. H., and Lundgren, H. P., Textile Research J . 21, 20-9
(1951). (3) Van Slyke, D. D., Jr., J . Biol. Chem. 83, 42547 (1929). (4) Tan Slyke, D. D., and Seill, J. AI., Ibid.. 61, 523-73 (1924).
