Color Comparator for Determination of Water in Cellophane

Abstract

143HE speed irith which cellophane reaches equilibrium with T atmospheric 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. APPARATUSThe 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. REAGENTSKarl 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. PROCEDUREMethanol 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 match...