THE EVALUATION OF WETTING AGENTS
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A Student Experiment I.A. MUNRO Queen's University, Kingston, Ontario, Canada
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official method of the American Association of Textile Chemists & Colorists for the comparison of wetting agents consists in finding the concentration of wetting agent required to cause the sinking of a 5-g. skein of standard two-ply cotton yarn in a standard time. The skein, carrying a hook of standard weight (3 g.), is held beneath the surface of the wetting solution by a 20-mm. thread attached to a heavier weight (40 g.) caUed the anchor. The concentration to cause sinking in a standard time of 25 seconds is obtained from a curve plotted from average sinking time for a range of concentrations. It is called the Draves index.', % The following experiment based on the Draves method has been found to give surprisingly good results in the colloid laboratory. Students have consistently obtained the same order of wetting for a series of commercial detergents and surface-active compounds as was previously obtained using the official method. The experiment described below employs undyed pipe cleaners, which cost
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The sinker consists of a 1.5-cm. piece of heavy capillary tubing through which is a triangular loop of Constantan or other wire. To the wire is attached a loop of linen thread 15 mm. long. The cylinders used are the tallest 100-ml. ones selected from stock. These give ample coverage of the immersed pipe cleaner. The experiment requires much lower concentrations and much smaller volumes of wetting agent than the official test. Figurn I Stock solutions containing 2 1 can therefore be used conveniently. PROCEDURE
Prepare 250 ml. of standard solutions of the assigned detergents or wetting agents, either by dilution of stock solution or by direct weighing, to contain 1.00 g./l. Pour gently 100 ml. of prepared solution into a graduated cylinder and allow to stand until all bubbles have come to the surface. Record the temperature of the solution. '"Year Book of the American Association of Textile Chemists and Colorists," 1942, p. 248. DIUVER. C . Z..' AND R. G. CLILRKMN. Am. Duestuff Revtr.. 23, 1938 (1934). Yomo, C. B. F., AND K. N. COONS, "Surface Active Agents," Chemical Publishing Co., Inc., Brooklyn, New York, 1945, p. 85.
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20 304050 100 Sinking time in seconds
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Bend the ei!d of a pipe cleaner so as to make a hook as shown in Figure 1. Pass the bend through the thread loop attached to the sinker. The pipe cleaner is then held by the tip with the anchor and thread immersed. Start the stop watch the instant the pipe cleaner is immersed. The pipe cleaner will now be floating vertically with the thread taut and bubbles of air escaping from the tip. Stop the watch the instant the pipe cleaner hits the sinker. The latter time is indicated instead of the first slackening of the thread in the Draves test, becauseit has been found that withlow concentrations (i. e., long wetting time) the escape of the second-last bubble causes a downward push by the displacing solution, and the buoyancy is such that the pipe cleaner neither sinks nor rises but the anchor line is slack. Handle the pipe cleaners as little as possible. Determine sinking times in triplicate or until consistent values are obtained. Discard any determinations giving values under two seconds and proceed to next dilution. Repeat, using concentrations of 0.75, 0.60,0.45,0.15,0.10 g./l. or other appropriate dilutions to give a range of times between approximately 2 and 300 seconds. Solutions for successive tests may be prepared by pouring off the required amount from the preceding test and diluting: 75 ml. of No. 1made up to 100 gives No. 2; 80 ml.of No. 2 gives No. 3, etc. Solutions containing 0.30 g./l. or less should be prepared by proper dilution of the stock solution. Place the used pipe cleaners on a cleanlaboratory towel. Plot on log-log paper average sinking time versus concentra-
tion. Repeat the test for any concentration which is obviously in error. Determine the concentration corresponding to 25 seconds sinking time for each of the assigned detergents.
Figure 2 shows typical student results for two differentdetergents. The pipe cleaners when dry are used again by the class in semimicro qualitative analysis.
