by dipping the teeth in an aqueous solution 0 5 by weight for 5 minutes and then air drying the treated teeth all of the teeth were then exposed to water by soaking them in two separate 5 microliter portions of water for 15 minutes each the fluoride remaining on the teeth after soaking was recovered by etching the teeth with a 5 microliter portion of 0 5 m perchloric acid which had been further diluted to 100 microliters by volume the amount of fluoride recovered was determined by the hanging drop fluoride electrode technique p venkateswarlu analytical chemistry 46 878 1974 the amount of fluoride retained on the coated teeth and the control teeth was then compared by dividing the amount retained on the coated teeth by the amount retained on the control teeth the fluoride treatments membrane forming materials and results of the hanging drop test are set forth in table ix table ix______________________________________ fluoride reten fluoride tion ratiotreat membrane forming coated ment material control ______________________________________4 naf c sub 8 f sub 17 ch sub 2 sub 10 cooh 2 35 min 4 naf protamine sulfate 185 min 4 naf c sub 16 h sub 33 o sub 2 pooh 1 75 min 1 44 naf ch sub 3 sub 2 choti ococ sub 16 h sub 32 ch sub 3 sub 3 1 85 min 5 snf sub 2 ch sub 3 sub 2 choti ococ.