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ANALYTICAL CHEMISTRY
placed in the dry box, the opening was sealed by folding the plastic and placing it between two rigid rods held tightly together by spring clamps. Dried nitrogen was usually passed through the bag for several hours prior to use. This is standard practice even with commercial dry boxes’ its purpose is to remove moisture which has been adsorbed inside the box. When the dry box is in use, a slow nitrogen f l o is ~ continuously passed through. The efficiency of the plastic dry box is shown in Table I. A weighed amount of magnesium perchlorate was placed inside the plastic dry box and a second sample was exposed to the atmosphere immediately outside of the dry box. At the end of 6 hours, the amount of water absorbed inside the dry box was 7% of that absorbed by the sample exposed to the atmosphere. Similar results have been obtained by the authors with commercial dry boxes under the same conditions.
Table I. Water Absorption Data
It is not necessary to sew the plastic bag together, although this does give added strength to the bag. A number of plastic cements are commercially available and several of these, such as cement for Plexiglas and Lucite and Pliobond cement, have been successfully used to seal plastic dry boxes together. The authors have used plastic dry boxes without gloves to perform simple transfers of materials. The plastic bag is sufficiently pliable to transfer a solid from one container to another. Rubber gloves have been used in place of the plastic ones, but they are more difficult to seal to the plastic bag than the plastic gloves. The advantage of rubber gloves is that they are commercially available from several sources. A “tinker toy” type frame can be mounted inside the plastic dry box. This arrangement keeps the bag from collapsing when the nitrogen flow is discontinued. In the initial work on plastic dry boxes a “flour sack” type of tie was used to close the opening of the plastic dry box. A piece of rubber or glass tubing was placed in the opening and the plastic bag was then tied around it. The piece of tubing through the opening was used as the exit tube in the early work. This arrangement was satisfactory, but is not as efficient as the method shown in Figure 1. The plastic dry box described is not intended to completely replace the metal and glass dry boxes that are commercially available today. Its extremely low cost (several dollars), however, should encourage the use of dry box techniques in procedures where they are desirable, but are not now used because of the high cost of commercial dry boxes. Other workers will probably modify the apparatus described in this article to satisfy their individual needs.
ethanol, dioxane, and other solvents. It is an inexpensive reagent, commercially available in high purity. PREPARATION AND PROPERTIES OF STARCH-FORMAMIDE SOLUTIONS
The starch solutions are prepared by first heating the formamide to 100” to 110’ C. (in a hood) and stirring in a slurry of the required amount of “soluble” starch. A 5% solution is prepared by pouring a slurry of 5 grams of soluble starch and 30 ml. of cold (room temperature) formamide with stirring into 65 ml. of hot (100’ to 110’ C.) formamide. The starch dissolves within 1 minute after addition. The solution is ready for use as soon as it cools to room temperature. Starch solutions as concentrated as 10% are very viscous and not readily handled by dropping bottles; 3 to 5% solutions are very suitable as an indicator solution, but solutions of different concentrations may be made up as desired. Formamide differs from water in that it dissolves the starch, forming a clear solution of medium viscosity. On the addition of a starch-formaniide solution to water, the starch is completely dissolved in the water, no doubt because of the highly polar characteristics of the formamide. When 10 ml. of a 10% starch solution was added to 50 ml. of water, there was no separation of the starch. DISCUSSION
The sensitivity of a starch-formamide indicator solution toward iodine is the same as for fresh aqueous solutions. Two drops of a 5% starch-formamide solution imparts an intense blue color (not violet or reddish violet) to the iodide-iodine test solution (1). I n order to test the reactivity of formamide with iodine, a series of iodine titrations was run using the same volume of iodine solution but adding varying amounts of formamide to the iodine solution before the titrations with sodium thiosulfate (Table I). Formamide does not combine with appreciable amounts of iodine, until approximately 10 ml. of the reagent are in excess. I n iodometry this would not be encountered and the error would be negligible, because a t most 0.5 ml. of the starch-formamide indicator solution would be used. The stability of the formamide-starch solution is excellent. After 8 months a 5% starch solution showed no evidence of mold growth, discoloration, or precipitation of the starch, and still retained its sensitivity toward iodine, giving an intense blue adsorption complex (not reddish violet or violet). A 5 % starch-formamide solution was inoculated with a mold (probably aspergillus) that was found growing in an aqueous starch solution. No growth of the mold was noted even after 2 months’ incubation.
Table I. Reaction of Formamide with Iodine Stable Starch Solutions for Iodometry Albert C. Holler, Twin City Testing and Engineering Laboratory, St. Paul 14, Minn.
indicator solution has wide usage in the analytical S laboratory. Because of its instability in aqueous solution TARCH
(mold growth, etc.) it is usually prepared fresh as needed. Rlany times this is inconvenient-Le., in the field. Preservatives recommended for aqueous starch solutions include mercuric iodide (5), thymol (Q), and glycerol (9). This paper describes a stable starch indicator solution made by using formamide (SCOT\“*) as the solvent for the starch.
(10.00 ml. of 0.0105N iodine solution taken) Sodium Difference Thiosulfate Compared with Amount of Starch Used, Aqueous Starch, Solution hf1. hfl. Fresh aqueous 1% starch solution (1 ml.) 10.30 .... 5 % starch-formamide solution 10.30 1’0,OO Fresh, 2 drops 6 months old, 2 drops 10.31 10.01 1 week old. 2 d r o m 10.31 +0.01 5 ml. formamideadded in excess 10.29 -0.01 10.20 10 ml. formamide added in excess -0 10 10.01 -0 29 30 ml. formamide added in excess
LITERATURE CITED (1)
REAGENTS
Soluble starch, Mallinkrodt Chemical Works. Formamide, obtained through the courtesy of the Polychemkale Department, E. I. du Pont de Nemours & Co., is a clear, slightly viscous liquid with a faint ammonia odor and a boiling point of 210’ C. It is completely miscible with water, methanol,
Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” p. 618, rev. ed., Macmillan, h-ew York, 1943.
Nordling, W. D., Chemist Analyst, 42, 70 (1953). (3) Tomicek, O., “Chemical Indicators,” p. 228, Butterworth Scientific Publications, London, 1951. ( 4 ) Vogel, A. I., “Quantitative Inorganic Analysis,” p. 332, 2nd ed., Longmans, Green, London, 1951. (2)
