Disinfection of Drinking Water under Field Conditions J. Carrell Morris, Shih Lu Chang, Gordon M. Fair, and G. H. Conant, Jr. DIVISION OF APPLIED SCIENCE, HARVARD UNIVERSITY, CAMBRIDGE, MASS.
In order to utilize effectively the
excellent germicidal properties of iodine for the emergency disinfection of water supplies, rapidly soluble tablet preparations were needed that would consistently furnish 5 to 10 p.p.m. of elemental iodine when dissolved in water. Tablet compositions containing appropriate amounts of stable, water-soluble polyiodides most nearly possess that combination of properties desirable in an emergency water disinfectant. Of the numerous compositions tested, tablets containing 20 mg. of tetraglycine hydroperiodide, 90 mg. of disodium dihydrogen pyrophosphate, and 5 mg. of talc were found to be most suitable. Such tablets will dissolve in less than 1 minute at room temperature, liberating 8 mg. of elemental iodine per tablet, a quantity that will satisfactorily treat 1 liter of most natural waters within 10 minutes. The treated water is palatable. The tablets, when packaged properly, are stable for extended periods of storage even under adverse climatic conditions.
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H E R E are numerous occasions when a reliable method for disinfecting individual drinking water supplies is needed. Natural disasters can cause a disruption of water supplies, so t h a t individual procurement and treatment of water are necessary. Campers and other outdoor sportsmen frequently rely on local natural waters. Workers and travelers in undeveloped areas of the world often find i t desirable t o treat their drinking water. A special need exists in connection with military operations in which individuals or small groups may be separated from normal supply points for considerable periods. A germicidal agent suitable for field use should destroy all forms of water-borne pathogens in all kinds of natural waters that might be used for drinking, and should accomplish this within a reasonably short time without imparting unpleasant taste or odor t o the water. The technique for use of the disinfectant should be simple and substantially foolproof; no testing of the water should be necessary. If a chemical agent is used, i t should not deteriorate appreciably over anticipated periods of storage and use. The requirement of simplicity in use is best satisfied if the germicidal agent is added as a single tablet per unit volume of water. It is difficult t o dispense small quantities of liquid reagents accurately and the use of multiple tablets is considered to be insufficiently foolproof. Solutions of aqueous iodine possess the chemical and biological characteristics desirable for individual treatment of water supplies ( 2 ) . A fixed dosage of 8 p.p.m. of iodine provides satisfactory treatment against all types of pathogens in nearly all types of waters within 10 minutes. The few waters not completely disinfected by this procedure can be treated by slight modifications in the technique-increasing the contact time or doubling the dosage.
Development of Suitable Iodine Preparations Elemental iodine is only very slowly soluble in water and so cannot be used as such in water-disinfecting tablets. However, soluble compositions providing requisite amounts of iodine can be prepared in several ways. The tablet may be composed of a water-soluble iodide together with a n oxidizing agent t o convert the iodide t o iodine when the tablet is added t o water. Preparations of this type containing iodide and iodate were used by the French army during World War I(6). They have two disadvantages: An acid solution is required for the reaction of the iodide and iodate, and the reaction is slow a t concentrations suitable for disinfection. Complete or constant liberation of iodine is obtained only if the tablets are al-
lowed to "pool" as they dissolve. Mixtures of iodides with oxidizing agents that react more rapidly and in neutral solution are extremely sensitive t o traces of moisture; the iodine is liberated prematurely and tends t o evaporate, leaving a deteriorated tablet. Iodine may be adsorbed or restrained in a water-soluble complex-for example, certain polyethylene glycols have a strong retentive power for iodine, as does sodium carboxymethylcellulose (3). The latter material releases the iodine when dissolved and exhibits germicidal activity corresponding to the iodine content. However, manufacturing and tableting problems connected with compositions of this type have yet t o be worked out. The iodine may be combined in the form of water-soluble triiodides or polyiodides. The equilibria of the reactions
I2 4-I- = I*IZ + Ia- = IS-,etc. are such that in concentrated solutions the iodine is almost wholly tied up in the form of polyiodide ions, whereas a t concentrations of a few parts per million dissociation into elemental iodine and iodide is essentially complete. I n general, the stability of solid polyiodides with respect to loss of iodine increases as the size of the cation increases. Unfortunately, this increase in stability is usually accompanied by a decrease in solubility, so that the highly stable quaternary ammonium polyiodides, for example, do not dissolve readily enough t o be used as ingredients of water-disinfecting tablets. However, polyiodides of complex cations which are themselves decomposed in dilute solution can be prepared and some of these materials are both stable and readily soluble. Some compounds of this type t h a t are suitable as active ingredients of water-disinfecting tablets are shown in Table I, together with pertinent physical and chemical properties, the compound tetramethylammonium triiodide being included for comparison purposes. I n general, the compounds can be prepared by mixing the necessary ingredients in just sufficient water to keep the product dissolved at 50" to 60" C. Slight excesses of iodide and of complexing material are desirable. Crystallization occurs on cooling, preferably t o 5" C., to obtain maximum yields. Recrystallization from an equal weight of water containing 1% iodide and 1% complexing compound gives products with compositions agreeing with the assigned formulas. They can be dried, after filtration, in a stream of warm air or in an oven at 60" C. for several hours without appreciable loss of active iodine.
Preparation and Properties of Tablets Tablets were prepared from each of the active ingredients listed i n Table I, such t h a t each tablet when dissolved in 1 quart of wa-
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Table 1. Characteristics of Some Polyiodides Active Formula Iodine, % (CHdaK'Ia 55.8 (NHzCHzCOOH)aHI.1.25Iz 42.3 (NHzCH&OOH)aKIs 35.3
Solubility, G,/LZ. a t 25 0.25 380 large
Iodine Vapor Pressyea a t 25 Relative hlm. X i O 3 Stability6 0.9 .. 1.3 50 0 . 5 (est.) 95
Substance Tetramethylammonium triiodide Tetraglycine hydroperiodide Pot&esium tetraglycine triiodide (4) Aluminum hexaurea sulfate tri- hl[CO(NHz)n]sSOaIa 29.4 590 0.4 100 iodide (11 Aluminum hexaurea dinitrate tri- A ~ [ C O ( S H ~ ) Z ] ~ ( N O ~ ) Z I2~8 , s 390 0.12 .. iodide ( 1 ) a Determined by equilibration of solid with CC14 a n d titration of dissolved iodine, assuming validity of Henry's law. b Percentages of initial active iodine remaining after 50 days when thin layers of powdered materials (
