SOME OBSERVATIOXS OX THE COLLOIDAL IMPURITIES I S DISTILLED WATER
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The Johnson ChemtcaZ Laboratories, Unaversaty of Adelazde, Adelaide, South ilustralia Received JuZy 29, 19%
Apart from thc small amounts of electrolytes that are prewit in distilled water, there are also present small quantities of organic and inorganic colloidal material. For ordinary purposes the latter niay be neglected, the specific conductivity being taken as a measure of the degree reciprocal ohms, the of purity. Whrn thc conductivity is less than I\ ater is referred to a i “conductivity water”, and is considered d f k i e n t l y pur(’ for mo5t purposeq, even the more exacting In some few cases, honever, this measure of purity is of little value. For instance, in the preparation of colloidal gold by reduction nith formaldehyde, trace.; of cc~tainforeign substances in the water h a w the effect of preventing tho formation of fine-grained stable sols The action of these substanres i- unknown, but apparently they slow dovn the velocity of nuclei formation with the result that crystallization, once it begins, is so rapid that the gold particle. become too large to remain in colloidal suspension The amount of idiibitory material bears no relation R hatb0e.i clr to the specific conductivity. Inderd, relatively large amounts of electrolytes are permissible, and actually are present, during the formation of the sol. In spite of this, it ii knoxn that certain specific electrolyte. are quite harmful, even in small amounts. For example, it has been shown by Zsigmondy and Thiessen (lo), Hiege (4),and Reitstotter (9) that the addition of ammonia, hydrazine sulfatc, or potassium ferro-, ferri-, or nickelo-cyanide u ill inhibit the formation of fine-graincd sols. More recently, Freundlich and Steiner (3) have .iliown that the same electrolytes have a n inhibitory action on the preparation of colloidal silver Furthermore, there evidence that certain colloidal substances have a similar harmful effect Zqigniondy and Thiessen (10) have denionqtrated the inhibitory action of traces of colloidal silica, which explains the impossibility of purifying water by redistillation through soft glass condensers. Also, in our own laboratories we have found that copper boilers contribute small amounts of colloidal copper oxide, which are decidedly harmful. A4 certain amounti of colloidal material are invariably carried over 681
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8. W. PENNYCUICX AND C. E. WOOLCOCK
during distillation, it is possible that the major inhibitory substances in distilled water are colloidal in nature. This view is supported by the results outlined below. THE STANDARD TEST FOR TEE WATER
All samples of water were subjected to a standard test in order to determine whether they would yield finegrained gold sols with fornddehyde as the reducing agent, the formaldehyde-reduced sol being probably the most sensitive to the retarding influence of traces of inhibitory material. The samples of water which yielded fine-grained homodispersed sols are in this paper referred to as positive, whilst those which failed to yield good sols are called negative. I n order to obtain comparable results, the details of preparation must be rigorously standardized. The method of Cruickshank (1) was adopted, special care being paid to the manner in which the formaldehyde was added, particularly a t the instant when the nuclei began to form. The conditions a t this instant seem to determine the subsequent stability of the sol. The usual shaking or swirling was replaced by motor-stirring a t constant speed. By standardizing the technique in every detail, it was possible to reveal quite sensitive variations in the water. THE EFFECT OF FREEZING
It is known that if a solution be frozen and then thawed, the various soluble substances, including the electrolytes, remain in solution, whereas the colloidal material, particularly the hydrophobic, is largely coagulated (2, 7). Experiments were conducted to see if the freezing treatment could be used to throw any light on the nature of the inhibitory substances in distilled water. It was found that, in many cases, negative water could be changed completely to positive simply by freezing and thawing. Owing to the very small amounts of impurities concerned, it proved necessary to observe the following precautions: (1) The water must be slowly frozen from the outside inwards, so that the colloidal material will be gradually concentrated towards the center and there finally coagulated. (2) The frozen solution must be kept at low temperature for a t least 24 hr. before thawing, thus making sure that the fine interstitial films have become completely crystalline. (3) Small quantities of water give the best results. The positive results obtained by this method showed that, in certain cases a t least, the inhibitory substances were hydrophobic colloids. I n those cases where the water failed to respond, the inhibitory substances may have been electrolytic in character, or, more probably, easily peptized hydrophilic colloids.
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THE EFFECT OF STANDING
A low colloid concentration plus an extremely low concentration of electrolyte is in general unfavorable to the stability of hydrophobic sols. From this it would appear that distilled water would gradually improve merely on standing. This proved to be the case. Various samples of water, particularly those which had been distilled from copper boilers (using block tin condensers), were changed from negative to positive by standing in stoppered Pyrex flasks for periods of about two weeks. I t so happened that these samples were the ones that gave best results by freezing, but with the latter process the improvement was always more pronounced. The action that proceeds on standing is not simply a gravimetric precipitation of the larger particles; it is more in the nature of an irreversible coagulation, since the inhibitory substances are not redispersed when the water is shaken. Again the results point to the presence of hydrophobic colloids. GRADES OF COLLOID-FREE WATER
It proved desirable to estimate the relative quantities of colloidal material in different samples of distilled water. The “Tyndallmeter” was of no use, since it failed to reach the limits desired, whilst the ultramicroscope revealed but a weak diffused cone with here and there an occasional bright particle, probably dust. The work in this paper concerns itself only with the colloi&l material which is harmful to the production of fine-grained gold sols. With this limitation, it is at least possible to assign grades to different samples of positive or negative water, either by ultramicroscopic observation of the size of the reduced gold particles, or more directly by observing the ease of formation of the particles and the particular color of the sol. I n this way it becomes possible to speak of the water as having been more or less improved by treatment. For instance, the freezing process invariably improves the water, although it does not necessarily change it completely to positive. REDISTILLATION
Distilled water can be further freed from both electrolytes and colloidal impurities by redistillation. A second distillation usually produces positive water, so long as extraneous impurities (from the condenser etc.) are excluded. This is the usual method adopted in preparing water for the production of reduced gold sols as used in the Lange test ( 5 ) , although the technique recently developed by Pennycuick, Woolcock, and Cowan (7) is to be preferred. Instructions concerning the preparation of gold sols (6) invariably
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W. PENNYCUICK AND C. E. WOOLCOCK
recommend the use of “freshly distilled” water. This direction is unnecessary, as our results show that distilled water, instead of deteriorating, always improves on keeping. Accordingly the water to be used in the preparation of gold sols, may, and in fact, should, be allowed to stand undisturbed in a stopprred Pyrex container for some weeks before use. THE ACTION OF STANNIC CHLORIDE
Stannic chloride in acid solution gives rise to tetravalent stannic ions. I t also gives rise, by hydrolysis, to small amounts of positively charged colloidal stannic oxide, whose constitution may be represented by [rSnOz.ySnO]*U+. Both the colloidal oxide and the stannic ions are good coagulants of negatively charged colloids. I n weakly basic solution the hydrolysis of stannic chloride proceeds to completion and the colloidal stannic oxide suffers a reversal of charge, being peptized by the free base to form negatively charged particles whose constitution may be represented by [rSnOZ.ySnO3]*~ -. Such particles are active coagulants of positively charged colloids. A series of experiments was carried out to see whether very small amounts of stannic chloride could be used to coagulate the inhibitory material in distilled water, and, if so, to determine the most favorable p H range. This was designed to give some insight into the nature of the charge carried by the inhibitory material. The results showed that when the p H of the water was kept below 7 , the addition of stannic chloride had no effect, but when the p H was adjusted to values above 7 , negative water was either markedly improved or else completely converted to positive. Very small amounts of stannic chloride, as small as one part in ten million, were sufficient, the most favorable p H range being from 9.7 to 10.3. Over this range the colloidal stannic oxide is undoubtedly negatively charged, and hence it is evident that the inhibitory material carries a positive charge. The adjustment of the p H of distilled water by the addition of a little hydrochloric acid or potassium hydroxide may appear anomalous, but it must be pointed out that the water is to be used solely for the preparation of colloidal gold, in which case its p H is always adjusted to about 10 before the addition of the formaldehyde. An cffort was made to obtain some measure of the actual quantity of inhibitory material in a given sample of water by determining the minimum amount of stannic chloride necessary to change the water from negative to positive. When quantities smaller than that stated above (one part in ten million) were used, positive results were often obtained, but the general behavior was variable and unreliable. On the other hand, when larger amounts were used, up to a limit of one part in one million, the results were quite satiqfactory. The latter amounts are obviously exces-
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sive, and the co~iclusioiimay be d r a w that negatively charged colloidal stannic oxide does not itself act as an inhibitory colloid. SrMJI.mT
Some at least of the substances in distilled water which inhibit the formation of formaldehyde-reduced gold sols are positively charged hydrophobic colloids. Their inhibitory effect can he reduced and somctime:: completely removed by ( a ) allowing the water to stand undisturbed for some weeks, ( b ) freezing the water, and (c) adding very sniall amounts of stannic chloride a t pH between 9.7 and 10.3. The authors are indebted to the Trustees of the Endowmcnt Fund of the Council for Scientific and Industrial Research of the Commoiirvealth of Australia for a grant for the purchase of the gold used in this work. REFERENCES (1) CRUICKSHANK: Brit. J. Exptl. Path. 1, 71 (1920). (2) EMSLANDER: Der Einfluss des Gefrierens auf Selenhydrosol, p. 21. (3) (4) (5)
(6) (7)
(8) (9) (10)
Dissertation, Stuttgart. FREUXDLICH AND STEIIFER: J. Chem. Soc. 1937, 1081. HIEGE:2. anorg. Cheni. 91, 145 (1915). LANGE:Z . Chemotherap. 1, 44 (1912). MELLANBY A N D ANWYI.-DAVIES: Brit. J. Exptl. Path. 4, 132 (1923). PENNYCUICK: J. Chem. Soc. 1938, 2108. PENNYCUICK, WOOLCOCK A N D COWAN: “The Successful Preparation of Gold Sols for the Lange Test”; in course of publication. REITSTOTTER: Kolloidchem. Beihefte 9, 222 (1917). ZSIGMONDY A N D THIESSEN:Das Kolloide Gold. Akadeniische Verlagsgesellschaft m.b.H., Leipzig (1925).
