Mercuric chloride, one of the most toxic of inorganic substances, does not ionize in aqueous solution, but in the presence of sodium chloride forms the double chlorides NaHgCl3 and Na2HgCl4 which ionize readily. It has been variously claimed that the ionization of mercuric chloride, thus effected, is accompanied by an increase of toxicity, by a decrease of toxicity, or that the toxicity of the solution remains unaltered.
In an investigation of the toxicity of mixed solutions of HgCl2 and NaCl to the minnow, Phoxinus phoxinus (L.), it is found that the addition, to HgCl2 solutions, of sufficient NaCl to convert the whole of the HgCl2 into the double chloride Na2HgCl4, and even ten times this quantity of NaCl effects no marked change in the toxicity of the solution.
On the addition of a considerable excess of NaCl a marked prolongation of the survival time occurs, the maximum antagonistic effect being evident when the solution is approximately isotonic. Almost exactly the same result is obtained if the NaCl is replaced by quantities of glucose sufficient to effect equal changes in the osmotic pressure of the solutions, and thus the antagonistic action of the sodium chloride appears to be due to the physical, rather than the chemical, changes its addition brings about in the nature of the toxic solution.
Mercuric chloride is generally recognized as one of the most toxic of inorganic substances, and one of the most powerful of antiseptics. Thus in a study by Martindale & Westcott (1921, pp. 348–9) of the antiseptic value of thirty-two substances, the germicidal power of mercuric chloride was found to be equalled only by mercuric potassium iodide. Bacillus coli was the organism employed.
The controversy regarding the toxic activity of these double chlorides has prompted the present study, in which the minnow is used as a test animal.
THE TOXICITY OF MIXED SOLUTIONS OF HgCl2 AND NaCl
In all the following experiments small minnows 18–20 mm. in length were used, and were found to give very consistent results provided that they were not kept in the laboratory for more than 5 days before use. The solutions, stock and experimental, were prepared with water distilled in glass; the salts were of “AnalaR” quality and the mercuric chloride was recrystallized before use. The fish were washed in two changes of glass-distilled water before being transferred to the solutions.
The survival curves for mercuric chloride and for sodium chloride are of normal type, the survival time increasing steadily on dilution. The curve for HgCl2 rises from 15 min. at 10−3M to 230 min. at 5 ×10−6. That for NaCl is drawn in Fig. 1.
In Fig. 1 four survival curves for mixtures of mercuric chloride and sodium chloride are drawn. In each series of experiments the molar concentration of HgCl2 is maintained at a selected value while that of NaCl is progressively increased from nil to 0·5–2·0 M. The NaCl concentration scale is logarithmic; since zero cannot be represented on a logarithmic scale NaCl nil is represented as 10−5M.
Along the fine XY the composition of the solution corresponds to the formula Na2HgCl4. It will be seen that the addition of sufficient NaCl to convert the whole of the HgCl2 into the complex salt has no appreciable effect on the toxicity of the solution, and that the only effect of adding ten times as much NaCl is a slight prolongation of the survival time. On the addition of a great excess of NaCl marked antagonism becomes evident, the survival time rising considerably, and each curve attains a maximum at approximately 0·15 M NaCl. Thereafter the curves descend rapidly, following closely the survival curve for NaCl. At concentrations of HgCl2 greater than 0·00015 M the addition of small quantities of NaCl remains ineffective, while the antagonism produced on the addition of an excess becomes less marked; at HgCl2 concentrations below 10−5M the antagonistic effect of an excess of NaCl becomes increasingly pronounced.
At 10−5M the solution volume employed contains only 0·02 mg. Hg per fish.
This quantity appears to be very small, but it is found that increasing the solution volume to 500 c.c. per fish results in no appreciable shortening of the survival times.
THE RELATION BETWEEN THE TOXICITY OF THE SOLUTIONS AND THEIR OSMOTIC PRESSURE
At 0·15 M, the concentration of sodium chloride at which the maximum antagonistic effect is evident, the osmotic pressure of the solution is approximately 6·3 atm. This solution is probably very nearly isotonic, for Ellis (1937, p. 408) states that the osmotic pressure of the blood of fresh-water teleosts is generally 6 atm. The results suggest that the prolongation of the survival time is connected, in some way, with the osmotic pressure of the solution, and this impression is materially strengthened by the fact that if increasing quantities of glucose, instead of sodium chloride, are added to mercuric chloride, survival curves of the same type are obtained. Two survival curves for HgCl2 plus glucose are drawn in Fig. 2; the survival curve for 5 × 10−5M HgCl2 plus NaCl is repeated in this figure for comparison. The upper curves indicate the osmotic pressure of the solutions; the values are calculated from the concentrations (and in the case of NaCl from G values), and are necessarily somewhat approximate.
It will be noted that in the HgCl2 plus NaCl and HgCl2 plus glucose curves the maximum survival time is attained at much the same osmotic pressure, (6·3 atm. NaCl, 6·7 atm. glucose). A greater molar concentration of glucose than of NaCl is necessary to effect the same rise of osmotic pressure because of the ionization of the salt. If the survival curves are plotted with osmotic pressures as abscissae, corresponding curves for HgCl2 plus NaCl and HgCl2 plus glucose are almost coincident.
In the strongly hypertonic solutions, whatever their composition, the death of the fish appears to result from rapid withdrawal of water from the gill tissues with the consequent arrest of the branchial circulation (Jones, 1939, pp. 431–2). At death the gills are clear of mucus film, are bright red and visibly shrunken; the body surface acquires an opalescent appearance which is particularly evident in the cornea and crystalline lens of the eye. In hypotonic, isotonic and slightly hypertonic solutions of HgCl2, HgCl2 plus glucose and HgCl2 plus NaCl the death of the fish is accompanied by precipitation of the gill secretions, with consequent asphyxiation, but when the osmotic pressure of the solution is 2–8 atm. the formation of the asphyxiating film of coagulated mucus appears to be delayed. The reason for this is not clear; chemical action between mercuric chloride and glucose does not appear to be a feasible explanation and experiment has shown that the toxicity of other heavy metal salts is similarly lowered when the solution is rendered isotonic by the addition of glucose or sodium chloride. The osmotic pressure which exists between the blood of the freshwater teleost and its normal medium results in a continual influx of water through the gill membranes, the excess of water being eliminated in the dilute and abundant urine (Smith, 1930). If the medium is rendered isotonic this absorption of water should cease and it is possible that under these conditions gill secretions coagulated by a heavy metal salt may be more readily dispersed.
The problem thus requires further investigation, but the results are interesting in that they present a case in which the antagonistic action of a salt appears to result from the physical, rather than the chemical, changes its addition brings about in the nature of the toxic solution.