Ca(OH)2 + CO3Na2 = CO3Ca + 2NaOH setting free a small quantity of alkali, to which the classic Labarraque's liquor owes its causticity. In Dakin's process, this alkali is neutralised by an excess of boric acid.
In Daufresne's process no caustic soda is formed; the liquor contains, in fact, a certain quantity of carbonic acid, feebly combined (that of the bicarbonate of soda), which attaches itself to the lime as soon as the two solutions come into contact
It is difficult to demonstrate with certainty what is the intimate mechanism of fixation of the lime, but it may be imagined. As a matter of fact, it is for the carbonic acid, amongst all the substances present, that lime possesses the greatest affinity. Henceforward, the harmful part played by the lime is suppressed, and the secondary reaction we have indicated is changed into one perfectly inoffensive:
Ca(OH)2 + 2CO3NaH = CO3Ca + CO3Na2 + 2H2O
D. Titration of the Solution of Hypochlorite.1 - Measure 10 c.c. of the solution, add 20 c.c. of ten per cent, solution of iodide of potassium, 2 c.c. of acetic acid, then drop by drop a decinormal solution of hyposulphite up to decoloration. The number of c.c. used, multiplied by 0.03725, will give the weight of hypochlorite of soda contained in 100 c.c. of solution.
1 See Daufresne, loc. cit.
In the first stage of the determination, hypochlorite displaces the iodine of the iodide of potassium according to the equation
ClONa + 2KI + H2O = I2 + 2KOH + NaCl which is only complete in the presence of a quantity of acid sufficient to saturate completely the liberated potash. The operation returns finally to an estimation of iodine by hyposulphite of soda:
I2 + 2S2O3Na2 = 2NaI + S4O6Na2
Examining the various reactions, we see that a single molecule of hypochlorite decomposes two molecules of iodide of potassium with liberation of two atoms of iodine, and that each atom of iodine transforms a molecule of hyposulphite into tetra-thionate of soda; thus:
1 mol. S2O3Na -> 1 atom of I -> 1/2 mol. ClONa 248 37.25
On the contrary, if as in the estimation of chloride of lime the result had to be determined in active chlorine (decolorising chlorine), it would have been necessary to take into account that one atom of chlorine only displaces one atom of iodine:
2C1 + 2KI = 2l + 2KCI and
2I + 2S2O3Na2 = 2NaI + S4O6Na2 and
1 mol. S2O3Na2 -> 1 atom of I -> 1 atom of CI 248 35.5
The equations (in the case of a sample of 10 c.c.) which give the activity of a solution of hypochlorite, will be different, according as the result is expressed, either directly in hypochlorite, or indirectly in the quantity of chlorine of equivalent activity.
Hypochlorite per cent. . .
Active chlorine per cent. . .
It is necessary to insist on this point, because the same coefficient of activity is sometimes wrongly attributed to the hypochlorite as to the chlorine.
Electrolytic hypochlorite of soda is neutral. It contains no foreign substance excepting an excess of chloride of sodium. A 0.5 per cent. solution of this hypochlorite possesses the same bactericidal power as Dakin's chemically prepared solution. Its solvent action on mortified tissues is less rapid, but it has the advantage of being less irritant to the skin.
Electrolytic hypochlorite has not been employed hitherto because it is unstable. In a few days, sometimes in a few hours in warm climates, the solution decomposes. We began to employ electrolytic hypochlorite of soda from the time when Daufresne found a way of stabilising it. Daufresne discovered that if an addition of I part in 20,000 of permanganate of potash, or 0.2 per cent. of a soda salt, such as silicate of soda, be made to the electrolytic solution, it keeps sufficiently well. We may therefore employ the electrolytic hypochlorite to-day in all cases where the special sensitiveness of the skin or the length of treatment makes it desirable to use a solution less irritating than Dakin's chemically prepared solution.