Having repeatedly examined similar solutions, Daufresne sought a remedy for these inconveniences by a more accurate mode of preparation which would give constant results. After numerous trials he decided on the following process, which necessitates the employment of three products, all easily procured: Chloride of lime, Carbonate of soda, anhydrous (Solvay), Bicarbonate of soda. A. Chloride of Lime and its Titration. - 1. The chloride of lime of commerce is obtained by the action of gaseous chlorine on powdered slaked lime. It presents great variations in composition; notably in its content of active chlorine. Its chemical constitution, in spite of the numerous discussions of which it has been the subject, is not yet established in any satisfactory manner. Whatever it may be, we know that under the solvent action of water it yields three substances, hypochlorite and chloride of calcium and small quantitias of lime, the residue being made up of excess of lime, partially carbonated.

It is an ordinary thing to meet with samples which differ greatly in richness. We have had occasion to examine a small number of lots whose titrage in active chlorine varied from 20.45 per cent. to 35.90 It will therefore, be of advantage to use only a chloride of lime of known titration, so that we may employ a quantity correctly calculated according to its titration.

2. Titration of Chloride of Lime. - The estimation of this quantity is made in the following manner by Dau-fresne: Weigh out an average sample of 20 grammes, stir it up in a litre of water as thoroughly as possible, and allow it to stand some hours. Measure off 10 c.c. of the clear liquid, add to it 20 c.c. of a 10 per cent. solution of iodide of potassium, 2 c.c. of acetic acid or hydrochloric acid, then to the mixture add drop by drop a decinormal solution of hyposulphite of soda (2.48 per cent.), up to decoloration. The number n of c.c. of hyposulphite employed, multiplied by 1,775, will give the weight N of active chlorine contained in 100 grammes of the chloride of lime.

This estimation must be carried out for each consignment received. The following table drawn up by Daufresne gives directly, according to the amount of active chlorine contained in the chloride of lime, the quantities of the reagents required to obtain automatically a correct solution:-

Titration of chloride of lime , (C1 %). (English degrees.)

Quantities to be used to obtain 10 litres of solution of hypochlorite of 0.475 %

Chloride of lime.

Carbonate of soda, anhydrous.

Bicarbonate of soda.

grms.

grms.

grms.

20

23O

115

96

21

220

II0

92

22

2I0

I05

88

23

200

I00

84

24

192

96

80

25

184

92

76

26

177

89

72

27

170

85

70

28

164

82

68

29

159

80

66

30

154

77

64

31

148

74

62

32

144

72

60

33

140

70

59

34

135

68

57

35

132

66

55

36

128

64

53

37

124

62

52

The determination of the quantity of soluble calcium in the chloride of lime would have a certain importance if one were not obliged, in order to obtain a product having some degree of stability, to use an amount of carbonate of soda far above that theoretically indicated. In fact, a solution prepared by the interaction of chloride of lime and salts of soda in theoretic quantities loses the whole of its hypochlorite in from 15 to 20 days.

B. Salts Of Soda

It is more convenient to use dry carbonate of soda {carbonate de sonde, Solvay, Fr.) which is to be preferred to the other commercial salts because of its being anhydrous, pulverulent, and free from caustic alkali.

When obliged to use the hydrated salt (crystals), the quantity needful will be 285 grammes for 100 of the dry salt.

Bicarbonate of soda is readily obtainable. It is always anhydrous. The solution should be made in the cold, because it commences to break up towards 500 C.

C. Preparation Of Dakin's Solution By Daufresne's Method

1. To prepare 10 litres of solution weigh out exactly the quantities of chloride of lime, carbonate of soda, and bicarbonate of soda determined when titrating the chloride of lime;

2. Place in a 12-litre flask the chloride of lime with 5 litres of tap-water; shake vigorously two or three times and leave it overnight;

3. Dissolve the carbonate and bicarbonate of soda in 5 litres of cold water.

4. Pour the solution of soda salts rapidly into the flask containing the chloride of lime, shake vigorously for about a minute, and put it aside for the carbonate of lime to settle;

5. After the lapse of about half an hour siphon the clear liquid and filter it through a double filter-paper, in order to obtain a perfectly clear product, which must be kept cool, and protected from the light.

An excellent means of stabilising hypochlorite of soda consists in adding 1: 200,000 of permanganate of potash (5 milligrammes per litre of filtered solution).

The antiseptic solution is then ready for surgical use. It would contain 0.35 to 0.50 per 100 of hypochlorite of soda, with small quantities of neutral soda salts. It is practically isotonic with blood-serum.1

1 The freezing-point of these solutions is very slightly higher than that of blood-serum. ∆ = -0.60° to - 0.65° Cent.

The action of water upon the chloride of lime not being instantaneous, the product often contains lumps, from which the hypochlorite does not readily diffuse. The following experiment of Daufresne's is extremely instructive in this connection:

10 grammes of chloride of lime giving on titration 28.25 per cent. of active chlorine are placed in a flask with 1000 c.c. of distilled water. The whole is shaken vigorously for two minutes, and the solution titrated. Similar titrations are made from time to time.

Original titration of the solution (in CI %).

Titration after contact of

1/2 hour.

1 hour.

2 hours.

6 hours.

12 hours.

0.089

0.I76

0.206

0.259

0.281

0.282

As will be seen, solution is complete only after some hours' maceration; for which reason we have prescribed, in our technical directions, a prolonged contact between the chloride of lime and the water.

When the solution of carbonate and bicarbonate of soda is poured into the maceration of chloride of lime, an abundant precipitate of carbonate of lime appears, the result of the double decomposition which takes place between the soluble constituents of the chloride of lime and the soda salts.

The two principal reactions are:-

(C1O)2Ca + CO3Na2 = CO3Ca + 2C1ONa CaCl2 + CO3Na2 = CO3Ca + 2NaCl but the chloride of lime always contains a residue of non-chlorinated lime, which may amount to 20 per cent. of the total weight, and of which a small quantity dissolves in water during the course of preparation. This lime in its turn intervenes in a secondary reaction, when the formula is only concerned with the carbonate of soda: