of the box is blackened. The flask or bottle containing the solution of silver to be estimated is placed on one side of, and close to, the screen which divides the box into two parts, and at such a height that the surface of the liquid in this flask is level with the centre of the round opening (3 centims. diameter) in the screen. A spherical flask containing a solution of potassium chromate is placed on the other side of the screen, and a lamp is placed behind this flask; this arrangement serves to throw a pencil of yellow light through the opening in the screen into the upper layers of the liquid in the flask.

Weigh out very carefully about 1 gram of pure silver into a flask of about 300 c.c. capacity; fit the flask with a caoutchouc cork previously boiled in caustic soda, and well washed, carrying a tube with a bulb blown on it and then bent twice at right angles; place the flask in water kept at 40o—45o, and allow the open end of the tube to dip into water in a beaker ; now add enough pure nitric acid, specific gravity 1.24, to dissolve the silver; when solution is complete, allow the flask to cool, and water to flow back into the flask; wash the exit tube once or twice with water. When the contents of the flask are cold add pure ammonia, drop by drop, until the liquid smells of ammonia, then add pure dilute acetic acid until the smell of ammonia is removed. Place the flask in the 'titration-box'. Weigh out very carefully a quantity of ammonium chloride rather less than that which the silver in solution is capable of completely decomposing (NH ClAq + AgNO2Aq = (NH)NO2Aq+AgCl; 107.66 Ag decompose 53.38NH CI); add this to the liquid in the flask; shake thoroughly and allow the silver chloride to settle. There now remains a small quantity of silver in solution; to estimate this, fill a 50 c.c. burette graduated to c.c. with the decinormal sodium chloride solution; arrange the burette so that the liquid can be dropped into the flask through the opening in the lid of the box; light the lamp, and arrange matters so that the upper layer of liquid in the flask is illuminated by the yellow light; add the sodium chloride solution one drop at a time; shake the flask well after each drop has run in, and allow to settle; continue the addition of sodium chloride until a drop of the solution ust ceases to produce an o palescence in the clear liquid in the flask.mgm. of silver in 1000 c.c. water may be thus determined with accuracy.

1

Calculate the weight of silver remaining in solution after

addition of the ammonium chloride; then find the weight of silver which has been used to precipitate the chlorine of 100 parts by weight of ammonium chloride. From these results calculate (1) the atomic weight of silver if Cl = 35.37; (2) the atomic weight of chlorine if Ag=107·66; assuming that the ratio of the atoms of silver and chlorine in silver chloride is 1 : 1.

CHAPTER II.

DISSOCIATION.

THE molecular weights of gasifiable elements and compounds are determined by finding the specific gravities of their vapours (s. Part II. Chap. III.

ex

Exp. 1). But some of the results obtained appear at first sight to be abnormal. The greater number of these apparently abnormal results are plained by the occurrence of dissociation. The phenomena of dissociation were briefly studied in Part II., but some other and more accurate Exps. must now be performed. (s. Pattison Muir's Elements of Thermal Chemistry, Chap. IV. Sect. 2.)

Exp. 1. Dissociation of hydriodic acid. Read Lemoine, Equilibres chimiques entre l'hydrogène et l'iode gazeux, Annales de Chim. et Phys. (5). 12, 145.

I. Lead a rapid stream of hydriodic acid gas for half an hour or so into a dry bulb (Fig. 54) of about 250 c.c. capacity, blown from the same glass as that from which pressure tubes are made. During the operation of filling, the bulb is surrounded by water of a known temperature. The neck of the bulb has been previously narrowed and thickened at a. The entrance of

b

Fig. 54.

moisture during the filling of the bulb with gas is prevented by suspending in the neck a small tube, b, filled with calcium chloride. The gas is conveniently prepared from a mixture of 120 grams iodine, 6 grams amorphous phosphorus, and 40 grams of a concentrated solution of hydriodic acid; the gas must be thoroughly dried by passage over phosphorus pentoxide. When the bulb may be presumed to be full of the gas it is sealed at a; at the same time the barometric pressure is read. The bulb is now heated for a couple of hours or more in the vapour of boiling sulphur (440°); and then cooled as rapidly as possible by wiping it with a damp cloth.

The sealed end of the neck is now broken off under a saturated solution of common salt which has been well boiled and cooled out of contact with the air. The iodine resulting from the dissociation of the hydriodic acid, as well as any undecomposed hydriodic acid, dissolves in the saline solution; the residual gas in the bulb is transferred to a graduated tube and its volume is read off, and reduced to 0° and 760 mm. (= v). It is necessary to transfer some of this gas to a eudiometer and analyse it, as it is impossible to get the bulb perfectly air-free. The volume of the bulb v' must also be accurately determined by weighing it full of water. We have now all the data necessary for determining the percentage dissociation of hydriodic acid under the conditions of the experiment.

The following example of the mode of calculation is taken from Lemoine's paper.

v = 35.48 c.c. This gas on analysis gave 96.7% H and 3.3% N. Hence v consisted of 34.31 c.c. H and 118 c. c. N (the latter number corresponds to 1.5 c.c. of air).

The bulb, which had a capacity v'394.8 c.c., had been filled with hydriodic acid gas at a temp. of 10.7° and under a pressure of 748.2 mm., hence it follows that the amount of gas operated on would occupy at 0o and 760 mm.

Now these 374.1 c. c. must have contained 1.5 c.c. of air. Hence the true quantity of HI used was 374.1-1.5=372.6 c.c. The half of this =1863 c. c. represents the hydrogen of this hydriodic acid; but on warming, the 32 c. c. of oxygen in the air present would oxidise ·64 c.c. of this hydrogen; hence there are only 185.7 c.c. of "disposable" hydrogen. Hence the ratio of free hydrogen to total disposable hydrogen (which is a measure of the dissociation)

374.1 623

or about

At the temperature of 350° at which this experiment was conducted it is easy to see that the pressure must have been X 2.15 atmospheres.

394.8 273

Now perform another experiment at 440°; but this time let the hydriodic acid gas be introduced into the experimental bulb at such a diminished pressure as will become about an atmosphere at 440°; this is done by connecting the neck of the bulb with a three-way tube, the other limbs of which are connected, respectively, with a vacuum-pump, and a large globe containing dry hydriodic acid. In this case it will be necessary to leave the bulb in sulphur vapour for 25 hours or The warming may be effected in periods of 12 hours each, or less, provided after each warming the bulb and contents are cooled rapidly.

SO.

The results of the two experiments ought to shew that the limit of dissociation of hydrogen iodide at 440° varies very slightly indeed with the pressure of the reacting gases.

II. Perform two experiments similar to the above, but heat the bulbs to a temperature of about 360°, for say 8 hours. In neither case will equilibrium between the two inverse

be reached, as was the case at 440°; but your experiments ought to shew that in the bulb which has been submitted to the lower pressure there has been more dissociation than in that subjected to the higher pressure.

The constant temperature of 360° may be obtained by immersing the bulbs in the vapour of boiling mercury, contained either in a cylindrical vessel of sheet iron of such a length that the mercury vapour is condensed in its upper cooler portions, or preferably in an iron bottle of the kind designed by Deville for use in vapour density determinations. (Annales de Chim. et Phys. (3), 58, 257.)

III. Find as accurately as possible the volumes of two globes. Into one of these put as much iodine as is equivalent to the amount of hydrogen the globe will contain under atmospheric pressure, and into the other just half as much iodine as is equivalent to the amount of hydrogen the globe will contain under ordinary pressure [100 c.c. H at 0° and 760 mm. are