A Text-book of Assaying: For the Use of Those Connected with Mines Part 12

Some hydrometers are graduated so as to show at a glance the percentage composition of the liquid they are intended to be used with. Gay-Lussac designed one to show the alcoholic strength of mixtures of alcohol and water; the construction of others upon the same principle is easy and perhaps useful. But when the principle is applied to complex liquids and mixed solutions, it is misleading.

The various methods of graduation ought all to give place to one showing a simple statement of the sp. g.

The method of determining sp. g. with the hydrometer is obviously inapplicable to the case of solids, and in the case of liquids it should not be used where exact figures are required. There are several other methods which may be used, but on the whole those with the specific gravity bottle are most convenient.

[Ill.u.s.tration: FIG. 35.]

~The specific gravity bottle~ (fig. 35) is a light flask of about 25 c.c. capacity, provided with a well-fitting perforated stopper. It is essentially a graduated flask, which measures a constant volume, but it does not much matter what the volume is.

_In taking the sp. g. of a liquid_ (_or, what is the same thing, a fused solid_) there is wanted the weights (1) of the flaskful of water and (2) of the flaskful of the liquid. Dividing the second by the first gives the required sp. g. The actual weighings required are--

(1) of the dry and empty flask,

(2) of the flask filled with water, and

(3) of the flask filled with the liquid.

The weighing of the flask once made need not be often repeated. It is well to do so now and then for safety's sake; but one weighing will serve for a large number of determinations. The same remarks apply to the weighing of the bottle filled with water. The bottle is dried by rinsing out first with alcohol and afterwards with ether; ether is very volatile, and a short exposure in a warm place will soon drive off the little remaining about the sides. The ether vapour should be sucked out through a gla.s.s tube. See that the bore of the stopper is dry as well as the bottle. Let the dry bottle stand in the box of the balance for a minute or two before weighing. The weight is, strictly speaking, not that of the empty bottle, but of the bottle filled with air. The empty bottle would weigh from 20 to 30 milligrams less. Correcting for this would, in most cases, only make a difference in the fourth place of decimals,[8] so that it is better to ignore the error.

The weight of the flask filled with water is got by filling it with distilled water, and inserting the stopper. The excess of water will overflow at the margin and through the bore. The bottle is wiped with a soft, dry cloth, taking care not to squeeze or warm the bottle. The bottle will remain filled to the top of the stopper. It is allowed to stand in the balance box for a minute or two, and then weighed.

Distilled water, as stated, should be used; the use of ordinary water may increase the weight by 5 or 6 milligrams. Many waters, if they have not previously been boiled, give off bubbles of air which render the weighing worthless.

The temperature of the water is of greater importance; lowering the temperature 2 will increase the weight by 10 or 12 milligrams. A beaker of water may be warmed or cooled to the required temperature; then the bottle is filled from it, and quickly weighed. If the balance-room is cooler than the water, the latter will draw back into the bottle, and a few small bubbles of air will enter; but even in extreme cases this will only increase the weight by a very small fraction of a milligram. There is more trouble caused when the room is warmer, for the liquid then expands and protrudes as a drop resting on the top of the stopper.

There will in this case be loss by evaporation, which in the case of the more volatile liquids, such as alcohol, is serious. To prevent this loss, as well as any that may arise by overflow, the stopper should be dilated above into a small cup, A (fig. 36), which may itself be stoppered. In a bottle of this kind the neck of the stopper is graduated, and the bottle is considered full when the liquid stands at the level of the mark in the neck. On inserting the stopper, the liquid rises into the cup, and is reduced to the level of the mark by absorption with pieces of filter-paper.

[Ill.u.s.tration: FIG. 36.]

For most purposes, however, there is no need for cooling and allowing room for subsequent expansion. The a.s.sayer, as a rule, can select his own standard temperature, and may choose one which will always necessitate warming. It will be handier in this case to have a bottle with a thermometer stopper. Of the two types shown in fig. 37, that with the external thermometer tube (A) is more generally useful.

[Ill.u.s.tration: FIG. 37.]

The bottle is filled at a lower temperature, and is then gently warmed so as to slowly raise the temperature to the required degree. The superfluous liquid is then at once wiped off, and the bottle cooled and weighed.

The weight of the flask filled with the liquid whose sp. g. has to be determined is ascertained in a similar way. Of course the temperature must be the same. If the liquid does not mix with water, the bottle should be dried before filling, but otherwise the flask need only be rinsed out two or three times with the liquid.

Having obtained the three weighings, deduct the weight of the bottle from each of the others to get the weights of the water and liquid respectively. Divide the latter by the former, the result shows the sp.

g. As an example, take the following, in which a rather large sp. g.

bottle was used:--

1. Weight of bottle 39.299 gram

2. Weight of bottle and water 81.884 "

3. Weight of bottle and paraffin 73.146 "

By subtracting 1 from 2 and 3 the result is as follows:--

81.884 grams 73.146 grams 39.299 " 39.299 "

------ ------ 42.585 of water. 33.847 of paraffin.

Divide the weight of the paraffin by that of the water--

42.585)33.8470(0.7948 29.8095 ------- .......

The sp. g. of the paraffin is 0.7948.

_The sp. g. of a fusible solid_ may be obtained in the same way at a temperature some degrees above its fusing point.

_The sp. g. of a solid in powder or gravel sufficiently fine to pa.s.s through the neck of the bottle_ is easily determined. If the bottle filled with water weighs 50 grams, and there is placed on the pan alongside of it 20 grams of a sand, the weight of the two together will of course be 70 grams. But if the sand is put in the bottle, it evidently displaces its own bulk of water; and if, on again weighing, the weight is found to be 62 instead of 70 grams, it is because the 20 grams of sand has displaced 8 grams of water. Bulk for bulk, the sand is 2-1/2 times as heavy.

In practice, the weight of the bottle filled with water will probably be already known; if not, it must be determined. A certain quant.i.ty, say 20 grams, of the powdered substance is then transferred carefully to the bottle. The bottle need not be dry inside, but its neck and outside must be. In making this transference a careful worker will make no loss, and the mode of working saves a little time. But it is better to weigh the dry flask; put into it 10 to 20 grams of the powder, and weigh again.

The increase in weight gives accurately the weight of powder in the bottle. About two-thirds fill the bottle with distilled water, and mix with the powder by gentle shaking. Air bubbles will disentangle themselves, and rise to the surface of the water. Wash back anything adhering to the stopper with a jet of water, and fill the bottle almost to overflowing. Allow it to stand for a minute or so; replace the stopper; warm to the required temperature; take off the superfluous moisture; wipe and weigh. As an example, take the following:--

1. Weight of bottle 12.681 grams 2. " " bottle filled with water 37.708 "

3. " " bottle with wolfram 40.821 "

4. " " bottle with wolfram and water 61.199 "

Subtract (1) from (3) to get the weight of wolfram taken:

40.821 grams 12.681 "

------ 28.140 "

add the weight of the wolfram to the weight of the bottle filled with water:

28.140 grams 37.708 "

------ 65.848 "

subtract (4) from this to get the weight of water displaced:

65.848 grams 61.199 "

------ 4.649 "

Divide the weight of the wolfram by the weight of the water displaced to get sp. g.:

4.649)28.140(6.053 27.894 ------ ......

_If the solid is soluble in water, or has a tendency to float_, some liquid other than water is used. Paraffin oil or oil of turpentine will do. The process is as follows:--The weight of the dry and empty bottle having been determined, add a sufficiency of the substance and weigh again to find how much has been added. Fill up with paraffin oil and weigh again. Clean out the substance by rinsing with paraffin; fill up and weigh. Calculate the sp. g. as if water had been used, and multiply by the sp. g. of the paraffin.

For example:

1. Weight of bottle 39.299 grams 2. " " bottle and nitre 57.830 "

3. " " bottle and paraffin 73.146 "

4. " " bottle and paraffin and nitre 84.665 "

5. " " bottle and water 81.884 "

First from (1),(3), and (5), calculate the sp. g. of the paraffin as already shown. It will be 0.7948. Deduct (1) from (2) to get the weight of the nitre:

57.830 grams 39.299 "