Preparation of the Normal Solution of Sea Salt, measuring it by Volume

—If the drum contains 110 litres, we should put only 105 into it, in order that sufficient space may be left for agitating the liquor without throwing it out. According to the principle that 100 centimetres cube, or 1 ⁄ 10 of a litre of the solution should contain enough of sea salt to precipitate a gramme of pure silver; and, admitting moreover, 13·516 for the prime equivalent of silver, and 7·335 for that of sea salt, we shall find the quantity of pure salt that should be di olved in the 105 litres of water, and which corresponds to 105 × 10 = 1050 grammes of silver, to be by the following proportion :— 13·516: 7·335 ∷ 1050 gramm.: x = 569·83 gr. And as the solution of the sea salt of commerce, formerly mentioned, contains approximately 250 grammes per kilogramme, we must take 2279·3 grammes of this solution to have 569·83 gram. of salt. The mixture being perfectly made, the tubes and the pipette must be several times washed by running the solution through them, and putting it into the drum. The standard of the solution must be determined after it has been well agitated, supposing the temperature to remain uniform. To arrive more conveniently at this result, we begin by preparing two decimes solutions; one of silver, and another of sea salt. The decime solution of silver is obtained by di olving 1 gramme of silver in nitric acid, and diluting the solution with water till its volume become a litre. The decime solution of sea salt may be obtained by di olving 0·543 grammes of pure sea salt in water, so that the solution shall occupy a litre; but we shall prepare it even with the normal solution which we wish to test, by mixing a measure of it with 9 measures of water; it being understood that this solution is not rigorously equivalent to that of silver, and that it will become so, only when the normal solution employed for its preparation shall be finally of the true standard. Lastly, we prepare beforehand several stoppered phials, in each of which we di olve 1 gramme of silver in 8 or 10 grammes of nitric acid. For brevity’s sake we shall call these tests. Now to investigate the standard of the normal solution, we must transfer a pipette of it into one of these test phials; and we must agitate the liquors briskly to clarify them. After some instants of repose, we must pour in 2 thousandths of the decime solution of sea salt, which, we suppose, will produce a precipitate. The normal liquor is consequently too feeble; and we should expect this, since the sea salt employed was not perfectly pure. We agitate and add 2 fresh thousandths, which will also produce a precipitate. We continue thus by succe ive additions of 2 thousandths, till the last produces no precipitation. Suppose that we have added 16 thousandths: the last two should not be reckoned, as they produced no precipitate; the preceding two were nece ary, but only in part; that is to say, the useful thousandths added are above 12 and below 14, or otherwise they are on an average equal to 13. Thus, in the condition of the normal solution, we require 1013 parts of it to precipitate one gramme of silver, while we should require only 1000. We shall find the quantity of concentrated solution of sea salt that we should add, by noting that the quantity of solution of sea salt, at first employed, viz. 2279·3 grammes, produced a standard of only 987 thousandths = 1000 - 13; and by using the following proportion: 987: 2279·3 ∷ 13: x = 30·02 grammes. This quantity of the strong solution of salt, mixed with the normal solution in the drum, will correct its standard, and we shall now see by how much. After having washed the tubes and the pipette , with the new solution, we must repeat the experiment upon a fresh gramme of silver. We shall find, for example, in proceeding only by a thousandth at a time, that the first causes a precipitate, but not the second. The standard of the solution is still too weak, and is comprised between 1000 and 1001; that is to say, it may be equal to 1000 1 ⁄ 2 , but we must make a closer approximation. We pour into the test bottle 2 thousandths of the decime solution of silver, which will destroy, perceptibly, two thousandths of sea salt, and the operation will have retrograded by two thousandths; that is to say, it will be brought back to the point at which it was first of all. If, after having cleared up the liquor, we add half a thousandth of the decime solution, there will nece arily be a precipitate, as we knew beforehand, but a second will cause no turbidity. The standard of the normal liquor will be consequently comprehended between 1000 and 1000 1 ⁄ 2 , or equal to 1000 1 ⁄ 4 . We should rest content with this standard, but if we wish to correct it, we may remark that the two quantities of solution of salt added, viz. 2279·3 gr. + 30·02 gr. = 2309·32 gr. have produced only 999·75 thousandths, and that we must add a new quantity of it corresponding to 1 ⁄ 4 of a thousandth. We make, therefore, the proportion 999·75: 2309·32 ∷ 0·25: x. But since the first term differs very little from 1000, we may content ourselves to have x by taking the 0·25 ⁄ 1000 of 2309·32, and we shall find 0·577 gr. for the quantity of solution of sea salt to be added to the normal solution. It is not convenient to take exactly so small a quantity of solution of sea salt by the balance, but we shall succeed easily by the following proce . We weigh 50 grammes of this solution, and we dilute it with water; so that it occupies exactly half a litre, or 500 centimetres cube. A pipette of this solution, one centimetre cube in volume, will give a decigramme of the primitive solution, and as such a small pipette is divided into twenty drops, each drop, for example, will represent 5 milligrammes of the solution. We should arrive at quantities smaller still by diluting the solution with a proper quantity of water; but greater precision would be entirely needle . The testing of the normal liquor just described, is, in reality, le tedious than might be supposed. It deserves also to be remarked, that liquor has been prepared for more than 1000 a ays; and that, in preparing a fresh quantity, we shall obtain directly its true standard, or nearly so, if we bear in mind the quantities of water and solution of salt which had been employed.