976

WATER.— Analysis. Nitrates and Nitrites.

permitting air to follow. If evaporation lias beencarried to dryness the residue is treated with 1 or2 c.c. of boiling distilled water, and the solutionthus obtained run into the tube. The beaker isrinsed once with hot distilled water and twice orthrice with strong sulphuric acid, using as little aspossible of water, and so much acid as to make up avolume which is to the total volume of water usedas 3 : 2. If any air has been admitted, or, as some-times occurs, any carbonic anhydride has beenevolved, it can be sucked out of the tube into thecup. As soon as the beaker is rinsed out, holdthe tube in the hand with the knuckles upwardand the wet thumb closing the tube, and shake vigor-ously with a motion upwards and forwards in thedirection of its length, so as to throw the mercuryup into the acid solution, which latter should notbe allowed to pass down to the thumb. Some ofthe mercury is soon finely divided, and if nitratesor nitrites are present nitric oxide is evolved; andwhen the pressure from within is considerable thethumb should be slightly, but not wholly, relaxed,some of the mercury being allowed to escape, but acertain amount of pressure still maintained withinthe tube. When no nitrates and nitrites are presentthe globules of mercury remain mobile and bright;but with a considerable quantity they break up onshaking into irregular masses, which adhere together,and look as if alloyed with some heavy metal. Inthe latter case it often happens that no gas isevolved for some little time; but it is at last pro-duced with great rapidity, and at the close of theoperation the finely divided mercury will, as inother cases, consist of minute spheres.

When the volume of gas ceases to increase it isdecanted into the laboratory vessel of the gas appa-ratus, a stream of mercury allowed to pass overfrom the measuring tube to remove acid from theentrance to the capillary connecting tube, and thegas then passed over and measured. The weight ofnitrogen present is calculated as in the determinationof organic nitrogen; but as the gas measured is nitricoxide, which contains half its volume of nitrogen,it will be necessary to divide the result by 2.

If the quantity of ammonia present is greater thanOT part per 100,000, it is desirable to guard againstloss of nitrogen by decomposition of ammonicnitrite during evaporation. Instead of employingthe residue from the determination of the total solidmatters, a fresh portion of the sample is taken, andto this, slightly acidified with dilute sulphuric acid,solution of potassic permanganate is gradually addeduntil a pink colour is obtained, which will endurefor about a minute. The liquid is then renderedslightly alkaline to test paper with sodic carbonate.Use as little as possible of each reagent. Thenitrites being thus converted into nitrates the evap-oration, with addition of sulphate of silver, may beproceeded with without fear of loss.

With waters containing an exceptionally largequantity of soluble matter (e.g. sea water), the pre-ceding method is often inapplicable, as the volumeof the liquid cannot be sufficiently reduced. If the

quantity of the organic nitrogen be small—say, lessthan 0T part per 100,000—the following methodmay generally be used:—Take 200 c.c. of the sample,add to it 10 c.o. of a solution of caustic soda in alitre of water, and boil rapidly in an open porcelaindish until the volume is reduced to about 70 c.c.Allow to cool, and pour it into a tall glass cylinderof about 120 c.c. capacity, rinsing it with water freefrom ammonia. Sink in this solution, by means of abent piece of heavy glass rod, a piece of thin cleanaluminium foil, close the cylinder with a cork per-forated with one opening, in which is fixed a smallglass tube filled with fragments of clean recentlyignited pumice, moistened with a few drops ofdilute hydrochloric acid free from ammonia. Thistube should be covered by a test tube or very smallbeaker. In from four to six hours the whole of thenitrates and nitrites are reduced, the nitrogen beingconverted into ammonia. The contents of thecylinder and of the pumice tube are washed into asmall retort, the cylinder and tube being well rinsedwith water free from ammonia; and the ammoniaproduced is distilled over and estimated in the usualway with Nessler’s solution. A correction basedon several blank experiments should be applied tocorrect for the small quantity of ammonia whichappears to be inevitably present in the reagents andapparatus. The caustic soda solution should beprepared as follows:—Dissolve 100 grams, of solidcaustic soda in a litre of distilled water; when cold,put the solution in a tall glass cylinder, and addabout 100 sq. c.m. of aluminium foil, keeping it atthe bottom with a glass rod. As soon as this isdissolved, boil until about one-third of its volumehas been drawn off, allow to cool, and make up tothe original volume with water free from ammonia.

Hydrochloric acid is sufficiently pure if two orthree drops do not contain an appreciable quantityof ammonia. If this is not already the condition ofthe ordinary pure acid, it should be rectified oversulphuric acid.

The nitrogen as nitrates may be more rapidlyestimated by means of a standard solution of indigoin presence of strong sulphuric acid at a hightemperature. This process, originally proposed byMarx and improved by Sutton, has recently beenvery elaborately investigated by Warington; andalthough it does not rival the mercury method inaccuracy, it will in many cases be sufficient for thepurpose in view, while its rapidity and simplicitygreatly recommend it. No gas apparatus is required,and it is not necessary to concentrate the sample tobe examined.

The indigo solution is made by dissolving puresoluble indigo-carmine (or according to Waring-ton, indigotine) in distilled water. A convenientstrength is one of which 10 e.e. is decolorized by10 c.c. of a standard solution of pure potassic nitratecontaining 002 gram, of nitrogen, or 0T4442 gram,of KNOg, per litre. The exact value of the indigosolution in terms of nitrogen as nitrates, must beascertained by experiments precisely similar to thoseby which the samples of water are examined.