Amount of N in the sample = 5 ∕ 25 = ·2 parts per 100,000.
(6) Determination of Organic Matter. Frankland’s combustion process involves the use of delicate and costly apparatus, and is seldom employed. In this process the organic carbon is evolved as carbonic acid, and the nitrogen as such.
Wanklyn’s ammonia process is based on the reduction of organic matter to ammonia. Part of this ammonia, free or saline ammonia, is simply combined with carbonic, nitric, or other acids, or is easily derived from the urea of urine, CH₄N₂O + 2H₂O = 2(NH₄)₂CO₃. Another part is only set free when the water is boiled with a strongly alkaline solution of permanganate of potassium. This is called the albuminoid ammonia.
In carrying out this method, a retort is taken, and after having been washed out, first with a little sulphuric acid, and then with some of the water to be analysed, 500 c.c. of the latter is put in, and the retort is connected with a condenser, and distillation begun; 50 c.c. of the distilled water is collected in a cylindrical glass tube called a Nessler glass. To this 1½ c.c. of Nessler’s reagent (mercuric iodide dissolved in a solution of potassic iodide and made alkaline by potass) are added. A rich brown colour is produced, if any ammonia is present in the distillate. The amount of ammonia in the distillate is determined by exactly imitating its colour by adding a known quantity of a standard solution of ammonium-chloride to 50 c.c. of ammonia-free distilled water, and then Nesslerising as before. Each c.c. of the dilute standard ammonium chloride solution is equivalent to ·00001 gramme of ammonia (NH₃).
If the first 50 c.c. of water distilled over gives only a slight colouration with the Nessler solution, no more water needs to be distilled over for free ammonia. If more is present, two more 50 c.c.’s must be distilled over, and the amounts of the standard solution required for imitating the test in each Nesslerised 50 c.c. added together. Thus, if 2 c.c. were needed. This
= ·00002 grm. NH₃, which is contained in 500 c.c. of the water
= ·00002 × 200 = ·004 parts saline NH₃ in 100,000 of water.
The free ammonia having been distilled over, 50 c.c. of an alkaline permanganate solution (containing 8 grammes KMnO₄ and 200 grammes of NaOH in 1100 c.c. of distilled water, boiled until the bulk is reduced to 1,000 c.c.) is poured into the retort, and distillation is begun again. Three successive 50 c.c.’s of water are collected, and then the distillation stopped. Each of these is Nesslerised, and the tint imitated as before with standard ammonia solution. The three amounts of ammonia thus found to be present are added together; and when multiplied by 200, we obtain the amount of albuminoid ammonia in 100,000 parts of water. This test is universally employed by water analysts along with the next test.
The amount of Oxygen Absorbed from permanganate of potassium is regarded as an approximate test of the amount of organic matter in water. Qualitatively this forms a favourite method of testing the purity of water. Two glass cylinders are taken, one filled with distilled water, one with the water to be tested. To each is added a given small amount of an acid solution of permanganate of potassium. The distilled water to which permanganate has been added will retain its pink colour; while, if the water being tested is very impure, it will speedily become decolourised. The rapidity and degree of decolourisation are a rough test of the amount of impurity. A rapid decolourisation proves the presence of organic matter having an animal origin, or of sulphuretted hydrogen, iron, or nitrites. Sulphuretted hydrogen is rarely present, and can be easily recognised by its smell; iron or nitrites are readily distinguished by their appropriate tests. In the absence of these, the rapid discolouration is an indication of animal contamination.
To Determine the Amount of Oxygen Absorbed, two glass-stoppered bottles, each holding about 350 c.c. are required. Into one, 250 c.c. distilled water, and into the other the same amount of the water under examination are placed. To each are then added 10 c.c. of standard permanganate of potassium solution[4] and 10 c.c. of a standard pure 25 per cent sulphuric acid solution. The two bottles, after being shaken, are placed in a water-bath at 27°C for four hours. At the end of this time add a few drops of potassium iodide solution to each bottle. The pink is now replaced by a yellow colour.[5] A standard thiosulphate solution (Na₂S₂O₃, 5H₂O)[6] is placed in a burette. From this the thiosulphate solution is run into the control bottle until the yellow colour almost disappears. Now a few drops of starch solution are added, and a blue colour is produced. The thiosulphate is then added cautiously until all the blue colour disappears. The amount of thiosulphate necessary for this is read off on the burette. The same process is repeated with the bottle containing the sample of water. The starch acts as an indicator. The amount of iodine liberated is an index of the amount of permanganate in the water, which has not been used up by its impurities. The amount of iodine liberated is measured by the amount of thiosulphate required to decolourise the solution. Thus—