and contains when filled a known weight of water (25-30 grammes) at 15° C. Some pycnometers are fitted with thermometers, used at the temperature of the air, whatever it happens to be, and then the weight of water in it calculated from the table on p. 5. The bottle is filled with the liquid the specific gravity of which is to be determined; it is weighed, and the specific gravity or density obtained by the formula : sp. gr.: = = ; the weight of the liquid, w' the weight of the water. The result is then obtained in comparison with water, which is taken as unity. In medical work it is often found more convenient to take water at 1000; a urine of specific gravity 1020 means one which is 1.02 times heavier than water bulk for bulk. DETERMINATION OF REACTION. ALKALIMETRY. Litmus papers, or a neutral litmus solution, are usually employed to determine whether a substance is neutral, acid, or alkaline. Neutral substances have no effect on either red or blue litmus, but in presence of organic materials a neutral solution will often turn delicate glazed blue litmus papers faintly red, and red ones faintly blue. Acid substances turn blue litmus red. In the case of volatile acid, the red colour passes off as the acid evaporates. Alkaline substances turn red litmus blue. At night it is best to examine the transition in colour by monochromatic (sodium light); the red colour appears colourless, the blue is blackish. In measuring the amount of acidity or alkalinity of a solution it is titrated with a standard solution of acid or alkali respectively; the indicator of the end of the process being the change in colour produced in the litmus. It is, in fact, a simple example of the volumetric method. Recently, however, more delicate indicators than litmus have been employed, and the following is a list of the principal ones: 1. Methyl orange, 1 gramme dissolved in a litre of water. This is only applicable to titration with mineral acids; it is not affected by carbonic or sulphydric acids in the cold. It is an admirable indicator for ammonia and its salts. The colour given is pink with acid, yellow with alkali. 2. Phenacetolin, 2 grammes dissolved in 1 litre of alcohol. The solution is dark brown; it gives a scarcely perceptible yellow with caustic soda or potash; with ammonia and the normal alkaline carbonates a dark pink; with the bicarbonates a brighter pink; and with the mineral acids a golden yellow. 3. Phenolphthalein, 1 gramme in 1 litre of 50 per cent. alcohol. A few drops of the indicator show no colour in the ordinary volumes of neutral or acid liquids, the faintest excess of caustic alkalis gives a sudden change to purple-red. It possesses the advantage of great delicacy, but the disadvantage of being useless for the titration of free ammonia or its compounds. 4. Rosolic acid, 2 grammes in 1 litre of 50 per cent. alcohol. Its colour is pale yellow unaffected by acids, but turning to violet-red with alkalis. It is not reliable for organic acids. 5. Lacmoid. This is prepared from resorcin, and behaves like litmus. Lacmoid paper is also prepared. Lacmoid, rosolic acid, phenacetolin and phenolphthalein are capable of showing change of colour with of the quantity of acid or alkali necessary in the case of methyl orange or litmus.' The normal solutions most frequently used in estimating acidity or alkalinity are those of sodium carbonate (53 grms. Na,CO, per litre), potassium carbonate (69 grms. K,CO, per litre), sulphuric acid (49 grms. H,SO, per litre), oxalic acid (63 grms. of C,H,O,.2H,O, or 45 grms. of C,H,O, per litre), hydrochloric acid (36-37 grms. HCl per litre), caustic soda or potash (40 grms. NaHO, or 56 grms. KHO per litre), and semi-normal ammonia2 (8.5 grms. NH, per litre). 100 c.c. of any of the acid solutions exactly neutralise the same volume of any of the alkaline solutions. except in the case of semi-normal ammonia, which requires only half the quantity of acid. THE CENTRIFUGAL MACHINE The separation of precipitates too fine to filter off, of corpuscles from serum, of cream from milk, &c. &c., may be facilitated by subjecting the fluid to the action of a centrifugal machine (see figure). The liquid is placed in tubes at the margin of a horizontal rotating disc, worked at a high rate of speed by machinery (1000 FIG. 11.-Centrifugal machine as made by Fr. Runne of Basel. Glass vessels containing the substances to be centrifugalised are placed within the six metallic tubes which hang vertically while the disc is at rest; when the machinery is set going they fly out into the horizontal position. A water motor is a very convenient motive power for these instruments. revolutions per minute). The tubes fly into the horizontal direction, and the heavy particles settle to the far end of the tube; the upper fluid can then be decanted or pipetted off. The time that this takes varies with the relative densities of the substances to be separated. Serum and blood corpuscles are usually separable by this means after about 30 to 60 minutes' whirling. 1 For full particulars see Thompson, Chem. News, vol. 47, pp. 123, 185, vol. 48, pp. 32, 119. 2 It is unsafe to use normal ammonia. C HEAT COAGULATION AND SATURATION WITH SALTS are processes used especially in connection with the proteids under which they are fully described. DETERMINATION OF RELATION OF SOLIDS AND WATER IN ANY SUBSTANCE If the substance is liquid, a weighed quantity is evaporated to dryness in a weighed crucible or capsule on a water-bath, and the residue is then thoroughly dried to constant weight in an air-bath (110° C.). If the substance is solid it is finely divided and weighed in a weighed crucible, then dried to constant weight at 110° C. In each case the total loss of weight is the amount of water, the weight of the residue is the amount of total solids. These numbers should be, for convenience sake, calculated out as percentages. The relation of organic to inorganic solids may then be somewhat roughly determined by incinerating the residue; the ash remaining after the burning of the organic substance gives the weight of inorganic, the loss of weight on ignition that of the organic substances. CHAPTER III ULTIMATE ANALYSIS OF ORGANIC COMPOUNDS for its object the A small number of THE ultimate analysis of organic compounds has determination of the elements contained in them. organic compounds consist of carbon and hydrogen, the greater number contain carbon, hydrogen, and oxygen, most of the rest carbon, hydrogen, oxygen, and nitrogen, and a small number sulphur, and a smaller number still sulphur and phosphorus in addition. The same method of analysis applies to compounds whether they contain oxygen or not; it is, however, necessary before commencing a quantitative analysis that the operator should know positively whether nitrogen, sulphur, or phosphorus is present or absent. TESTS FOR NITROGEN 1. Burn the substance; if it contains a tolerably large amount of nitrogen, the characteristic odour of burnt hair or feathers is given off. If the smell is distinctly perceptible no further test is necessary, but if not, a more delicate method must be adopted. 2. The substance is mixed with potassium hydrate in powder, or with soda-lime, and the mixture heated in a test-tube. If the substance contains nitrogen, ammonia will be evolved, which may be detected by its odour, reaction, and fuming with hydrochloric acid; or the products of combustion may be conducted into dilute hydrochloric acid; evaporate the acid on the water-bath, dissolye the residue in water, mix the solution with platinum tetrachloride, evaporate nearly to dryness, and treat the residue with alcohol. If the residue dissolves without leaving any double chloride of ammonium and platinum, the substance may be considered free from nitrogen. TESTS FOR SULPHUR 1. Solids are fused with about 12 parts of pure potassium hydrate and 6 of potassium nitrate; the mass is allowed to cool, dissolved in water, acidified with hydrochloric acid, and tested for sulphates with barium chloride. Care must be taken that the reagents used are free from sulphuric acid. 2. Liquids are treated with fuming nitric acid, or a mixture of nitric acid and potassium chlorate, at first in the cold, and finally with the application of heat; the solution is evaporated nearly to dryness, diluted, filtered, if necessary, and then tested for sulphates. 3. The following test serves to detect sulphur in organic compounds in the unoxidised state only. The substance is boiled with a strong solution of potassium hydrate, and evaporated nearly to dryness. The residue is dissolved in a little water, poured into a small flask A, which is then loosely corked. Through the cork a funnel tube c passes, which is allowed to dip into the fluid at the bottom. A slip of paper b, moistened with lead acetate and then with a few drops of ammonium carbonate, is allowed to hang down the neck of the flask; dilute sulphuric acid is poured down the funnel; if sulphur is present the slip of paper is turned brown from the action of the sulphuretted hydrogen which is evolved; or the sulphide of potassium may be detected by a solution of lead oxide in soda, which is turned black or brown. IFG. 12. TESTS FOR PHOSPHORUS The methods 1 and 2 just described in testing for sulphur may also be employed for phosphorus. The solution obtained is examined for phosphates, either by a mixture of magnesium sulphate, ammonium chloride and ammonia, which gives a white precipitate, or preferably by the yellow crystalline precipitate given by a nitric acid solution of ammonium molybdate. If method 2 is used the greater part of the nitric acid must be first removed by evaporation. QUANTITATIVE ANALYSIS OF SUBSTANCES CONSISTING OF CARBON AND HYDROGEN, OR OF CARBON, HYDROGEN, AND OXYGEN The principle first proposed by Liebig for the analysis of these compounds was as follows. The substance is burnt and carbonic acid and water are formed; these products are separated from one another and weighed; the carbon is calculated from the weight of carbonic acid, the hydrogen from the amount of water. If the sum of the carbon and hydrogen is equal to that of the original substance, that substance contains no oxygen; if it is less than the weight of the substance, the difference expresses the amount of oxygen present. Methods have been proposed for the direct estimation of oxygen, but the oxygen is generally obtained by difference. |