ULTIMATE ORGANIC ANALYSIS The limited number of elements (carbon, hydrogen, oxygen, nitrogen, sulphur, halogens and occasionally phosphorus and some metals) employed by nature in building up her vast store of organic compounds would suggest that the analyses of such substances are simple operations, and this is true in so far as the determination of the elements present, their relative proportions, and the molecular complexity of the compound are concerned. Naturally such information tells us nothing of the structure of the compound and, indeed, leaves us much in the same position as we should be in if we were interested in some building and were supplied with the builder's estimate for material while denied ac cess to the architect's drawings and plans. The problem of deter mining the manner in which the elements present in organic com pounds are arranged, important as it is, lies outside the scope of the present article, which is limited to the description of the ultimate analysis. With the exception of oxygen, all the elements present in organic compounds can be estimated readily; oxygen is nearly always determined by difference, and in view of this fact it is of great importance that a careful qualitative examination be made of a substance, otherwise the results of the quantitative analysis may be interpreted erroneously.
Nitrogen, Halogens, Sulphur.—When organic compounds con taining any or all of these elements are heated strongly with me tallic sodium they are decomposed with the formation of sodium cyanide, halide and sulphide, respectively (Lassaigne's test). To test for these elements, therefore, 1mg. of the substance and a small pellet of sodium (or potassium) are heated in a narrow test-tube, gently at first and afterwards more strongly, until the glass softens ; the hot tube is shattered by plunging it into water, which converts any residual sodium into sodium hydroxide, yield ing an alkaline solution of the sodium salts, which is boiled and filtered, and the filtrate is used in the following (a) Nitrogen.—A portion of the filtrate is mixed with a solu tion of ferrous sulphate containing a small quantity of the ferric salt and boiled, thereby converting any sodium cyanide present to ferrocyanide, which gives a bluish-green solution and a precipi tate of Prussian blue on adding hydrochloric acid.
(b) Halogens.—A second portion of the alkaline filtrate is acidified with nitric acid and silver nitrate added, any precipitate of chloride, bromide or iodide, or a mixture of these, is examined in the usual manner. If the original substance contain also nitro gen or sulphur, the acid solution must be boiled to expel the hydrogen cyanide or hydrogen sulphide before adding the silver nitrate.
(c) Sulphur.—A third portion of the alkaline filtrate is tested for sodium sulphide (i.) by the formation of a deep violet colour when treated with a few drops of a freshly prepared solution of sodium nitroprusside; (ii.) by the evolution of sulphuretted hydrogen when acidified—this is readily detected by the odour and also by lead acetate test paper.
Phosphorus.—A small portion of the organic compound is in timately mixed with about ten times its bulk of a mixture of sodium carbonate and peroxide (2 :3) and heated to fusion in a nickel crucible. The cold water extract of the melt is filtered, acidified with nitric acid, and tested for phosphoric acid with ammonium molybdate.
Metals.—(i.) Volatile metals. (a) Arsenic.—The presence of arsenic in an organic compound is usually revealed by the forma tion of a dull grey mirror of the metal on the walls of the test tube when the compound is fused with sodium as in Lassaigne's test. (b) Arsenic and Antimony.—A small quantity of the com pound is fused with a mixture of sodium carbonate and peroxide as in the test for phosphorus. The fused melt is extracted with water, acidified and tested with sulphuretted hydrogen. (c) Mercury.— When an organic compound containing mercury is mixed with soda lime and heated in a long test-tube, a mirror of metallic mercury is formed.
(ii.) Non-volatile Metals.—Any of these metals that may occur in organic compounds are found as such or in the form of their carbonate or oxide in the ash left after the compound has been ignited on porcelain or silica. The ash is dissolved in dilute hydro chloric acid and the solution examined for metals in the usual manner (see section on "Inorganic Analysis").
Quantitative Analysis of Organic Compounds.—Estima tion of Carbon and Hydrogen.—The carbon and hydrogen in an organic compound are estimated by completely oxidizing a weighed quantity of the substance and weighing the carbon diox ide and water thus formed. The simple method devised by Liebig in 1831 for carrying out this operation is in all essential principles the one still employed. A weighed quantity of the substance is heated in a tube filled with dry copper oxide, and the water and carbon dioxide are absorbed in weighed tubes filled with dried calcium chloride and potassium hydroxide solution or soda lime respectively. A current of dry air or oxygen is passed through the tube to sweep the products of combustion into the absorption ap paratus. Fig. i shows the essential details of the apparatus re quired for these estimations; a and b are closely fitting, short spirals of copper gauze which serve to hold the copper oxide in position ; the copper oxide is prepared by oxidizing copper wire; c and d are longer copper oxide spirals ; f is the platinum or porce lain boat containing the weighed quantity of the compound.
In the absorption apparatus, the water is collected in the glass stoppered U-tube g, filled with calcium chloride (carefully freed from lime). The carbon dioxide is absorbed by the soda-lime con tained in the U-tubes h and h', the second limb of each tube con taining a short filling of calcium chloride. The two soda-lime tubes may be replaced by the apparatus k, of which the bulbs are charged with concentrated potassium hydroxide solution and the side tube filled with soda-lime and calcium chloride. The appara tus for purifying the current of air or oxygen is connected with the anterior end of the tube and consists of a series of washing cylinders charged with potassium hydroxide or sulphuric acid, and U -tubes filled with soda-lime or calcium chloride to remove moisture and carbon dioxide from the incoming gas. This ap paratus is arranged in duplicate, one for the air and one for the oxygen supply. Any simple form of gas-heated furnace may be employed (fig. 2), or the tube may be heated electrically.
The combustion tube and its copper oxide charge are dried by heating the tube to a dull red heat for about an hour while a cur rent of dry air is passed, the posterior end of the tube being pro tected by a calcium chloride tube. At the end of that time the fore part of the tube containing the oxidized spiral and the first six inches of copper oxide is cooled. While this process is in operation, the weighings are made. About o.2gr. of the pure dry compound to be analysed is weighed into the plati num or porcelain boat if the compound is a solid or a non-volatile liquid. Volatile liquids are weighed in small thin glass bulbs drawn out to a capillary opening. The tubes for collecting the water and carbon dioxide are also weighed, and attached to the posterior end of the combustion tube. The boat and its contents are then introduced into the cooled anterior end of the combustion tube.
A slow current of air is passed through the tube, and the burn ers under the oxidized spiral in the rear of the boat are lighted. The heating is gradually extended towards the boat by moving forward the heated screen over the tube, or increasing the flame surface under the tube, the object being to distil the contents of the boat, or the volatile products of decomposition, on to the cold copper oxide where the combustion is completed. This most criti cal part of the operation must be carried out very slowly, and is finally completed in a current of oxygen which burns any residue of carbon left in the boat, and re-oxidizes the reduced copper oxide. After displacing by air the oxygen in the apparatus, the absorption tubes are removed, closed, cleaned and allowed to cool in the balance room before being re-weighed. From the weight of carbon dioxide and water yielded by the definite weight of the compound, the percentages of carbon and hydrogen in the com pound are readily calculated.
The simple method described above applies to compounds con taining carbon, hydrogen and oxygen only; if elements other than these are present the filling of the tube must be modified as follows : (a) Nitrogen present. The copper oxide spiral (d) is re placed by a similar spiral of freshly reduced copper to reduce any oxides of nitrogen that may be formed during the combustion. It is introduced into the end of the tube that has been previously cooled, before attaching the absorption apparatus. (b) Halogens, sulphur or arsenic present. The first six inches of copper oxide after the boat are replaced by a lead chromate cartridge, made by filling a hollow cylinder of copper gauze with freshly ignited fused lead chromate. The copper container becomes oxidized in the preliminary heating of the tube, non-volatile lead compounds being formed with the sulphur or arsenic in the compound, and sparingly volatile compounds with the halogens. A short spiral of silver gauze should, therefore, be introduced at the posterior end of the tube when the compound contains halogens.
Estimation of Nitrogen.—The two methods in use for the esti mation of nitrogen are (a) the absolute method (Dumas), in which the nitrogen in a known weight of the compound is elimi nated in the gaseous form and estimated by direct measurement ; (b) the Kjeldahl method, in which the nitrogen in a known weight of the material is reduced to ammonia and estimated volumetri cally by titration. (a) The absolute method consists in the com plete oxidation of a known weight of the compound by heating it with copper oxide in a tube in a current of carbon dioxide, pass ing the gaseous products of combustion over a heated copper spiral and collecting the nitrogen in a gas burette over strong po tassium hydroxide solution. The gas is afterwards transferred to a calibrated eudiometer tube and measured over water. The method of filling the tube is indicated in fig. 2. The three grades of coarseness in the copper oxide used are obtained by crushing the ordinary copper oxide made from wire, and passing it through two copper gauze sieves, 20 strands and seven strands to the centimetre respectively; the first retains the coarse and medium grades, and the second the coarse. The amount of the substance used in an analysis should be sufficient to give about 20 cu.cm. of nitrogen. The charged tube is placed in the furnace and connected with an apparatus for generating carbon dioxide and with a Schiff's nitrometer filled with a concentrated solution of potas sium hydroxide. While the air in the apparatus is being displaced by carbon dioxide, the part of the tube containing the coarse cop per oxide and reduced spiral is heated to dull redness. When no insoluble gas collects in the charged nitrometer, the current of carbon dioxide is stopped, and the nitrometer is filled with the po tassium hydroxide solution which is allowed to overflow into the thistle funnel seal attached to the end of the capillary transfer ring tube. The burners under the copper oxide spiral are then lighted, and when this part of the tube is at a dull red heat, the heating is extended towards the mixture of the substance with copper oxide, and so regulated that not more than three bubbles of gas are ascending the nitrometer tube at the same time. Finally, when all the burners are lighted, the tube has attained a uniform dull red heat, and there is no further evolution of gas, a slow cur rent of carbon dioxide is again passed through the tube to sweep out the remainder of the nitrogen. The nitrometer is then re moved, the gas reduced to atmospheric pressure, by bringing the liquid in the open arm of the nitrometer to the same level as that in the closed arm, and after about 15 minutes transferred to a eudiometer tube over boiled-out water, which is then completely immersed in a tall glass cylinder of water containing also a ther mometer. After about ten minutes the volume of the nitrogen (v) the temperature (t) and the barometer (p) are read. If p' is the tension of aqueous vapour under these conditions and w denotes the weight of substance taken, the percentage of nitrogen in the compound is given by the expression (b) The Kjeldahl method is a modification of Will and Varren trapp's method and is used for the estimation of nitrogen in such materials as foodstuffs, therapeutic substances, fertilizers, etc., in which the amount of nitrogen is small and many determinations have to be carried out simultaneously and rapidly. The method consists in decomposing the organic material by heating it with concentrated sulphuric acid ; the carbon is oxidized and the nitro gen converted to ammonia which is retained as the sulphate, and afterwards estimated in the usual manner by distillation (see AMMONIA). A weighed quantity of the material (o.5 to 5.ogr.) is placed in a long-necked, pear-shaped flask of 500 cu.cm. capac ity, 20 cu.cm. of concentrated sulphuric acid are added, the flask is clamped in an inclined position and the contents heated gradu ally to avoid frothing. After the acid has been boiling for about 15 minutes, mgr. of potassium sulphate are added, and the heat ing is continued until the contents of the flask are clear and a faint straw colour. In cases of not easily oxidizable materials, small quantities of certain metallic salts such as copper or mer curic sulphate or mercurous iodide may be added. Finally the cold acid liquid is transferred, with the washings of the flask, into the distilling flask of an ammonia distillation apparatus ; after the contents of the flask have been made alkaline, the ammonia is dis tilled into a measured volume of standard sulphuric acid and estimated by titrating the excess of acid.
The Kjeldahl method can also be applied to the analysis of pure organic compounds, provided that those compounds which contain nitrogen combined directly with oxygen or with nitrogen as in nitro-, nitroso-, azo- or diazo-compounds or hydrazines, undergo a preliminary reducing operation.
Estimation of Halogens.—The estimation of halogens involves the complete decomposition of the compound, with the conver sion of the halogen element into the hydracid or one of its salts, which is subsequently precipitated and estimated as the silver salt. (a) Liebig's is the oldest method and the one applicable to the largest number of organic compounds. It consists in decom posing a weighed quantity of the compound by heating it to red ness with a large excess of pure lime in a glass tube sealed at one end, plunging the hot tube into cold water, dissolving the lime and calcium halide in cold dilute nitric acid, filtering, and estimating the halogen acid in the filtrate in the usual manner. Piria and Schiff modified the method in the direction of shortening the time of the operation, by replacing the lime by a mixture of lime and sodium carbonate, and the glass tube by two nickel crucibles, the smaller one containing the mixture of the substance being inverted in the larger, and the annular space between the two filled with the lime and sodium carbonate mixture. (b) Stepanoff's method, as modified by Bacon, is applicable to compounds in which the halogen is readily eliminated as hydracid by hydrolysis or by re duction. It consists in heating a weighed quantity of the com pound with sodium and alcohol, and estimating the sodium halide in the resulting solution. (c) The method of Carius, as modified by Kuster, is the one most generally employed; it consists in the de structive oxidation of a weighed quantity of the compound (about o.2gr.) by means of fuming nitric acid (2 cu.cm.) in the presence of silver nitrate in a sealed tube at a temperature of C. The halogen is converted into the silver salt and weighed as such.
Estimation of Sulphur.—The estimation of sulphur in organic compounds involves the destructive oxidation of the substance whereby the sulphur is converted to sulphuric acid and weighed as the barium salt. The two methods in general use The method of Carius, in which the operation is carried out as in the preceding case, no silver nitrate is required, but as the oxida tion is facilitated by the presence of a small quantity of bromine, a crystal of potassium bromide is placed in the tube containing the compound. The contents of the tube are diluted with water, the nitric acid removed by evaporation and the sulphuric acid precipitated and weighed as the barium salt. (b) The method of Asboth, in which the oxidizing agent employed is a mixture of sodium peroxide (three parts) and sodium carbonate (two parts). A weighed quantity of the substance (0.2 to o.5gr.) is mixed with about I sgr. of the oxidizing mixture in a nickel crucible which is then carefully heated until the contents become liquid. When cold, the melt is extracted with water, and the soluble sulphate estimated in the usual manner.
Estimation of Phosphorus.—Organic compounds containing phosphorus are oxidized by fuming nitric acid, by concentrated nitric acid and potassium permanganate, or by chromic and sul phuric acids. The resulting phosphoric acid is precipitated as am monium phosphomolybdate, and subsequently weighed as mag nesium pyrophosphate.
Estimation of Metals in the Salts of Organic Acids and Bases. Gold, silver and platinum in organic salts are weighed as the me tallic residue left after the organic material has been destroyed by igniting a weighed quantity of the dry salt in a platinum or silver crucible. Sodium, potassium, barium and calcium salts are de composed by igniting a weighed quantity of the dry salt in the presence of pure sulphuric acid and weighing the sulphate of the metal.
Later modifications have aimed at reducing the time required for a combustion by a corresponding reduction in the amount of material analysed and the magnitude of the apparatus employed, and the micro-analytical methods devised by Pregl, Dubsky, Funk and others are now fairly extensively employed in schools of or ganic research. Using 5-15mg. of the substance, a carbon and hydrogen estimation can be completed within an hour.
H. ter Meulen and his school have recently described new methods for the ultimate analyses of organic compounds which include the direct estimation of oxygen. The analyses are carried out by the combustion (oxidation), or the destructive hydrogen ation method, usually with the help of catalysts. The operations can be conducted with great rapidity, as about 5omg. of the com pound are employed in each estimation, and many of the final operations are volumetric instead of gravimetric. Thus nitrogen is reduced quantitatively to ammonia and estimated by titration; the halogens are reduced by hydrogen containing ammonia to the ammonium halide and estimated volumetrically by the Volhard method.