(2.) The Soluble Ash.-The ash is boiled up two or three times with water in the same platinum dish; filtered, and the filtrate evaporated to dryness, heated to dull redness, and weighed. (3.) The Ash Soluble in Acid.-The portion of ash insoluble in water is boiled up with HCl, and filtered from the sand; the latter is washed, dried, and weighed. (4.) The Alkalinity of the Ash.-The solution in water from (2) is coloured with cochineal, and titrated with d. n. acid: the result may be usually expressed as potash. (5.) The Percentage of Chlorine.-The determination of chlorine in the ash usually gives results too low, especially if the substance burnt is one, like bread, of difficult combustion, or containing substances which decompose chlorides at a red heat. Notwithstanding this defect, in a series of ashes burnt under similar circumstances, the amount of chlorine found gives fair comparative results. Should there be any special necessity for an accurate determination of chlorine, no volatilisation will occur in the combustion of most articles of food, if they are simply well carbonised and not burnt to a complete ash, and if the charcoal be finely powdered and extracted with plenty of boiling water. The chlorine may be determined gravimetrically by nitrate of silver, or more conveniently by a standard solution of nitrate of silver, using as an indicator neutral chromate of potash. Should alkaline phosphates be present, they must be first removed by baryta water. (6.) The Phosphoric Acid.-The usual method of determining phosphoric acid is to dissolve the ash in hydrochloric acid, evaporate to dryness, remove the silica, mix the acid filtrate with ammonia in excess, redissolve the precipitated earthy phosphates by acetic acid, filter off and estimate the insoluble phosphate of iron (and alumina, if present), precipitate the lime with oxalate of ammonia, and then in the fluid (free from lime and iron) precipitate the phosphoric acid, by the addition of ammonia and magnesia mixture. § 66. General Method of Determining all the Constituents of an Ash. The best method of determining all the constituents of an ordinary ash is perhaps as follows:-A sufficient quantity of the ash (from 5 to 10 grms.) is placed in a flask, about 25 cc. of water added, and saturated with CO2; the liquid is now evaporated to dryness, heated with a small quantity of water to dissolve the alkaline salts-the solution is filtered through a small weighed filter, the filtrate evaporated to dryness, the saline residue treated with a small quantity of water, and the calcium sulphate which separates out filtered through a weighed filter, and estimated; the filtrate from this is put in a tared flask, made up to any convenient weight, and divided into five portions by weight, viz. :(1.) For CO-This is most accurately determined by the G Fig. 19. of the tube G (which it is convenient to furnish with a Bunsen's valve*). G is then placed under the mouth of a graduated measuring tube filled with mercury, and it is at once seen whether all the air is expelled. The flame is withdrawn for a second, and the glass rod, which moves quite airtight, is pulled a little up, so as to allow the acid tube to fall down and empty its contents into the alkaline fluid. The flame is again placed under the flask, and the CO, boiled out into the measuring apparatus, and measured in the ordinary way. Those who are not provided with gas apparatus will find it convenient to jacket their eudiometer with a tube open at both ends. The lower end is closed by the mercury in the bath; the upper is placed under a water-tap, and a syphon is adjusted so as to prevent overflow. In this way the gas is rapidly cooled, and the whole determination, from first to last, need not take more than a quarter of an hour.t Instead of boiling the solution in this way, those who possess the mercury pump described and figured page 70 (fig. 6), will find it more convenient to make the flask vacuous, then upset the acid, and collect the gases expelled. at Bunsen's valve is made as follows: Take a piece of rather thick-walled india-rubber tubing, say, three inches in length; work it, by the aid of a little spirit, on to any wooden rod which is of sufficient size to stretch it well; then with a sharp chisel, by a single blow, cut a longitudinal slit; if well made, it allows air to go one way with the greatest ease, but effectually prevents a return. + Much ingenuity has been expended on the estimation of carbon dioxide, and the old method of estimating in light glass apparatus, by the loss of weight, is quite forsaken. The method given in the former edition of this work was to absorb the CO2 in a clear solution of ammoniacal calcium chloride, collect the precipitated calcium carbonate, and titrate it with d. n. acid, (2.) For the sulphuric acid, determined by chloride of barium. (3.) For the phosphoric acid, determined as magnesian pyrophosphate. (4.) For the chlorine, by precipitation as silver chloride. (5.) For the alkalies, by boiling in a platinum dish with slight excess of baryta water, filtering, getting rid of the excess of baryta by ammonia and ammonium carbonate, evaporating the filtrate to dryness, converting the alkalies into chlorides, and determining their relative proportion from their total weight and their content in chlorine. This completes the analysis of the soluble portion of the ash. The insoluble will contain lime, magnesia, ferric oxide, alumina if present, silica, phosphoric, sulphuric, and carbonic acids. The main portion of the insoluble ash is dissolved in nitric acid, freed from silica in the usual way, evaporated again to dryness in a porcelain basin, dilute nitric acid added until the bases are completely dissolved, and strong fuming nitric acid added, until the solution begins to be turbid from the separation of calcic nitrate. The turbidity is now destroyed by a few drops of dilute nitric acid, the solution warmed, and tinfoil added in small portions at a time, in weight about equal to the amount of ash taken. When the tin is fully oxidised, the solution is evaporated nearly to dryness, water is added, and the solution filtered; the phosphoric acid is retained in the precipitate-the bases are all in the filtrate. The precipitate is dissolved in strong potash solution, acidified with sulphuric acid, and freed from tin by hydric sulphide, concentrated to a small bulk, filtered if any further sulphide of tin separates, and the phosphoric acid determined by magnesia mixture and ammonia.* The filtrate from the tin phosphate must be freed from lead (if the tin originally contained lead) by hydric sulphide, concentrated, the iron and alumina separated and determined by ammonia, the manganese separated as binoxide by brominewater, the lime by oxalate of ammonia as oxalate, and the magnesia determined in the usual way as pyrophosphate. A weighed portion of the insoluble ash must also be taken for the carbon dioxide, sulphuric acid, and sand. The carbon dioxide is determined in the manner already described. using as an indicator cochineal. Another method which has been proposed, is to make a combustion of the substance with potassic bichromate, and absorb the CO2 in the usual way in potash bulbs. A third, is to let the acid drop from a separating funnel on to the carbonates, and absorb the CO as in the last, an aspirator and drying tubes being used. (Annalen der Chemie, clxxvi. 136-144.) All of these methods, in the author's opinion, are inferior to that given above in the text. *Thorpe's "Quantitative Analysis." Lond., 1877. * The process just given is not quite accurate with regard to the estimation of the alkalies; for Bunge has shown that since the alkalies form insoluble compounds with the alkaline earths, a watery extract of the ash gives low results. For example, Bunge incinerated 300 cc. of cow's milk; from a watery extract of the ash he obtained K,O 5436, Na,O 0700; while from a subsequent nitric acid extract of the same ash, K2O 0937, Na2O 0·1162. If chlorides of the alkalies be heated with tribasic phosphate of lime, the soda is specially likely to combine with the lime in insoluble combination-in far less proportion the potash. Bunge recommends the following method:-The watery extract is decomposed with baryta water until a film forms on the surface of the solution, the mixture is warmed and filtered hot. The excess of baryta is got rid of by CO,, subsequent warming, and filtration; the filtrate is evaporated in a platinum dish, the residue gently ignited, dissolved in a little water, filtered through a small filter, and evaporated with HCl in a small platinum dish. The chlorides are then ignited, weighed, and separated by platinum chloride. The hydrochloric or nitric solution of the insoluble portion of the ash is evaporated to dryness in a platinum dish, the residue again dissolved in a little of the acid and water, treated like the former with baryta water, and filtered hot. Ammonia and carbonate of ammonia are now added, the liquid filtered, and the filtrate evaporated in a platinum dish, and ignited at the lowest possible temperature. The residue still containing a trace of alkaline earth, is extracted with water, evaporated with oxalic acid, ignited again, taken up with water, filtered, evaporated in a small platinum dish, ignited again, dissolved in a little water, and lastly, evaporated with HCl, and the alkaline salts separated by bichloride of platinum. Since a determination of the ash only gives those mineral substances which are fixed in the fire, and destroys nitrates, and changes oxalates, citrates, and tartrates into carbonates, while other constituents, under the influence of heat, undergo a new arrangement, it becomes a question whether the ingenious method recommended by E. Laugiert in the analysis of sugar, would not be applicable in several cases. *Liebig's Annalen der Chimie u. Pharmacie, April 15, 1874. M. Laugier takes two portions of sugar, one for the ash, the other for the organic acids, the latter being exactly double the quantity of the former. To the larger quantity of the sample, dilute sulphuric acid is added drop by drop to set free the organic acids; the acidified sugar is mixed with pumice stone and exhausted with ether; half of the ethereal solution is added to the ash obtained from the smaller portion, and evaporated down upon it and weighed. By this means M. Laugier thinks that he reconstructs the original salts in the sugar. This, however, cannot be entirely true. The other half of the ether solution is titrated with an alkali. METHODS OF ESTIMATING NITROGEN AND NITROGENOUS SUBSTANCES IN FOODS. § 66a. A complete analysis of foods, especially with the view of ascertaining their dietetic value, necessitates the following determinations : 1. Total nitrogen. 2. Nitrogen as albumen. 3. Nitrogen as acid-amines. 4. Nitrogen as amido acids. 5. Nitrogen as nitrates and nitrites. Total Nitrogen.-The method of burning organic substances with copper oxide and copper, so as to obtain all the nitrogen as a gas, and also the other well-known method, by which the substance is mixed with soda lime and burned, so as to decompose the nitrogenous substances into ammonia, are both too well known to be farther described. At the present time, nitrogen is most frequently determined by Kjeldahl's process. From 1 to 2 grms. of the substance are placed in a flask of hard glass, of about 500 cc. capacity, and moistened with 20 cc. of a mixture of 3 volumes of concentrated sulphuric acid and 2 volumes of Nordhausen acid, and a bead of quicksilver is added. The flask is closed with a glass marble, and heated on an asbestos millboard, or on a sandbath, until the mass boils; it is kept gently boiling until a clear solution is effected; this usually takes from one to several hours; the flask is then allowed to cool, the contents are diluted with double their bulk of water, and alkalised with 80 cc. of soda lye, of sp. gr. 1.35; 25 cc. of a 4 per cent. solution of potassium sulphide, or so much of the latter as will precipitate all mercury in the form of sulphide, are also added, and, lastly, a little fine zinc powder. The liquid is now distilled, with special precautions, into a flask containing from 10 to 20 cc. of normal sulphuric acid. The special precautions mainly consist in adapting a tube with a bulb to the condensing |