405. Determination of Ash.—No method of treatment is known by means of which vegetable proteid matters may be obtained entirely free of mineral matters. The mineral bases may be naturally present in the proteid matter as organic and inorganic salts, or they may be mechanically entangled therewith, having been derived either from the other tissues of the plant or from the solvents employed. It is necessary in calculating the analytical data to base the computation on the ash free substance. The percentage of ash is determined by any of the standard processes or by heating the sample in a combustion tube, to very low redness, in a current of oxygen. The total residue obtained is used in calculating the percentage of ash, and the weights of material subsequently used for the determination of carbon, hydrogen, nitrogen and sulfur are corrected for the calculations by deducting the quantity of mineral matter contained therein.
By reason of the highly hygroscopic nature of the dry proteid bodies, they must be kept over a desiccating material and weighed quickly on a balance, in an atmosphere which is kept free of moisture by the usual methods.
406. Carbon and Hydrogen.—Carbon and hydrogen are estimated in proteid matters by combustion with copper oxid. Osborne prefers to burn the sample in a platinum boat in a current of air or of oxygen free of moisture and carbon dioxid.[382] It is advisable to use also a layer of lead chromate in addition to the copper oxid and metallic copper. The method of conducting the combustion has already been described.[383] The analyst should have at his disposal a quantity of pure sugar, which may be used from time to time in testing the accuracy of the work. In beginning a series of combustions this precaution should never be omitted. The addition of the lead chromate is to make more certain the absorption of oxidized sulfur produced during the combustion.
407. Estimation of Nitrogen.—In most cases it is found convenient, during the progress of separating vegetable proteids, to determine the quantity of each kind by estimating the nitrogen by moist combustion and computing the quantity of proteid matter by multiplying the nitrogen by 6.25. The estimation of the nitrogen is made either on an aliquot part of the extract or by direct treatment of the residue.
In the pure extracted proteid matter the nitrogen is most conveniently determined by moist combustion, but it may also be obtained either by combustion with soda-lime or with copper oxid, or by other reliable methods.[384]
The percentages of nitrogen found in the principal proteid bodies, together with the factors for computing the weights of the proteid bodies from the weights of nitrogen found, are given below:
| Name of body. | Percentage of nitrogen. | Factor. |
|---|---|---|
| Mucin | 13.80 to 14.13 | 7.25 to 7.08 |
| Chondrin | 14.20 to 14.65 | 7.04 to 6.83 |
| Albuminates | 13.87 | 7.21 |
| Oat proteids | 15.85 | 6.31 |
| Serum globulin | 15.63 | 6.40 |
| Egg albumin | 15.71 to 17.85 | 6.37 to 5.60 |
| Maize proteids | 16.06 | 6.22 |
| Casein | 15.41 to 16.29 | 6.49 to 6.13 |
| Serum albumin | 15.96 | 6.27 |
| Syntonin | 16.10 | 6.21 |
| Keratin | 16.20 to 17.70 | 6.17 to 5.65 |
| Fibrinogen | 16.65 | 6.01 |
| Peptones | 16.66 to 17.13 | 6.00 to 5.84 |
| Elastin | 16.75 | 5.97 |
| Wheat proteids | 16.80 to 18.39 | 5.95 to 5.44 |
| Fibrin | 16.91 | 5.91 |
| Flax seed proteids | 17.70 to 18.78 | 5.65 to 5.33 |
408. Determination of Sulfur.—Sulfur is a characteristic constituent of the proteid bodies, existing in quantities approximating one per cent of their weight.
In the estimation of sulfur, it is first converted into sulfuric acid, which is thrown out by a soluble barium salt and the sulfur finally weighed as barium sulfate.
All the sulfur existing in the organic state in a proteid may be obtained by burning in a current of oxygen and conducting the gaseous products of combustion through solid sodium or potassium carbonate at or near a red heat.[385] The organic sulfur may also be converted into sulfuric acid by fusing the proteid body with a mixture of sodium hydroxid and potassium nitrate. The fused mass, after cooling, is dissolved in water, the solution acidified with hydrochloric, evaporated to dryness to decompose nitrates and remove excess of hydrochloric acid and dissolved in a large excess of water. After standing for a day, the solution is filtered and the sulfuric acid thrown out of the hot filtrate with a slight excess of barium chlorid solution. The usual precautions in precipitating, filtering and igniting the barium sulfate are to be observed.[386]