Table I.—Multipliers and Standard Weights for the Principal Alkalies and their Carbonates. (Cooley.)
| NAMES, &c. | Factors or Multipliers for converting the weight of carbonic acid expelled into real strengths. | Quantity (in grains) to be taken, so that the per-centage value of the sample tested shall be shown in the terms of any of the denominations given, by the weight of the evolved Carbonic Acid reckoned— | ||||
| In tenths of a grain. | ||||||
| Whole numbers and decimals. | Nearest common numbers. | in quarter-grains. | in half-grains. | in grains. | ||
| Ammonia (pure, gaseous) | ·77273 | 7·727 | 73⁄4 | 191⁄3 | 385⁄8 | 773⁄0 |
| Carbonate of ammonia (neutral, anhydrous) | 1·77273 | 17·727 | 173⁄4 | 445⁄16 | 885⁄8 | 1771⁄4 |
| Carbonate of ammonia (neutral, crystallised) | 1·9773 | 19·773 | 193⁄4 | 497⁄16 | 987⁄8 | 1973⁄4 |
| Sesquicarbonate of ammonia (translucent) | 2·6818 | 26·818 | 2613⁄16 | 671⁄10 | 1341⁄10 | 2681⁄5 |
| Bicarbonate of ammonia (crystallised) | 3·5909 | 35·909 | 359⁄10 | 8913⁄16 | 1795⁄8 | 3591⁄10 |
| Potassa (anhydrous) | 2·1364 | 21·364 | 211⁄2 | 531⁄2 | 107 | 2133⁄8 |
| Hydrate of potassa | 2·54546 | 25·455 | 255⁄11 | 635⁄8 | 1271⁄4 | 2541⁄2 |
| Carbonate of potassa (anhydrous) | 3·1364 | 31·364 | 313⁄8 | 781⁄2 | 157 | 3131⁄2 |
| Carbonate of potassa (granulated) | 3·7727 | 37·727 | 371⁄2 | 943⁄10 | 1885⁄8 | 3771⁄4 |
| Carbonate of potassa (crystallised) | 3·9545 | 39·545 | 395⁄8 | 99 | 198 | 3951⁄2 |
| Bicarbonate of potassa (crystallised) | 4·5454 | 45·454 | 451⁄2 | 1133⁄4 | 2271⁄2 | 4541⁄2 |
| Soda (anhydrous) | 1·4091 | 14·09 | 141⁄10 | 351⁄4 | 701⁄2 | 141 |
| Hydrate of soda | 1·8182 | 18·182 | 181⁄5 | 451⁄2 | 91 | 182 |
| Carbonate of soda (anhydrous) | 2·4091 | 24·091 | 241⁄10 | 601⁄4 | 1201⁄2 | 241 |
| Carbonate of soda (crystallised) | 6·5 | 65· | 65 | 1621⁄2 | 325 | 650 |
| Sesquicarbonate of soda (dry; theoretical) | 2·9091 | 29·091 | 291⁄10 | 721⁄2 | 145 | 290 |
| Sesquicarbonate of soda (Ph. L., 1836) | 3·7273 | 37·273 | 371⁄4 | 931⁄4 | 1861⁄2 | 373 |
| Sesquicarbonate of soda (average commercial) | 3·7954 | 37·954 | 38 | 947⁄8 | 1893⁄4 | 3791⁄2 |
| Bicarbonate of soda (crystallised) | 3·8182 | 38·182 | 381⁄5 | 951⁄2 | 191 | 382 |
| Lithia (pure, anhydrous) | ·6818 | 6·818 | 613⁄16 | 171⁄20 | 341⁄10 | 681⁄5 |
| Baryta (pure, caustic) | 3·4773 | 34·773 | 344⁄5 | 867⁄8 | 1737⁄8 | 3473⁄4 |
| Lime (pure, caustic) | 1·2727 | 12·727 | 123⁄4 | 313⁄4 | 635⁄8 | 1271⁄4 |
| Magnesia (pure, anhydrous) | ·90909 | 9·091 | 91⁄11 | 223⁄4 | 451⁄2 | 91 |
In this ingenious method of alkalimetry it is absolutely necessary that the whole of the alkali in the specimen tested should be in the state of neutral carbonate. If a sample of potash contains any caustic alkali (as the potashes and pearlash of commerce generally do), Fresenius and Will direct it, previously to being tested, to be triturated with its own weight of pure quartzose sand, and about one third of its weight of carbonate of ammonia; and the resulting mixture, placed in a small iron capsule, or a porcelain crucible, to be moistened with water, and exposed to a gentle heat until it becomes quite dry, and all the ammonia is expelled. If the sample contains any bicarbonate or sesquicarbonate, it must be heated to dull redness before being placed in the apparatus and tested. In the case of crude soda (particularly soda ash), the proportion of carbonate of ammonia should be equal to at least one half the quantity operated on. With both alkalies, if the sample contains sulphides, sulphites, or hyposulphites, the same method is to be followed, except that solution of ammonia,
instead of water, is to be employed for moistening the powder. To remedy the error which would arise from the apparent amount of carbonic anhydride liberated during the assay, being swelled by the disengagement of ‘sulphuretted hydrogen’ or sulphurous acid from these substances, a small quantity of neutral (i. e. yellow) chromate of potash may be added to the alkaline solution in the flask (A); by which they will be converted into sulphates, sulphur, and water, which will remain in the apparatus, the carbonic acid only being evolved. “As most sorts of soda of commerce contain one or other of the substances (just) named, and as it is far more simple to add at once some chromate of potassa to the soda solution, than to test the latter for either of the three salts, it is always advisable to make it a rule, in the examination of SODA, to add some chromate of potassa.” (Fresenius.)
If the sodium or other carbonate under analysis contains much chloride, the addition of more sulphuric acid than necessary must be avoided, and the carbonic anhydride expelled by gently heating over a warm bath, and not by the addition of excess of acid.
To obviate the difficulties, and to give greater precision and delicacy to volumetrical assays, the instrument known as Mohr’s ALKALIMETER, or Mohr’s BURETTE, and which is figured in the margin, may be employed. By means of it the test-acid in the graduated tube (a) may be added to the alkaline solution in (f), in any quantity at a time, however minute, by merely pressing the handles of the clamp (d) with the thumb and finger. The terminal tube (e) has its lower orifice very small, and it is connected with the burette by means of a small piece of vulcanised india-rubber tube, on which the clamp (d) acts. (See engr.) The inner cylindrical part of the arm (b) is lined with cork, to prevent injury to the glass burette, and to hold it the more firmly.
Generally the alkali in the specimen examined may be in either the caustic or carbonated state, or it may consist of any mixture of caustic alkali, or carbonates; but it is absolutely necessary for accurate results, that it should be free from sulphides, sulphites, and hyposulphites, as sulphuric acid acts upon these substances as well as on carbonates. The presence of chlorides does not interfere with the accuracy of the assay, unless a higher degree of heat is employed than that necessary for the expulsion of the absorbed carbonic acid. The SODA-ASH of commerce generally contains all these substances besides common salt, sulphate of soda, and insoluble matter, which do not interfere. Rough samples of POT-ASHES and PEARL-ASH also generally contain some sulphides, though not a large quantity. Various plans have been proposed to avoid this source of error. The best is that of MM. Fresenius and Will, given above, in which the value of the carbonates is estimated by their yield of carbonic anhydride.
The difference between an assay of a sample of the unprepared alkali and of another which has been treated as above, indicates the quantity of impurities contained in them under the forms just referred to. The presence of these substances in the commercial alkalies may be detected by the following tests:—
Sulphides. The addition of sulphuric acid causes the evolution of an odour like that of rotten eggs. The sample in solution yields a black precipitate with acetate of lead. But the most delicate test is the splendid violet-blue colour with nitro-prusside of sodium.