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Carbonates in Limestone and other materials.

Design by J.F. Barker, N.Y. Agr. Exp. Station

the funnel-shaped top, until immersed to zero point. Raise hydrometer out of water and open stop-cock D until acid drops slowly into reaction chamber, decomposing the limestone; then release so apparatus floats freely. As the reaction proceeds the instrument slowly rises and at the conclusion the point on stem at surface of water gives the percentage of calcium carbonate equivalent to the carbon dioxide in the sample. This figure is the calcium carbonate equivalent so often mentioned in connection with limestone analyses. A Fahrenheit thermometer accompanies each instrument and is hung inside the floating cylinder. Its reading is taken before and after each determination to allow for any error due to change in temperature. To the figure for calcium carbonate equivalent add 0.5 for each degree rise or subtract 0.5 for each degree fall in temperature between the two readings. This temperature change need seldom amount to more than a fraction of a degree.

The right amount of acid to be used is the amount that will cause the hydrometer with 10-gram weight to sink somewhere along the graduated space at C. This should be approximately 40 c.c. Some limestones or other materials will froth badly and in such cases the acid must drop very slowly.

DEGREE OF ACCURACY.

The accuracy and practicability of the method are well illustrated by the following results of analyses of limestone. The figures for this method were obtained by a farm boy without chemical training or special aptitude for such work. They do not simply represent a few of his best determinations, but are all he had made up to date. The other column gives results on the same samples obtained by a chemist using a standard laboratory method.

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SOME DETAILS.

For those who care to examine the method critically it should be said that sources of error have been eliminated until the method is accurate to the degree that the graduated stem can be read. This means about of one per ct. The limestone can be weighed to an accuracy of .02 gram. The weight of CO2 remaining in the apparatus tends to offset the loss due to moisture escaping with the gas, but the difference, together with any other important sources of error, has been accounted for in the graduation of the reading stem. This graduation has been verified by the author, checking against samples of calcite, and any necessary corrections made so that the instrument as now obtainable gives readings which need no corrections save for changes in temperature occurring during the progress of a determination.

Each outfit of this apparatus includes a glass cylinder for floating the hydrometer, a small Fahrenheit thermometer, 500 c.c. bottle of hydrochloric acid (sp. gr. 1.15), 50 c.c. graduate, small dropping pipette and scoop for convenience in transferring the sample. The manufacturer's price for a single outfit, it seems now, will be less than ten dollars. Hydrochloric acid of the given strength can be obtained at any time from a local drugstore.

COMPOSITION OF LIMESTONE.

For the benfit of those who are not chemists it should be explained that limestones are composed of calcium carbonate, or calcium and magnesium carbonates, and a varying percentage of clay-like material. In good grades of limestone the carbonates make up 85 to nearly 100 per ct. of the stone. Calcium carbonate (CaCO3) is composed of 40 parts calcium, 12 parts carbon and 48 parts oxygen, by weight. When calcium carbonate is treated with acid, as in this determination, all of the carbon and 32 parts of the oxygen pass off in the form of carbon dioxide gas (CO2), and this 100 parts of calcium carbonate loses 44 parts as gas.

Magnesium carbonate (MgCO3) is made up of 24 parts magnesium, 12 parts carbon and 48 parts oxygen, by weight. When treated with acid, carbon dioxide again passes off and 84 parts of the carbonate loses 44 parts as gas.

Now 24 parts, by weight, of magnesium may be said to be equivalent to 40 parts of calcium; for the two elements in these proportions neutralize the same amount of acid, either in the laboratory or in the soil. With this in mind a rereading of the above statements will make clear the fact that equivalent amounts of calcium and magnesium carbonates give off the same amount of gas.

In limestone analyses, then, the weight of gas given off is 44 per ct. of the weight of calcium carbonate equivalent to the total carbonates in the sample. With this fact in mind the stem of our hydrometer

has been so graduated that with any given loss of carbon dioxide the figure for calcium carbonate equivalent is read directly.

Limestones having a high percentage of magnesium carbonate are decomposed by acid more slowly than others and this fact will usually enable a person making this determination to recognize such stones.

APPLICATION OF THIS METHOD.

For analyzing carbonates other than limestone and similar materials, this instrument is fitted with a graduated stem which reads percentage of carbon dioxide; an arrangement which greatly extends its use. It will be found especially suited to determining the comparative strength of baking powders, more suitable, considering speed and accuracy, than any other device now employed for that purpose.

The method is adapted for use in all chemical laboratories: college, experiment station, or commercial laboratories. However, it is devised for use outside of chemical laboratories, also. Limestone companies can employ it to check up on every carload of their goods, or to locate the stone in their quarry most suitable for grinding. County agricultural agents will use it to keep posted on the quality of limestone being sold in their territory and to determine the composition of local limestone deposits. Many an individual farmer or some member of his family will no doubt find it a source of satisfaction to own this outfit with the object of examining purchases of limestone or other carbonates.

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