Assay Of Manganese

184. Assay Of Manganese. Take a short necked, flat bottomed flask of three ounces capacity, and fit into it a cork through which pass two glass tubes of about 1/8 inch bore. One of the tubes must extend nearly to the bottom of the flask; the other, about three inches in length, is bent to a right angle about an inch from one end, the shorter limb being passed through the cork. Now take the barrel of a small glass syringe, such as is used for medical purposes, the orifice of which must be enlarged by cutting off a portion of the beak. Fit a cork to the larger end, and through it pass the disengaged end of the bent tube, the other end of which is in communication with the flask. Place in the syringe some pieces of pumice stone, and moisten them with sulphuric acid; attach to the bent tube by means of the cork. A test tube suspended from the neck of the flask completes the apparatus, which is represented by the accompanying cut.

Weigh 99 grains of the. finely pulverized manganese which place in the flask. Add one ounce of cold water, and then 150 to 160 grains of sulphuric acid. Replace the cork in the neck, with the glass tubes, and, applying the lips to the orifice of the syringe, suck air through the flask. This is done for the purpose of removing any gas which the acid may cause to be evolved from impurities in the manganese. Now weigh 150 grains of oxalic acid which place in the test tube, put the entire arrangement upon the pan of a balance which will indicate 1/10 grain, and counterpoise it.

Remove the apparatus from the balance, throw the oxalic acid from the test tube into the flask, cork in-stantly and set aside for a short time. When effervescence ceases, warm slightly, which will renew it, and when no more gas is evolved draw air through the flask as before. Place the entire apparatus again on the scales. It will be found to be lighter than before, and the number of grains required to restore the equilibrium will be the percentage of binoxide in the sample.

This method, while not rigidly accurate, is sufficiently so for practical purposes. It depends on the circumstance that oxalic acid, in presence of sulphuric acid and manganese binoxide, is converted into water and carbonic acid gas, (carbon dioxide). The water remains in the apparatus. The gas escapes through the tube containing pumice imbued with sulphuric acid, which retains any vapor of water which might otherwise pass off, the last portion of the gas being drawn out by the mouth as directed. Thus nothing but the carbonic gas escapes, and as 99 grains of pure binoxide produce almost exactly 100 grains of the gas, it follows that the weight lost corresponds to the percentage of binoxide in the sample.

185. Having assayed the salt and manganese, the relative proportion in which they should be used remains to be considered. It has already been intimated that practice does not conform strictly to theory, not because the theory is incorrect, but because the conditions attainable in working do not admit of its complete realization. Hence it is found necessary to use sulphuric acid in excess of the chemical equivalent of the manganese, and as sulphuric acid is only useful in conjunction with a sufficient quantity of salt, the latter must also be in excess of the manganese.

For this reason, instead of only 27 pounds of pure salt to 20 pounds of manganese binoxide, we use 36 pounds; these quantities usually suffice for the chlori-nation of three tons of roasted ore. In order to ascertain how many pounds of the impure materials must be used, it is only necessary to divide the quantity of the pure substance required by the fraction representing the percentage found in the assay.

Example: Suppose the assay of the salt gave 87 percent, it is required to know how much of it is equal to 36 pounds of pure salt, then 36 % 87 = 41.37 pounds; again, if the manganese is 66 1/2 per cent, then 20%665=30 pounds of the manganese to 41 pounds of the salt, which is about the proportion generally used, but either or both of the substances may vary from this supposition.

As to the acid, the quantity required depends so much on the temperature of the generator, and the nature and quantity of the impurities in the other materials, that no other rule can be given than this; it must be added in small portions as long as it continues to develope a useful quantity of chlorine. It will soon be discovered how much is required for a given quantity of salt and manganese.

The strength of the acid is indicated by its density, as measured by a Beaume hydrometer, which is simply a floating gauge - the higher it floats the stronger is the acid, and the reverse.