A Text-book of Assaying: For the Use of Those Connected with Mines Part 67

~Direct Gravimetric Method.~--Fit up the apparatus shown in the diagram (fig. 79). The various tubes are supported by a fixed rod with nails and wire loops, and connected by short lengths of rubber-tubing. The first tube contains soda-lime. The small flask is fitted with a rubber-stopper perforated with two holes, through one of which pa.s.ses the tube of a pipette holding 25 or 30 c.c. This pipette is to contain the acid. The substance to be determined is weighed out into the flask. The second tube contains strong sulphuric acid; the third, pumice stone, saturated with copper sulphate solution, and dried until nearly white (at 200 C.); the fourth contains recently fused calcium chloride; and the fifth, which is the weighed tube in which the carbonic acid is absorbed, contains calcium chloride and soda-lime,[120] as shown in fig. 80. The sixth also contains calcium chloride and soda-lime; its object is to prevent the access of moisture and carbonic acid to the weighed tube from this direction; it is connected with an aspirator.

Having weighed the ~U~-tube and got the apparatus in order, weigh up 1, 2, or 5 grams of the substance and place in the flask. Fill the pipette with dilute acid, close the clamp, and cork the flask. Then see that the apparatus is tight. Open the clamp and allow from 10 to 20 c.c. of the acid to run on to the a.s.say. Carbonic acid will be evolved and will be driven through the tubes. The gas should bubble through the sulphuric acid in a moderate and regular stream. When the effervescence slackens the clamp is opened and the greater part of the remaining acid run in.

When the effervescence has ceased the clamp is opened to its full extent and a current of air drawn through with an aspirator. A gentle heat is applied to the flask; but it should not be prolonged or carried to boiling. After the removal of the heat a gentle current of air is drawn through the apparatus for 30 or 40 minutes. The weighed ~U~-tube, which in the early part of the operation will have become warm if much carbonic acid was present, will by this time be cold. It is disconnected, plugged, and weighed. The increase in weight is due to the carbon dioxide of the sample.

_Example._--Ore taken 1 gram.

Weight of tube, before 42.6525 grams " " after 43.0940 "

------- Increase equals CO_{2} 0.4415 "

[Ill.u.s.tration: FIG. 81.]

~Indirect Gravimetric, or Determination by Loss.~--Take a Geissler's carbonic-acid apparatus (fig. 81) and place in the double bulb some strong sulphuric acid. Put into the other bulb, the stopc.o.c.k being closed, 3 or 4 c.c. of nitric acid diluted with water. Leave the apparatus in the balance-box for a few minutes and weigh. Introduce into the flask (through A) about 1 gram of the powdered substance and again weigh to find the exact amount added. Allow the acid to run gradually on to the carbonate, and when solution is complete, heat and aspirate. Cool and again weigh; the loss in weight is the carbonic acid.

For _Example_:--

Weight of apparatus and acids 85.494 grams " " marble 86.879 "

------ Equal to marble taken 1.385 "

Weight of apparatus and marble 86.879 grams " " minus carbonic acid 86.2692 "

------- Equal to carbonic acid 0.6098 "

1.385 : 100 :: 0.6098 : _x_ _x_ = 44.03 per cent.

The substance contains 44.03 per cent. of carbonic acid; a duplicate experiment gave 43.73 per cent.

This method is quicker, but less exact, than the direct gravimetric determination.

VOLUMETRIC METHOD.

This, which is of somewhat limited application, is based upon the determination of the quant.i.ty of acid required to decompose the carbonate. It consists in adding to a weighed quant.i.ty of the mineral a known amount of standard solution of acid which is in excess of that required to effect the decomposition. The quant.i.ty of residual acid is then determined by t.i.trating with standard solution of alkali. This method has been described under _Lime_.

GASOMETRIC METHOD.

This method is the quickest of all, and the least troublesome after the apparatus has been once prepared. It yields fairly accurate results when worked in the manner described below; but if greater precautions are taken the results are exact. It depends on the measurement of the volume of gas given off on treating the weighed sample with acid. The apparatus described, page 52, is used. Weigh out a portion of the mineral which shall contain not more than 0.15 gram of carbonic acid (or 0.4 gram of carbonate of lime) and put it in the bottle. Put in the inner tube 10 c.c. of dilute hydrochloric acid (1--1), cork tightly, and read off the level of the liquid in the burette after adjusting the pressure. Turn the acid over on to the mineral. Run out the water so as to keep the level in the two burettes the same. When effervescence has ceased, rotate the contents of the bottle; finally, adjust the level in the burettes and read off the volume. The increase in volume is due to the evolved carbon dioxide. At the same time read off the "volume corrector."

Some of the carbon dioxide remains dissolved in the acid in the generating bottle, and the quant.i.ty thus dissolved will depend on the amount of carbonate as well as on the amount of acid present.

Consequently, a measured quant.i.ty of acid should be used in each a.s.say and a comparative experiment made with a known weight of pure carbonate of lime which will yield about the same volume of gas. The number of c.c. of gas got in the a.s.say multiplied by 4.7 will give the number of milligrams of pure carbonate of lime that must be taken for the standard. With ordinary work the error rarely exceeds half a c.c.

The following example will ill.u.s.trate the calculations:--

One gram of a mineral was taken, and yielded 49.0 c.c. of gas. The "volume corrector" reading was 100.4 c.c.

0.2405 gram of pure carbonate of lime was then taken, and treated in the same way; 50.5 c.c. of gas were got. The volume corrector still read 100.4 c.c.

0.2405 gram of carbonate of lime is equivalent to 0.1058 gram of carbon dioxide; then,

50.5 : 49.0 :: 0.1058 : _x_ _x_ = 10.26 per cent.

~Estimation of Carbonic Acid in the Air of Mines.~--According to a series of a.n.a.lyses by Angus Smith, the proportion of carbonic acid in the air of underground workings varied from 0.04 to 2.7 per cent. by volume. In places where men are working the proportion ought not to reach 0.25 per cent.

A simple method of determining whether a sample of air reaches this limit (0.25 per cent.) is described by Dr. C. Le Neve Foster in the "Proceedings of the Mining a.s.sociation and Inst.i.tute of Cornwall" for 1888. The apparatus used is an ordinary corked 8-ounce medicine bottle.

This is filled with the air to be examined by sucking out its contents with a piece of rubber-tube. Half-an-ounce of dilute lime-water[121]

(tinted with phenolphthalein) is poured in. If, on corking the bottle and shaking, the colour is not discharged, the air contains less than 0.25 per cent. of carbon dioxide. "If the colour fades slowly, and does not finally vanish till after a great deal of shaking, it may be a.s.sumed that the percentage of carbon dioxide does not greatly exceed one quarter; whereas, if the disappearance is rapid after a very few shakes, the contrary, of course, is the case." The dilute lime-water is measured out and carried in ordinary half-ounce phials. This method does not pretend to great accuracy, but as a method of distinguis.h.i.+ng between good and bad air it is very convenient, and will be found useful.

For determining the actual proportion in the air the following plan is adopted:--Take a bottle which will hold about 50 ounces, and measure its capacity; fill the bottle with the air to be examined, pour in 100 c.c.

of lime-water, and shake up for some time; add phenolphthalein, and t.i.trate the remaining calcium hydrate with standard solution of oxalic acid.

The solution of oxalic acid is made by dissolving 2.25 grams of re-crystallised oxalic acid (H_{2}C_{2}O_{4}.2H_{2}O) in water and diluting to 1 litre. One c.c. = 0.001 gram of lime (CaO), or 0.0007857 gram of carbon dioxide.

Take 100 c.c. of the same lime-water, to which add the same amount of phenolphthalein as before. t.i.trate. The difference between the two readings gives the amount of "acid" equivalent to the lime-water neutralised by the carbon dioxide. The number of c.c. thus used up, when multiplied by 0.3989, gives the number of c.c. of carbon dioxide (at 0 C. and 760 mm.) in the volume of air taken. This volume, which is that of the bottle less 100 c.c., must in accurate work be reduced to the normal temperature and pressure.[122] The percentage by volume can then be calculated.

PRACTICAL EXERCISES.

1. In a gasometric determination 71.3 c.c. of gas were obtained from 0.2055 gram of mineral. The "volume corrector" reading was 102.2 c.c.

0.3445 gram of pure carbonate of lime gave 74.1 c.c. The "volume corrector" reading was 100.6. What is the percentage of carbon dioxide in the substance?

2. What volume of dry gas at 0 C. and 760 m.m. pressure should be obtained from 0.3445 gram of carbonate of lime? 1 c.c. of CO_{2} under these conditions weighs 1.97 milligrams.

3. A sample of coal is reported on as follows:--

Specific gravity 1.315 Moisture 1.001 Volatile matter 35.484 Fixed carbon 50.172 Ash 12.028 ------- 100.000

What is there about this requiring explanation?

4. Calculate the percentage of carbonic acid in a mineral from the following data:--

Weight of apparatus and acids 87.0888 grams " " " plus mineral 88.8858 "

" " " after loss of carbonic acid 88.1000 "

5. A sample of pig iron contains 1.43 per cent. of "combined" and 2.02 per cent. of "free" carbon. Taking 2 grams of it for each determination, what weight of CO_{2} will be got on burning the residue from solution in ammonium cupric chloride, and what from the residue after solution in hydrochloric acid?

BORON AND BORATES.

Boron occurs in nature as boric acid or sa.s.soline (H_{3}BO_{3}); borax or tincal (Na_{2}B_{4}O_{7}.10H_{2}O); ulexite or boronatrocalcite (2CaB_{4}O_{7}.Na_{2}B_{4}O_{7}); borocalcite (CaB_{4}O_{7}.4H_{2}O); boracite, 2Mg_{3}B_{8}O_{15}.MgCl_{2}, and some other minerals. Boric acid is also a const.i.tuent of certain silicates, such as tourmaline, axinite, and datholite.

The natural borates are used in the preparation of borax, which is largely employed as a preservative agent, for fluxing, and for other purposes.

There is only one series of boron compounds which have any importance.

These are the borates in which the trioxide (B_{2}O_{3}) acts the part of a weak acid. The addition of any acid liberates boric acid, which separates out in cold solutions as a crystalline precipitate. Boric acid is soluble in alcohol and in hot water. On evaporating these solutions it is volatilised, although the anhydrous oxide is "fixed" at a red heat. The borates are mostly fusible compounds, and are soluble in acids and in solutions of ammonic salts.

~Detection.~--Boron in small quant.i.ties will escape detection unless specially looked for, but there is no difficulty in detecting its presence. Heated in the Bunsen-burner flame with "Turner's test," it gives an evanescent yellowish-green colour, due to fluoride of boron (BF_{3}). "Turner's test" is a mixture of 5 parts of bisulphate of potash and 1 part of fluor spar. Boric acid itself imparts a characteristic green colour to the flame, which gives a spectrum made up of four well-marked and equidistant lines, three in the green and one in the blue. Solutions of boric acid give with "turmeric paper," which has been dipped into it and dried, a characteristic red tint. This is a very delicate test, but in trying it a blank experiment should be carried out alongside with a solution made up of the same re-agents which have been used in liberating the boric acid in the sample.

~Solution and Separation.~--The solution presents no difficulty, but the separation is troublesome. The best method is that of Gooch; who, if necessary, first fuses with carbonate of soda, and after the removal of chlorides and fluorides (by nitrate of silver or a lime salt), evaporates the aqueous extract with nitric or acetic acid to dryness in a retort and, subsequently, with repeated doses of 10 c.c. each of methyl alcohol. The distillate contains the boron as boric acid. Half a gram of the trioxide (B_{2}O_{3}) is completely carried over by two evaporations, each with 10 c.c. of the alcohol; but if water or foreign salts are present, more than this is required. In ordinary cases six such evaporations are sufficient for 0.2 gram of the oxide.[123]