bb. The Water expelled has an acid reaction.

The fluoride under examination is, in the first place, treated with sulphuric acid as directed in II., b, a, to determine the metal on the one hand, and the water + fluorine on the other. Another weighed portion is then mixed, in a small retort, with about 6 parts of recently ignited oxide of lead; the mixture is covered with a layer of oxide of lead, the retort weighed, and the water (which is now entirely free from admixture of hydrofluoric acid) expelled by the application of heat, increased gradually to redness. The weight of the expelled water is inferred from the loss. The first operation having given us the exact amount of water + fluorine, the quantity of the latter substance may now be readily calculated, by simply subtracting from the combined weight of the water + fluorine, the weight of the water expelled in the second operation.

In the fifth section we shall have occasion to speak of another method of determining fluorine (in the chapter on the separation of fluorine from silicic acid).

Fourth Division of the First Group of the Acids.

CARBONIC ACID-SILICIC ACID.

§ 139.

1. CARBONIC ACID.

I. Determination.

a. In a mixture of Gases.

Measure the gases accurately, in a graduated tube over mercury, insert into the tube a moistened ball of hydrate of potassa, cast on a platinum wire in a pistol bullet-mould, and leave this in the tube for 24 hours, or until the volume of the gas ceases to show further diminution; withdraw the ball, and measure the gas remaining; the amount of carbonic acid gas originally present is inferred from the difference, provided the gaseous mixture contained no other gas liable to absorption by potassa (compare also, §§ 13-16).

b. In Aqueous Solution.

a. Mix solution of chloride of barium or chloride of calcium* with solution of ammonia in excess, heat the mixture to boiling, let the precipitate deposit, and then filter. Have ready several flasks of about 300 cubic centimetres capacity each, provided with tight-fitting corks, and pour into each about 50 c.c. of the filtrate. The method to be adopted for adding, without loss of gas, certain definite quantities of solution of carbonic acid to the filtrate in the flasks, depends upon whether the carbonic acid water is contained in a stone bottle, or flowing from a pipe, or whether it can be taken directly from the spring. In the former case weigh the flasks, with the ammoniacal chloride of barium solution in them, separately, together with the cork; then let the water run into each flask, with proper care, until nearly full,† cork instantly,

Solution of chloride of barium is preferable in cases where the after process is to be conducted according to aa; solution of chloride of calcium in cases where bb to be resorted to.

+ If the carbonic acid water is in a stone bottle, it should always be transferred to the flask by means of a siphon, after being cooled first to about 39.2° F. were poured in direct from the bottle, some carbonic acid gas might also into the flask, together with the water.

If the water

find its way

and weigh again; this will give the exact quantity of water contained in each flask. Or, pour into each flask, in the first instance, exactly 50 cubic centimetres of the ammoniacal solution of chloride of barium, or chloride of calcium, then fill in the water, in the way just stated, mark with a diamond, or by means of a small paper slip pasted on the outside of the flask the point to which it reaches; measure afterwards the contents of the flask up to the mark, and deduct the 50 cubic centimetres of chloride of barium or chloride of calcium solution, when the difference will show the quantity of carbonic acid water added.

In the latter case, where the water can be taken direct from the spring, you will find the estimation of the volume more convenient by making use of a siphon or large pipette, dipped into the spring, so that the water enters through the lower orifice. The siphon or pipette used for the purpose must have a mark cut in the glass a little below the upper orifice; the exact volume which it holds up to this mark may be determined either before or after the operation, by filling it with water up to the mark, and measuring in a graduated glass. When the siphon or pipette is quite full, lift it out of the spring, wipe the outside dry, with proper expedition; let some water flow out, until the contents reach just up to the mark; then empty into one of the flasks containing the ammoniacal solution of chloride of barium, or chloride of calcium, and cork instantly. Proceed in the same way with the other flasks. The ammoniacal chloride of barium or chloride of calcium solution in the flasks generally turns turbid as soon as the carbonic acid water is poured in ; but, it is only after standing some time on the water bath, or even after actual ebullition, that the whole of the carbonic acid separates in form of carbonate of baryta or carbonate of lime.* When this point has been attained, allow the precipitate to subside, with exclusion of air, and then proceed by one of the following methods :—

aa. Estimation of the Carbonic Acid by the Gravimetrical Method. Decant the supernatant fluid rapidly on to a filter, secluded as much as possible from access of air; fill the flask with warm water, and insert the cork; shake, let the solid particles deposit, decant again, and repeat this washing by decantation once more; transfer the precipitate now to the filter, wash until the last rinsings remain clear upon addition of solution of nitrate of silver, dry, ignite gently, and weigh (§ 101, 2, a). The amount of the carbonic acid may now be calculated from the weight of the carbonate of baryta, provided the analysed solution contained, besides carbonic acid, no other substance liable to be precipitated by ammonia and chloride of barium. But should the latter be the case, and the precipitated carbonate of baryta contain an admixture of carbonate of lime, phosphate of baryta, sesquioxide of iron, or other similar substances, the carbonic acid must be determined in the gently ignited, but not weighed precipitate, as directed in II., d. The filter, freed as completely as possible from adhering matter, should be incinerated, and the ashes added to the precipitate. If the quantity of the precipitate is very large, it is best first to weigh the

• The tardiness of this reaction is explained, as is well known, upon the assumption that carbonic acid and ammonia, coming in contact, form, in the first place, carbamate of ammonia, CO, + 2 NH, =c/c

{N H., N H, O.

whole of it, and then to determine the carbonic acid in a weighed portion of the uniformly mixed powder.

If the last particles of the precipitate cannot be removed from the flask by mechanical means, they are dissolved in a little dilute hydrochloric acid (the glass having previously been thoroughly washed), the solution is precipitated with carbonate of soda, and the trifling precipitate formed filtered off on a separate small filter, which is incinerated with the larger one.

bb. Estimation of the Carbonic Acid by the Volumetrical Method.

Filter as in aa; there is no necessity, however, to collect the whole of the precipitate on the filter, as the last particles adhering to the inside of the flask may be left and washed by decantation. Put the funnel with the filter containing the precipitate on the flask in which the precipitation has been effected, make a small hole in the point of the filter, and rinse the thoroughly washed precipitate into the flask, with the aid of a washing bottle. Dry the filter, incinerate, and add the ash to the precipitate in the flask. Add now a little tincture of litmus, and then, from a Mohr's burette (see § 21), standard (or, according to circumstances, decimal standard) nitric or hydrochloric acid, until the fluid shows a distinct red color; expel the carbonic acid by heat, and then add solution of soda of known strength until the fluid just appears blue. After noting the number of cubic centimetres of the acid and soda, add again about 1 cubic centimetre of acid, and, after heating to boiling, solution of soda until the fluid again just appears blue. This operation may be repeated several times. By subtracting the volume of acid corresponding to the volume of solution of soda used in the process from the entire volume of acid added in the several experiments, you find the quantity of acid which has served to expel from the carbonate of baryta the carbonic acid, and is accordingly equivalent to the latter. For the details of this expeditious and very accurate method, see § 223. As the coloring matter of the litmus is sometimes thrown down, along with silicic acid separating from the precipitate, it is occasionally found necessary to add again some tincture of litmus. If this should fail to lead to the desired result, solution of soda is added until the reaction is almost complete; the amount of the soda solution left in the burette is then read off, the fluid diluted to a certain definite volume, and filtered; one-half the volume taken of the clear filtrate and solution of soda very cautiously added until the fluid appears blue; the quantity of soda solution required to effect this purpose is then doubled, and the result added to the quantity first used.

B. In cases where rigorously accurate results are not required, and it is simply wished to ascertain approximately and comparatively the amount of free carbonic acid in a mineral water, R. Kersting's method (" Annal. d. Chem. u. Pharm.," 94, 112,) may be resorted to. This method is based upon the fact that tincture of litmus is colored violet by free carbonic acid, but not by bicarbonate of soda. If, therefore, to a solution of carbonate of soda, colored blue by litmus, dilute sulphuric acid is cautiously added, the fluid will acquire a violet tint as soon as Na O, SO, and Na O, 2 CO, have been formed in it, and another drop of sulphuric acid is added, which disengages some carbonic acid. The experiment requires, a, standard solution of dilute sulphuric acid, containing in 1 cubic centimetre 10 milligrammes of S O,, corresponding to 11 milligrammes of carbonic acid; this is prepared by mixing 1 volume

of standard sulphuric acid (§ 215) with 3 volumes of water; b, a concentrated solution of soda containing some carbonic acid, as is, moreover, usually the case.

aa. Add to about 450 cubic cent. of pure water 1 cub. cent. of tincture of litmus, prepared by digesting in the cold equal parts of litmus and water; add exactly 5 c.c. of the solution of soda, dilute with water to 500 c.c. of fluid; take out, by means of the pipette, 3 several portions of 100 c.c. each, pour them respectively into 3 beakers, standing on a white ground, and add to each successively, with proper care, dilute standard sulphuric acid, until the color appears distinctly violet. The experiment must be made in the daytime, and by a good light; towards the termination of the process, you must always wait 1 or 2 minutes after each addition of acid, to allow the change of color to become distinctly visible. Bestow increased care and attention upon the experiment with the fluid in the second beaker, and in the third. Multiply by 5 the volume of acid used in the most accurate of the three experiments, and mark the result as the quantity of acid corresponding to 5 c.c. of the concentrated solution of soda.

bb. Add to about 450 to 500 cubic centimetres of the carbonic acid water under examination, carefully measured, either before the experiment or after, 5 c.c. of the soda solution b,* and mix. Of this mixture, which is generally turbid, owing to the separation of carbonates of alkaline earths, take out successively 3 several portions of 100 c.c. each, add to each portion 4 drops of litmus tincture, and then, cautiously and with stirring, sulphuric acid, until the fluid shows a violet tint. Bestow increased care and attention upon the experiment with the second portion and with the third. By determining now the quantity of carbonic acid water used, if you have not done so before, you find the last factor still required for calculating the results.

Suppose the experiment has been made with 455 c.c. of mineral water, and you have added to this 5 c.c. of solution of soda; 100c.c. of your mixture required 6 c.c. of sulphuric acid; 460 c.c. would, accordingly, have required 27.5 c.c. Now suppose 5 c.c. of soda solution have required 90.6 c.c. of acid, the 455 c.c. of the carbonic acid water contained, accordingly, as much free carbonic acid as corresponds to 90·6-276-63 c.c. of sulphuric acid, that is to say, 63 × 0·011 0·693 grammes; as for every 2 equivalents of carbonic acid, which are ultimately present in the fluid in the state of bicarbonate of soda, there is required 1 equivalent less of sulphuric acid; 1 equivalent of S 0,= 40 corresponds, consequently, to 2 equivalents of CO, 44, or 10 milligrammes SO, (i.e. the quantity contained in 1 c.c.) correspond to 11 milligrammes CO.

=

3

To obtain properly corresponding results by this method requires a practised eye; the method has, therefore, rather a subjective than an objective value; and, in my opinion, ought never to be employed in cases intended for publication.

II. Separation of Carbonic Acid from the Bases, and its estimation in Carbonates.

a. Separation from Alkalies.

aa. If the salts contain 1 equivalent of carbonic acid to 1 equivalent

* If the water is in a bottle, cool it down to about 39.2° F., then remove the stopper, take out a little of the water, and instantly add the solution of soda. If 5 c.c. is not sufficient to make the water alkaline, add 10 c.c.

of base, and there is no other salt with alkaline reaction present, the most convenient way is to determine the quantity of the base by the alkalimetrical method (§§ 219, 220), and to calculate for each equivalent of base 1 equivalent of carbonic acid.

bb. If the salt contains more than 1 equivalent of carbonic acid to 1 equivalent of base, mix the solution of a weighed portion with a mixture of chloride of barium, or chloride of calcium and ammonia, heated to boiling and filtered, and proceed exactly as directed in I., b, a.

cc. If the salts contain less than 1 equivalent of carbonic acid to 1 equivalent of base, mix the dilute solution of a weighed portion with an excess of pure and neutral solution of chloride of barium or chloride of calcium; heat, let deposit, and treat the washed precipitate as in I., b, a, aa, or bb.

b. From the Alkaline Earths.

If the compounds are neutral and contain no other acids forming with the alkalies salts with alkaline reaction, the bases in them may be determined by the alkalimetrical method (§ 223), and 1 equivalent of carbonic acid reckoned for each equivalent of base.

c. Separation of Carbonic Acid from Bases which upon Ignition readily and completely yield the Carbonic Acid with which they are combined.

Such are, for instance, the carbonates of the oxides of zinc, cadmium, lead, copper, magnesium, &c.

a. Anhydrous Carbonates.

The

Heat the weighed substance, in a platinum crucible (carbonates of cadmium and lead in a porcelain crucible), to ignition, and keep in that state until the weight of the residue remains constant. results are, of course, very accurate. Substances liable to absorb oxygen upon ignition in the air are ignited in a bulb-tube, through which a stream of dry carbonic acid gas is conducted during the process. The carbonic acid is inferred from the loss.

B. Hydrated Carbonates.

The substance is ignited in a bulb-tube through which dried air or, in presence of oxidizable substances, carbonic acid is transmitted, and which is connected with a chloride of calcium tube, by means of a dry, close-fitting cork. During the ignition, the posterior end of the tube is, by means of a small lamp, kept sufficiently hot to prevent the condensation of water in it, care being taken, however, to guard against burning the cork. The loss of weight of the tube gives the amount of the water + the carbonic acid; the increase of weight gained by the chloride of calcium tubes gives the amount of the water, and the difference accordingly that of the carbonic acid. A somewhat wide glass tube may also be substituted for the bulb tube, and the substance introduced into it in a little boat, which is weighed before and after the operation.

d. Separation of Carbonic Acid from all Bases without exception, in Anhydrous Carbonates.

The carbonate under examination is weighed in a platinum crucible, and about 4 times its weight of fused biborate of soda (§ 64, 6) added, which has been heated again just previous to weighing. The mixture is weighed, and heat is then applied, which is gradually increased to intense ignition, and maintained at this temperature until the contents of the crucible are in a state of calm fusion. The crucible is now