Air, Analysis of

Priest ley’s discovery of oxygen gas in 1774 prepared the way for the knowledge of the real composition of air, which was discovered about the same time by Scheele and Lavoisier. Scheele’s method of operating was by exposing some atmospheric air to a solution of sulphide of pota ium. Lavoisier effected the same object by the combustion of iron wire and phosphorus, and subsequently by heating mercury on a flask filled with air for some time, just below its boiling point. These, however, were but elementary methods, which, however creditable to the ingenuity of the great founders of modern chemistry, not only failed in accuracy, but took no account of the presence and amount of two most important constituents in the atmosphere, viz. carbonic anhydride (acid) and ammonia. Determination of Aqueous Vapour. To effect this an aspirator must be used (see Aspirator ). This instrument is easily made, and is not expensive. The accompanying figure will illustrate the arrangement generally adopted: a is an aspirator made of galvanised iron or sheet zinc. It holds from 50 to 200 litres (from 11 to 44 gallons). By this means a known volume of air is drawn through the tubes marked b , c , d , e , which may be filled with pumice-stone moistened with strong sulphuric acid; but if the carbonic acid is to be estimated as well, b and c are filled with moist hydrate of lime (potash used to be employed, but hydrate of lime is to be preferred, as the potash absorbs oxygen), and d and e as above. Each of the tubes is accurately weighed previously to connecting them with the apparatus. IMG:596405606043057898_i062-1.png: It is imperative to have each of the tubes connected by perfectly air-tight joints. The gain of weight in d and e gives the water in b and c the carbonic acid. Determination of Carbonic Acid. A better and perhaps more exact means of determining the carbonic acid is that invented by Pettenkofer . It may be briefly described as follows:—Baryta water of definite strength is prepared and accurately standardised by a standard solution of oxalic acid. A portion of this baryta water is then made to act upon a definite quantity of air. It will absorb the whole of the carbonic acid in that air. The alkalinity of the liquid will in consequence be diminished; it will take le of the oxalic-acid solution than before, which shows so much le caustic baryta, and from which the carbonic acid absorbed may be easily calculated. The actual Analysis. Two kinds of baryta water may be used, the one containing 7 grammes to the litre, the other three times that strength; 1 c. c. of the stronger = 3 m. grms. of carbonic acid; 1 c. c. of the weaker = 1 m. grm. The baryta water is best kept in the bottle represented below. IMG:596405606043057898_i062-2.png: The bottle ( a ) contains the baryta water. It has an accurately-fitting double-perforated stoppered caoutchouc. The left-hand tube is connected with the tube ( b ) containing pumice-stone moistened with potash, while the right-hand one is a syphon. When required for use the stop-cock ( f ) is opened, and suction applied by a gla tube to F . The syphon is thus filled and the stop-cock closed. If a pipette is required to be filled its nozzle is inserted at F, the stop-cock compre ed, and the fluid immediately rises into the pipette. The air entering the bottle as the fluid decreases in a is, of course, thoroughly deprived of its carbonic acid by the tubes at b . The first thing to be done is to standardise the baryta solution by a solution of oxalic acid, containing 2·8636 grammes of crystallised oxalic acid to the litre. Thirty c. c. of baryta solution are run into a small flask, and the oxalic acid run in from a Mohr’s burette with float, the vanishing-point of the alkaline reaction being ascertained by delicate turmeric paper. As soon as a drop placed on turmeric paper does not give a brown ring the end is attained. The actual analysis is performed by filling a bottle of known capacity, with the aid of a pair of bellows, with the air to be analysed, then distributing over its sides 45 c. c. of the baryta water it is left for half an hour. The turbid water is poured into a cylinder, closely secured, and allowed to deposit; then take out 30 c. c. by a pipette of the clear fluid, run in the solution of oxalic acid, multiply the volume used by 1·5, and deduct the produce from the c. c. of oxalic acid used for 45 c. c. of the fresh baryta water. A different method has been suggested by Dr Angus Smith, viz. to measure the carbonic anhydride by the turbidities of the baryta water; this is, in fact, a colorimetric test. For rough approximative results Dr Smith’s proce will be found a very useful and convenient one. It depends upon the fact that the amount of carbonic acid in a given quantity of air will not produce a precipitate in a given quantity of lime or baryta water unle the carbonic acid is in exce . The following is one of his tables:—Columns 1 and 2 give the rates of carbonic acid in the quantity of air which will produce no precipitate in half an ounce of lime water. Column 3 is the same as column 2; but 14·16 c. c. (half an ounce) is added to give the corresponding size of the bottle, and column 4 gives the size of the bottle in ounces. To be used when the point of observation is “no precipitate.” Half an ounce of baryta water contains about ·08 gramme of baryta. Air at 0° C. and 760 millims. Bar. | Carbonic Acid in the Air, per cent. | Volume of Air in cubic centimètres. | Size of bottle in cubic centimètres. | Size of bottle in ounces Avoirdupois. | | ·03 | 185 | 199 | 7·06 | | ·04 | 139 | 154 | 5·42 | | ·05 | 111 | 125 | 4·44 | | ·06 | 93 | 107 | 3·78 | | ·07 | 79 | 93 | 3·31 | | ·08 | 70 | 84 | 2·96 | | ·09 | 62 | 76 | 2·69 | | ·10 | 56 | 70 | 2·46 | | ·11 | 51 | 65 | 2·29 | | ·12 | 46 | 60 | 2·14 | | ·13 | 43 | 57 | 2·01 | | ·14 | 40 | 54 | 1·90 | | ·15 | 37 | 51 | 1·81 | | ·20 | 28 | 42 | 1·48 | | ·25 | 22 | 36 | 1·29 | | ·30 | 19 | 33 | 1·16 | | ·40 | 14 | 28 | 1·04 | | ·50 | 11 | 25 | ·89 | | ·60 | 9 | 23 | ·89 | | ·70 | 8 | 22 | ·78 | | ·80 | 6 | 20 | ·72 | | 1·00 | 5·5 | 19·7 | ·70 | Mr Wanklyn’s proce for the determination of carbonic acid in the atmosphere is as follows:—A solution of carbonate of soda is first made as follows: 4·47 grammes of gently-ignited carbonate of soda are di olved in one litre of water, giving a solution of such a strength that 1 c. c. contains exactly 1 c. c. of carbonic acid (= 1·97 milligrammes of CO 2 ); a large quantity of baryta water (strength about 0·1 per cent.) is prepared. If now 100 c. c. of clear baryta water be treated with 1 c. c. of carbonate of soda, just described, a certain degree of turbidity is produced. If 2 c. c. of the solution be taken another degree of turbidity is produced, and so on. If, then, a bottle capable of holding 2000 c. c. of air, together with 100 c. c. of baryta water, be filled with the sample of shaking it up. Having got the air to expend itself on 100 c. c. of baryta water the degree is to be found by comparison with another 100 c. c. of baryta water, in which a like turbidity has been induced by means of the standard solution of carbonate. Every c. c. of soda solution counts for a c. c. of carbonic acid in two litres of air. A consumption of 1 c. c. will correspond to ·05 volumes of carbonic acid per cent. Good air should accordingly not take more than 1 c. c. of soda solution, air which takes already 2 c. c. being already bad. In order practically to carry out this method of estimating carbonic acid the following apparatus is required:—Several bottles capable of holding 2·210 c. c., and well stoppered (failing bottles of exactly the right capacity Winchester quart bottles will answer); a small pair of bellows; several colourle gla cylinders marked at 100 c. c. capacity—the Ne lerising cylinders will answer for this purpose—a graduated pipette or burette to deliver tenths of a c. c. of solution, the standard solution of carbonate of soda, and the baryta water, which may be of moderate strength. The testing is managed thus: Winchester quart bottles having been made clean are rinsed with distilled water, and allowed to drain a little. They are then closed with their stoppers, and are ready for use. The operator having provided himself with two or three of these bottles and a small pair of bellows enters the room the air of which is to be tested. The stopper is then removed from one of the bottles, and some air of the room blown through with the bellows, and then the stopper is replaced, and the bottle carried away to be tested. The testing is done by pouring into the bottle 100 c. c. of clear baryta water, shaking up for two or three minutes, and then pouring out into a cylinder of colourle gla , and observing the depth of the turbidity in various lights and against various backgrounds. The turbidity is to be exactly imitated by means of the standard solution of carbonate of soda. good air only 1 c. c. of this solution of carbonate of soda is required. If 2 c. c. or more than 2 are required, the air is bad and the ventilation is defective. In place of the first c. c. of solution of carbonate of soda the carbonic acid naturally present in a Winchester quart of good average air may be used, and a little practice and intelligence will suggest the nece ary precautions. Estimation of the Oxygen. —To determine this Angus Smith has recourse to the endiometer. Five or six of Bunsen’s endiometers were used at once and the mixed gases were exploded by means of a powerful battery and a Ruhumkorff’s coil. In his ‘Inorganic Chemistry,’ Miller thus explains the principle upon which the action of the endiometer is based: “By means of the endiometer various gaseous mixtures may be analysed with great exactne . Many different forms of this instrument are in use. One of the most convenient is Hoffmann’s. It consists of a stout syphon tube. (See next figure.) Into the sides of the tube, near the sealed end, two platinum wires ( a , b ) are fixed for the purpose of transmitting an electric spark through the cavity of the tube. The sealed limb is accurately graduated to tenths of a c. c. or other suitable divisions. Suppose it be desired to ascertain the proportion of oxygen in atmospheric air. The instrument is first filled with mercury, after which a small quantity of air is introduced; the bulk of the air is accurately measured, taking care that the liquid metal stands at the same level in both tubes, which is easily effected by adding mercury, or by drawing off the mercury if needed, through the caoutchouc tube, which is fixed upon the small inlet tube just above the bend, and which is closed by means of a screw tap ( c ). IMG:596405606043057898_i064-1.png: A quantity of pure hydrogen, about equal in bulk to the air, is next introduced, and the bulk of the mixture is then accurately measured. The open extremity of the tube is now closed with a cork, below which a column of atmospheric air is safely included. This portion of air acts as a spring, which gradually checks the explosive force, when the combination is effected by pa ing a spark acro the tube by means of the platinum wires. The mixture is then exploded by the electric spark. The remaining gas now occupies a smaller volume, owing to the condensation of the steam which has been formed. Mercury is, therefore, again poured in the open limb until it stands at the same level in both tubes, and the volume of the gas is measured a third time. One third of the reduction of the bulk experienced by the gas will represent the entire volume of oxygen which the mixture contained. Liebig’s method is as follows. It is based upon the fact that an alkaline solution of pyrogallic acid absorbs oxygen: 1. A strong measuring tube holding 30 c. c., and divided into one fifth or one tenth c. c., is filled to two thirds with the air intended for analysis. The remaining part of the tube is filled with mercury, and the tube is inverted over that fluid in a tall cylinder widened at the top. 2. The volume of air confined is measured—a quantity of solution of potash of 1·4 sp. grf. (1 part of dry hydrate of potash to 2 parts of water), amounting from 1 ⁄ 40 th to 1 ⁄ 50 th of the volume of the air, is then introduced into the measuring tube by means of a pipette with the point bent upwards (see drawing ), and spread over the entire inner surface of the tube by shaking the latter. When no further diminution of volume takes place the decrease is read off. The carbonic acid is thus removed. IMG:596405606043057898_i064-2.png: 3. A solution of pyrogallic acid containing 1 gramme of the acid in 5 or 6 c. c. of water is introduced into the same measuring tube by means of another pipette similar to the above. The mixed fluid (the pyrogallic acid and the solution of potash) is spread over the inner surface of the tube by shaking the latter, and when no further diminution of volume is observed the residuary nitrogen is measured. 4. The solution of pyrogallic acid mixing with the solution of potash of course dilutes it, causing thus an error from the diminution of its tension; but this error is so trifling that it has no appreciable influence upon the results. It may, moreover, be readily corrected by introducing into the tube, after the absorption of the oxygen, a small piece of hydrate of potash, corresponding to the amount of water in the solution of the pyrogallic acid. There is another slight error on account of a portion of the fluid adhering to the inner surface of the tube, so that the volume of the gas is never read off with absolute accuracy. In conducting these endiometric experiments the nece ary corrections for temperature and barometric pre ure must, of course, be made. Estimation of the Nitrogen. The amount of this gas is usually determined by deducting the aqueous vapours, oxygen and carbonic acid, from the volume of air examined. Determination of Ammonia and Organic Matter. These are best determined by drawing a known volume of air through absolutely pure water. To obtain this latter it is best to redistil distilled water, to reject the first portions, then to add an alkaline solution of permanganate of potash, and to discard any portions of the distillate which give the slightest reaction with the Ne ler test. The water through which the air is drawn must be kept cool, and afterwards submitted to the proper tests, which will be found under Ammonia and Water Analysis . Mr Blyth says, “Solid bodies such as vibrionic germs, dust, fungi, ., may be obtained by using an aspirator, and drawing the air either through a drop of glycerine or water. Organic matter may also be obtained by suspending gla ve els filled with ice water, over or in the places to be investigated, and submitted to the microscope. High powers, such as immersion lenses, are requisite for the investigation of germs,” . Of these germs Dr Angus Smith says:—“They may probably be divided into many kinds—the useful and the deleterious, those which promote health and those which bring disease. The idea of any of them bringing health is not founded on anything positive, but we can scarcely imagine these numberle forms to be all usele . The idea that they bring disease is, I think, one well confirmed.” See a paper by the same author “On the Air and Rain of Manchester.” ‘Memoirs of the Literary and Scientific Society of Manchester,’ vol. x. See Air , Vitiated .