The risk of sulphuroua acid going away unabsorbed is certainly so small as not to enter into tho calculation.
If it be desired to make a further test, a new measure of iodine solution can be introduced without trouble, and the operation recommeneed immediately. When several repetitions have been made it will be observed that the liquid in A when decolorized will colour itself anew after a tirne, because it has then come to eontain so much hydriodie sold that it decomposes of itself and free iodiao is liberated. The liquid roust then be poured out of A, and the vessel be refilled with elean water containing a little starch. Such a test can be made in a very short time when the gas ie rich in oxygen.
Wheu the volume of gases is found to contain much more than 11 per cent. of sulphurous acid, the draught must be increased, and in the opposite ease it must be reduced.
5. The Oxygen in Vie Escaping Gases.—We have already seen that the gases escaping from the chambers should eontain about 5 per cent. by volume of oxygen and 95 per cent. of nitrogen. Fixing the amount of this proportion in practioe may often assist very materially in regulating the process, and will serve as a check upon the estimation of the sulphurous a.cid, and to some extent even render it unnecessary. A very simple arrangement is in nse for absorbing the gaaes which gives results sufficiently accurate for all ordinary purposes, though not absolutely so. The volume of gases to be tested, which consist, besides oxygen and nitrogen, of some steam and a small proportion of sulphurous acid and oxides of nitrogen, is generally aspirated from the exit flue of the last chamber by the agency of a vessel alternately filled and emptied with water. A small gasometer may be conveniently used for the purpose, and should be furnished with a tap for regulating the outflow. In lieu of this, in some works a amall bellows made entirely of indiarubber is used, hold ing an exactly ascertained volume of the gas, which it is made to give up by closing the inlet, opening the outlet, and squeezing the bellows together. A simple hollow indiarubber ball fitted with tubea and which may be squeezed in the hand, answers just as well. In using this it is first tightly compressed in the hand so as to eject all the air, then it is put into connection with the interior of the flue by passing the indiarubber tube over the glass tube in the flue, aad the hand is opened. The moment the pressure is removed the ball fills with the gas. The ball is filled and emptied several times iu succession in order to be sure that no air remains, but that it ie entirely filled with gas, and finally the tubes are shut with pincheocks.
The inclosed gas ie now put into a graduated glass cylinder surrounded by water in a pneumatic trough for examination. By the passage of the gas through the water, the small portion of sul phurous'aeid and the traces of the nitrogen compounds contained in it will be absorbed. Tbe volume of the remaining gas is then noted, and a small stick of phosphorus on a wire ie introduced into the cylinder above the water level. .After twenty-four hours the phosphorus remaining
nnoxidized by the oxygen ie removed, and the volume of the gases ia estimated anew. The difference is the volume of oxygen. Aa the absorption of the oxygen by the phosphorus only takea place at 12° C., and under certain conditions not till a temperature of 15°-20° C. is reached, it ia necessary to notice at the commencement of the operation whether the phosphorus becomea coated with a film of phosphorous acid, and, if needed, the water must be warmed.
A concentrated alkaline aolution of pyrogallate of potaah ia now more generally used than phosphorus. Such a solution absorbs a considerable amount of oxygen. According to Dobereiner 1 grm. of pyrogallic acid in an ammonia solution absorbs 260 cc. of oxygen. In the proceas, the gas to be tested is collected in a graduated cylinder over mercury and the solution added.
When estimating the oxygen by pyrogallate of potaah, it ia advisable to allow the gas to be for some time previously in contact with a solution of bichromate of potash, whereby the aulphurous acid is turned into aulphuric acid, and the binoxide of nitrogen and the nitrous and hyponitric acids are converted into nitric acid and completely removed from the volume of the gas.
By employing auch tests as these in conjunction with a constant regulation of the draught and temperature it is poasible to render the process almost exact, and iu the event of irregularities occurring they can be checked immediately they commence. When, however, auch irregularities are allowed to continue for a long time their rectification ia more difficult, as by reducing the formation of sulphuric acid the relative proportion of sulphurous acid ia abnormally increased. This causes an interruption of the draught and the temperature rises in the kiln in consequence, till at last the sulphur sublimes. When the mischief has once got so far, there remains no cure but to atop the working and recommence anew.
In watching the conduct of the procesa it muat not he forgotten that various causes will ahow similar symptoms. Thus the reduction in strength of the acid may be caused as much by a cessation of production as by an exceas of steam. A check in the draught may also be due to reduced formation of aulphuric acid as well as to atmospheric influences, or to atoppage of the pipes and connections through which the gases pass. The conversion of the aulphurous acid into sulphuric acid may be hindered as much by want of air consequent upon alow draught as by excess of air from too atrong a draught. This evil may also be due not only to excess of ateam causing a condensation of the nitric acid from the volume of the gasea, but likewise to a lack of steam permitting the combination of the nitrous acid with the aulphuric acid to form chamber crystals. All theae crimes may reduce the strength of the chamber acid.