CONCENTRATION OF SULPHURIC ACID.—The sulphuric acid made in the chambers is not strong enough for many of the purposes to which it is applied. The acid can be concentrated by boiling, however, which causes the evaporation of a part of the water with which it is combined. This may be performed in leaden pans up to a strength of 1.750 ap. gr.; but the higher the concen tration the greater the difficulty in disengaging the combined water, so that the temperature at which evaporation takes place rises rapidly and an increasing proportion of acid is distilled over at the same time. The acid cannot he concentrated to monohydrate by simple evaporation of the water, but moderately strong acid will be distilled and must be afterwards condensed. As acid of more than 1.750 sp. gr. attacks lead very powerfully, and the boiling point of monohydrated acid is very nearly equivalent to the melting point of lead, the concentration is not carried beyond that point in leaden vessels, but in retorts of platinum or glass.
When the acid is to be concentrated in platinum vessels, it must first be perfectly purified from nitrogen compounds, as that metal is very rapidly destroyed by them. Nitrous acid, as we have already said, can only be present in a form of combination with the sulphuric acid in cry.tal lizable proportions, from which it cannot be eliminated by simple heating ; on the contrary, nitric acid can be so removed from the sulphuric acid when the latter contains sufficient water. Opinions have differed as to whether the sulphuric acid should be freed from nitrous and nitric acids before or during the process of concentration. A simple method consists in treating the acid with sul phurous acid during the concentration Fig. 79 shows a vertical cross-section of the arrangement of the apparatus H. It consists of a leaden vessel similar to the pan A, except that it is deeper, shorter, and narrower. A cover is formed over the vessel, and in the -space above the acid the sulphurous acid introduced from the kiln through the pipe B can circulate freely. Its passage is directed by the two partitions a, the first of which extends from the nearer side of the pan to within a very short distance of the farther side, while the second starts from the farther side and reaches almost to the nearer side, and through these spaces the aulphurous acid has to pass. The excess of
sulphurous acid finds its way to the chambers through a pipe provided for the purpose. The sulphuric acid enters the pan by the pipe E.
Instead of the foregoing arrangement a leaden pan may be built into the kiln flue and domed over. Into this the weak acid is run, and the kiln gases are passed over it on their way to the chambers. As these gases are very hot the acid will be considerably concentrated without the aid of any other fire, whilst the steam liberated will effect a certain economy in the consumption of fuel in the steam-boiler for supplying the chambers.
This plan of denitrating the acid by means of sulphurous acid is not worth very much, unless the acid be much weaker than that made in the chambers when the process is well conducted. The reason of this is the before-mentioned fact that the crystallizable combination of sulphuric and nitrous acids when dissolved in strong sulphuric acid is very easily decomposed by water, but that the operation by means of sulphuroua acid is very difficult.
The most reliable plan of denitrating the sulphuric acid during concentration in leaden pans is by the addition of a small quantity of sulphate of ammonia. With tolerably good working the acid will contain only so much nitrogen compounds that •1—•5 per cent. of the ammonia salt will suffice.
One authority suggests that sulphur may be used for the same end. Flowers of sulphur are best, and may be introduced in little capsules of hard stoneware into the first pan, in which the temperature does not reach the fusing point of sulphur, and where the acid is richest in water. Great caution must be exercised, however, that no sulphur finds its way into the other pans, as strong hot sulphuric acid is reduced to sulphurous acid by the presence of sulphur. As 1 part of sulphur will decompose 6* parts of monohydrated acid, the loss may he very considerable. For the &Me reason, precautions must be taken that flowers of enilphur formed in the chambers through sublimation in the kilns shall not find their way into the concentrating apparatus. Organic sub stances, suoli as sugar, have also been proposed, but tho same care must he exercised that uo excess should pass into the subsequent pans. Another recommends crystallized oxalic acid.