Nature of certain Solutions. 89 



the best, is reddened by the contact of any substance which 

 will deprive it of its alkali. Methyl orange, which is by some 

 considered more sensitive than litmus, has this disadvantage, 

 that it seems to be decomposed by some metallic salts, so that 

 its colour is rendered lighter by them instead of darker, although 

 free acid may be present. This tendency greatly diminishes 

 its usefulness. 



Another method has been proposed for the detection of 

 free sulphuric acid in the presence of combined. The solution 

 is heated to 100°, and evaporated to dryness in the presence 

 of a small quantity of organic matter, which is blackened if 

 free sulphuric acid is present. As what we want is to deter- 

 mine the nature of the solution at ordinary temperatures, such 

 a method is quite worthless. 



* But by means of a new reaction the conditions of such 

 solutions can be examined, and free sulphuric acid can be 

 detected in the presence of sulphates with great accuracy and 

 sharpness, even when only a trace is present, 



The method is as follows: — Taking the well-known po- 

 larizing-salt discovered by Herapath, the sulphate of iodo- 

 quinia, it is possible to remove the whole of the sulphuric acid 

 without breaking up the molecule. This may be done with 

 either barium carbonate or barium hydroxide. The mode of 

 operation is as follows. The barium compound is placed in a 

 beaker and covered with weak alcohol of about 70 per cent. 

 Into this the crystallized herapathite is dropped. In cold 

 alcohol of this strength it is but very slightly soluble ; but in 

 the presence of the barium compound it dissolves with facility, 

 and in large quantity, forming a deep sherry-wine coloured 

 liquid. This solution, when allowed to dry spontaneously, 

 leaves an amber-coloured varnish without a trace of crystalli- 

 zation. If, however, we add to this solution a minute quantity 

 of sulphuric acid, there is left behind, on evaporating, a cha- 

 racteristic bluish-black film and isolated crystals of iodo- 

 quinia sulphate. In forming this reagent it is more convenient 

 to employ barium carbonate, as the decomposition is effected 

 equally well, and with it we are certain that the solution con- 

 tains no barium compound. Barium carbonate decomposes 

 iodoquinia sulphate with slow disengagement of carbonic 

 anhydride ; it seems therefore probable that the resulting 

 solution contains a free base. 



From this solution the sulphate is regenerated with great 

 facility by free, but not by combined sulphuric acid. And as 

 the herapathite thus formed is a well-characterized substance, 

 we obtain a most useful means of deciding as to whether 

 sulphuric acid in certain solutions is free or combined. It is 



