Investigation of errors arising in iodometric determination of free sulphurous acid by the acetone procedure

Apart from difficulties in the observation of the end‐points, errors arise bemuse the determinations involve disturbance of the equilibrium between free and combined SO 2 , leading to production of more free SO 2 . The change following dilution is not rapid enough to cause serious error. The change following removal of all the free SO 2 by titration with iodine is so rapid as to be troublesome in accurate work. Addition of 20% of acetone (the optimum proportion as reckoned by volume on natural strength) does not take up all the free SO 2 in a reasonable time, but allowance can be made for this error. The most serious defect lies in the fact that with some juices (perhaps those damaged by overheating) the acetone titration does not correctly measure the reducing power of the components of the juice other than SO 2 , and the difference may be equivalent to an over‐estimate of free SO 2 by as much as 50 p.p.m.