Sulfur Dioxide Content of Wine I. Iodometric Titration

Abstract

Several of the factors affecting the reproducibility of the Ripper titration for total sulfur dioxide content of white table wine were determined. The end-point in the titration was found to be objectionably fleeting even under conditions that were not expected to lead either to an appreciable content of unchanged residual sulfite-addition products or to appreciable rate of hydrolysis of a-hydroxysulfonic acids. The concentration of starch present during titration was found to affect the sharpness of the end-point. The speed of titration with dilute iodine also noticeably affected the reproducibility of the results. Dilution of the aliquot used improved the end-point. The amount of alkali used and the period of storage could be greatly reduced without noticeably affecting results. Sufficient alkali to adjust the pH of the mixture to 12 resulted usually but not always in liberation of practically all the sulfur dioxide in 2 minutes. Under these conditions oxidation of the liberated sulfite tending to give low results and production of non-sulfur dioxide iodine reducing substances tending to give high results could be reduced. Non-sulfur dioxide iodine reducing substances were found to be present in both the alkali-treated wines and in distillates from wines affer acidification with hydrochloric acid. These could be estimated by the addition of either formaldehyde as a sulfite binding agent or hydrogen peroxide as a sulfite oxidizing agent. The latter gave sharper and less fleeting end-points. The direct iodine titration, however, did not correspond with the direct distillation into iodine or with reflux distillation into hydrogen peroxide. The latter procedure on the two wines examined gave unexpectedly high results. Modified electrometric titration was not found to be an improvement over iodinetitration using starch as indicator.