In the third place, the error may be eliminated by substituting for an alkali which acts upon the glucose one which does not, _viz._, ammonia. At the temperature of boiling water ammonia does not have any decomposing effect upon reducing sugars. It is important, however, that the reduction take place in an inert atmosphere in order to avoid the oxidation of the dissolved cuprous oxid and the temperature need not be carried beyond 80°. The end of the reaction can be easily distinguished in this case by the disappearance of the blue color. When one reaction is finished the copper may be completely reoxidized by conducting through it a current of air or oxygen for half an hour, when an additional quantity of ammonia may be added to supply any that may have evaporated, and a new reduction accomplished with exactly the same quantity of copper as was used in the first. The solution used by Gaud contains 36.65 grams of crystallized copper sulfate dissolved in water and the volume completed to one liter with ordinary aqueous ammonia.[83] =120. Permanganate Process for the Estimation of Reducing Sugars.=—Dextrose, invert sugar, and other reducing sugars can also be determined with a fair degree of accuracy by an indirect volumetric process, in which a standard solution of potassium permanganate is used as the final reagent.[84] The principle of the process is based upon the observation that two molecules of dextrose reduce from an alkaline cupric tartrate solution five molecules of cuprous oxid. The five molecules of cuprous oxid thus precipitated when added to an acid solution of ferric sulfate, will change five molecules of the ferric sulfate to ten molecules of ferrous sulfate. The reaction is illustrated by the following equation: { 5Cu₂O } + { 5Fe₂(SO₄)₃ } + { 5H₂SO₄ } = { 10CuSO₄ } {715 parts} { 2000 parts } {490 parts} {1595 parts} + { 10FeSO₄ } + { 5H₂O } {1520 parts} {90 parts} The ten molecules of ferrous sulfate formed as indicated in the above reaction, are reoxidized to ferric sulfate by a set solution of potassium permanganate. This reaction is illustrated by the equation given below: { 10FeSO₄ } + { K₂Mn₂O₈ } + { 5H₂SO₄ } = { 5Fe₂(SO₄)₂ } {1520 parts} {316.2 parts} {784 parts} { 2000 parts } + { 2MnSO₄ } + { K₂SO₄ } + { 8H₂O } {302 parts} {174.2 parts} {144 parts}. By the study of the above equations it is seen that two molecules of dextrose or other similar reducing sugar, are equivalent to one molecule of potassium permanganate, as is shown by the following equations: { 2C₆H₁₂O₆ } = { 5Cu₂O } = { 10FeSO₄ } = { K₂Mn₂O₈ } { 360 parts } {715 parts} {1520 parts} {316.2 parts}