A Correction on the Benedict Test The addition of aaueous sodium carbonate to a solution containins distilled water. a water soluble coooer(I1) salt. an .. aldehyde of any tyyr, and v~diumcitratr rausrs the rvulut~onofmrhun dioxldr which is indilatd by rxriny or efirrvcsrmrr.' This also i s similar to the reactirm invoh ing an nqumus nlknhne mrtnl r x h m a t e adution and either an iron(1ll1 call, rhromium(ll1, salt or an aluminum salt in aqueous r;dutionAexcept that no hydroxide precipitate is formed. The fiming ceases after the entire solution is heated over a water bath for several minutes, hut the characteristic blue color remains. The addition of sodium hydroxide to this solution after a few extra minutes of heating causes the desired brick red precipitate, eopper(1) oxide, to be formed. The equation in ionic form for the reaction is
P + 2Cu2+ + COa2- + 30H-
RC-H
A
a0 + COz t + 2H20 + CuzO
RC-0-
Na citratD
This hrirk rrd solid alw is h a i n e d by substituting sodium hydroxide for sodium rarbcmorc altogether, nnd the equntion , ~ ~fur the Frhlini: test. for the proress id a n a l ~ gtu~ that 'The evolut~onof rnrhon dioxide and the fmnation uf brirk red or nenr brick red ropperll) oxid? RIW W P W ub~erved as the process was executed with starch and other carbohydrates. However, the heating process is not as long as it is with straight out aldehydes otherwise a variety of colors for copper(1) oxide will he obtained. It should he evident now that aqueous sodium carbonate is not adequate for the production of brick red copper(1) oxide as has been s~eeifiedin man". oreanic - ehemistrv teats and organic chemistry lab hooks. Sodium hydroxide also is needed for the produetion.
' Weaver and Foster, "Chemistry for Our Times," McGraw-Hill B w k Company Inc., New York, 1947, p. 203.
Hogness, Johnson and Armstrong, "Qualitative Analysis and Chemical Equilibrium," 5th Ed., Holt, Rinehart and Winston Ine., New York, 1966, p. 214. North Carolina Central University, William D. Hill, Jr. Durham, NC 27701
334
Journal of Chemical Education
