Alkaline Cupric Citrate

Reducing Substances Transfer 1.0 g of sample into a 250-mL conical flask, dissolve it in 20 mL of water, and add 25 mL of alkaline cupric citrate TS. Cover the flask, boil the contents gently for 5 min, accurately timed, and cool rapidly to room temperature. Add 25 mL of 0.6 A acetic acid, 10.0 mL of 0.1 A iodine, and 10 mL of 3 A hydrochloric acid, and titrate with 0.1 A sodium thiosulfate, adding 3 mL of starch TS as the endpoint is approached. Perform a blank determination (see General Provisions), make any necessary correction, and note the difference in volumes of 0.1 A sodium... 

D. Add 4 mL of alkaline cupric citrate TS to 1 mL of a 1 50 aqueous solution. Boil vigorously for 2 to 4 min. Remove from heat, and allow the precipitate (if any) to settle. The supernatant liquid is blue or blue-green. 

Algin, 24, 54,312, 351 Alginates Assay, 768 Algin Derivative, 331 AlginicAcid, 19 Alkaline Borate Buffer, 848 Alkaline Cupric Citrate TS, 851 Alkaline Cupric Tartrate TS, 849 Alkaline Mercuric-Potassium Iodide TS, 850... 

Benedict s Qualitative Reagent See Cupric Citrate TS, Alkaline. 

Cupric Citrate TS, Alkaline (Benedict s Qualitative Reagent) With the aid of heat, dissolve 173 g of sodium citrate (C6HjNa307 2HiO) and 117 g of sodium carbonate (Na2-CO3 H2O) in about 700 mL of water, and filter through paper, if necessary. In a separate container, dissolve 17.3 g of cupric sulfate (CuS04-5H20) in about 100 mL of water, and slowly add this solution, with constant stirring, to the first solution. Cool the mixture, dilute to 1000 mL, and mix. 

Benedicts reagent (an alkaline solution containing a cupric citrate complex ion) and Tollens solution [Ag (NH3)20H] oxidize and thus give positive tests with aldoses and ketoses. The tests are positive even though aldoses and ketoses exist primarily as cyclic hemiacetals. 

The nonstoichiometric sugar oxidation process in the presence of alkali is used for both qualitative and quantitative determination of reducing sugars (Fehling s reaction with alkaline cupric tartrate Nylander s reaction with alkaline trivalent bismuth tartrate or using Benedict s solution, in which cupric ion complexes with citrate ion). Hydroxyaldehydes and hydroxyketones are formed by chain cleavage due to retroaldol reaction under nonoxidative conditions using dilute alkali at elevated temperatures or concentrated alkali even in the cold. 

An impure hydrosol of copper was first prepared by Lottermoser 2 as an adsorption-compound with stannic oxide by heating a slightly alkaline solution of a cupric salt with a similar solution of stannous chloride in presence of an alkali-metal citrate or tartrate. With water the black precipitate yields a reddish-brown liquid, rapidly oxidized by atmospheric oxygen, with production of a greenish coloration which soon becomes yellow. 

Aldoses reduce Tollens reagent, as we would expect aldehydes to do. They also reduce Fehling s solution, an alkaline solution of cupric ion complexed with tartrate ion (or Benedict s solution, in which complexing is with citrate ion) the deep-blue color of the solution is discharged, and red cuprous oxide precipitates. These reactions are less useful, however, than we might at first have expected. 

In one such method (details of which are found on the Evolve site that accompanies this book), using Benedict s reagent (cupric ion complexed to citrate in alkaline solution), reducing substances convert cupric to cuprous ions, forming yellow cuprous hydroxide or red cuprous oxide. 

Benedict s reagent contains an alkaline solution of cupric ion (Cu ) complexed with citrate anion. A positive test is detected by the disappearance of the blue color of cupric ion and the formation of a red precipitate. The red precipitate is CU2O which forms when cupric ion is reduced to cuprous (Cu ). 

We studied the use of Tollens silver mirror test in Section 16.13B. Benedict s solu tion and the related Fehling s solution (which contains a cupric tartrate complex ion) give brick-red precipitates of CUgO when they oxidize an aldose. [In alkaline solution ketoses are converted to aldoses (Section 22.5A), which are then oxidized by the cupric complexes.] Since the solutions of cupric tartrates and citrates are blue, the appearance of a brick-red precipitate is a vivid and unmistakable indication of a positive test. 

Citrates, oxalates, salicylates, glycerol, and cane sugar also stabilize alkaline solution of cupric salts. Some of these, citrates in particular, have been used in the preparation of copper solutions for sugar analysis. 

A test for the presence of reducing substances in urine. Benedict s reagent contains cupric ion complexed with citrate in alkaline solution. Glucose or other reducing substances reduce cupric ion to cuprous ion, resulting in the formation of yellow cuprous hydroxide or red cuprous oxide depending on the amount of reducing substance present. 

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