BENEDICT'S REAGENT: WHAT IT IS FOR, COMPONENTS, PREPARATION - CHEMISTRY - 2025

The Benedict 's reagent is a blue copper solution which is used to detect the presence of reducing sugars: aldehydes, alpha-hydroxy ketones and hemiketals. It was developed by Stanley R. Benedict (1884-1936).

Alpha-hydroxy-ketone sugars are characterized by having a hydroxyl group in the vicinity of the ketone. Meanwhile, a hemiketal is a compound that results from the addition of an alcohol to an aldehyde or ketone. Benedict's reagent reacts indiscriminately with all these reducing sugars.

The colorations of the test tubes after adding Benedict's reagent allow us to know semi-quantitatively how many reducing sugars are dissolved. Source: Thebiologyprimer

Benedict's method is based on the reducing action of sugars on Cu ²⁺, which is blue in color, which transforms it into Cu ⁺. Cu ⁺ forms a brick-red precipitate of cuprous oxide. However, depending on the concentration of sugars, a spectrum of colors appears (upper image).

Note that if Benedict's reagent is added to a test tube without reducing sugars (0%), it does not experience any change in its bluish color. Thus, when the concentration is higher than 4%, the test tube is stained brown.

History

The reagent was created by the American chemist Stanley Rossiter Benedict in 1909, who published his scientific article A reagent for detection of reducing sugars, in the journal J. Biol. Chem.

Furthermore, Lewis and Benedict (1915) published a method for the determination of reducing sugars in blood, using picrate as an indicator; but it was discontinued due to its lack of specificity.

Benedict's reagent is very similar to Fehling's. They differ in that Benedict uses the citrate ion and the sodium carbonate salt; while Fehling uses the tartrate ion and sodium hydroxide.

The Benedict test is qualitative, that is, it only detects the presence of reducing sugars. However, Benedict's reagent can be quantitative if it has potassium thiocyanate in solution, which forms a white precipitate of copper thiocyanate that can be titrated by using glucose standards.

What is the Benedict reagent for?

Detection of glucose in urine

Benedict's reagent is still used to detect the presence of glucose in urine and is an indication of diabetes disease in the patient, whose urine is subjected to the Benedict test or test. Although, it cannot be ruled out that glucosuria has a different origin.

For example, increased glycosuria is found in conditions such as: pregnancy, primary renal glycosuria, renal tubular acidosis, primary or secondary Fanconi syndrome, hyperaldosteronism, and acute pancreatitis or pancreatic cancer.

Benedict's reagent is blue in color due to the presence of Cu ²⁺, which is reduced to Cu ⁺ by the action of reducing sugars; in this case, glucose, forming a brick red copper (I) oxide precipitate.

Solution coloring

The coloration and formation of the precipitate in the Benedict test applied to urine varies depending on the concentration of the reducing sugar. If the glucose concentration in the urine is less than 500 mg / dL, the solution turns green and there is no precipitate formation.

Glucose concentration in urine of 500-1000 mg / dL causes a green precipitate in the Benedict test. At a concentration greater than 1,000 to 1,500 mg / dL, it causes the formation of a yellow precipitate.

If the glucose concentration is 1,500 - 2,000 mg / dL, an orange precipitate will be seen. Finally, a concentration of glucose in urine is greater than 2,000 mg / dL, it will cause the formation of a brick-red precipitate.

This indicates that the Benedict test has a semi-quantitative character and the result is reported using crosses. Thus, for example, the formation of a green precipitate corresponds to a cross (+); and the formation of a brick red precipitate, corresponds to four crosses (++++).

Detection of various monosaccharides and disaccharides

Benedict's reagent detects the presence of reducing sugars that possess a free functional group or a free ketone functional group, as part of their molecular structure. This is the case of glucose, galactose, mannose and fructose (monosaccharides), as well as lactose and maltose (disaccharides).

Sucrose and starch do not react with Benedict's reagent because they have free reducing groups. In addition, there are compounds that interfere with the Benedict test in urine, giving false positivity; such is the case of salicylate, penicillin, streptomycin, levodopa, nalidixic acid and isoniazid.

There are chemicals in the urine that can reduce the Benedict reaction; for example: creatinine, urate, and ascorbic acid.

Components

The components of Benedict's reagent are as follows: copper sulfate pentahydrate, sodium carbonate, trisodium citrate, and distilled water.

Copper sulfate pentahydrate, CuSO ₄ · 5H ₂ O, contains Cu ²⁺: it is the compound that gives Benedict's reagent its blue color. The reducing sugars act on Cu ²⁺, causing its reduction to Cu ⁺ and the formation of a brick-red precipitate of cuprous oxide (Cu ₂ O).

Sodium carbonate generates an alkaline medium, necessary for the reduction of copper to take place. Sodium carbonate reacts with water, generating sodium bicarbonate and the hydroxyl ion, OH ^-, responsible for the alkalinity of the medium necessary for the reductive process to occur.

Sodium citrate forms a complex with copper (II) that prevents it from undergoing a reduction to Cu (I) during storage.

Procedure of use

5 mL of Benedict's reagent is placed in a 20 x 160 mm test tube and 8 drops of urine are added. The test tube is gently shaken and placed in a container of boiling water for 5-10 minutes.

After this time, the tube is removed from the hot water bath and its surface is cooled with running water to finally have the reading of the result obtained when performing the Benedict test (the colors).

Benedict's test reaction

The reduction of Cu (II) during the Benedict test can be schematized as follows:

RCHO + 2 Cu ²⁺ (in complex) + 5 OH ^- => RCOO ^- + Cu ₂ O + 3 H ₂ O

RCHO = aldehyde; RCOO ^- = (carboxylate ion); Cu ₂ O = cuprous oxide, a brick red precipitate.

Preparation of Benedict's reagent

173 grams of sodium citrate and 100 grams of sodium carbonate are weighed out and dissolved together in 800 mL of warm distilled water. If traces of undissolved substances are observed, the solution must be filtered.

On the other hand, 17.3 grams of cupric sulfate pentahydrate are dissolved in 100 mL of distilled water.

Subsequently, the two aqueous solutions are gently mixed and continuous stirring is continued, making up to 1,000 mL with distilled water.

References

1. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10 ^th edition.). Wiley Plus.
2. Clark J. (2015). Oxidation of aldehydes and ketones. Recovered from: chemguide.co.uk
3. Wikipedia. (2020). Benedict's reagent. Recovered from: en.wikipedia.org
4. Editorial team. (January 9, 2019). Benedict's Test: Principle, Reagent Preparation, Procedure and Interpretation. Recovered from: laboratoryinfo.com
5. Dr. Whitson. (sf). Benedict's reagent: A Test for Reducing Sugars. Recovered from: nku.edu