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Benedict’s Solution Essay

What is Benedict’s Solution?
Benedict’s solution, deep-blue alkaline solution used to test for the presence of the aldehyde functional group, – CHO. The substance to be tested is heated with Benedict’s solution; formation of a brick-red precipitate indicates presence of the aldehyde group. Since simple sugars (e.g., glucose) give a positive test, the solution is used to test for the presence of glucose in urine, a symptom of diabetes. One liter of Benedict’s solution contains 173 grams sodium citrate, 100 grams sodium carbonate, and 17.3 grams cupric sulfate pentahydrate. It reacts chemically like Fehling’s solution; the cupric ion (complexed with citrate ions) is reduced to cuprous ion by the aldehyde group (which is oxidized), and precipitates as cuprous oxide, Cu2O.

Benedict’s Test
Using Benedict’s test will reveal the presence of reducing sugars * It results in an orange-red precipitate.
* The more reducing sugar there is present, the more precipitate will be formed, and the more Benedict’s solution (copper sulphate) will be used up.
* The precipitate is filtered out then the concentration of the remaining solution can be measured. * This will tell you how much Benedict’s solution has been used up allowing you to estimate the concentration of reducing sugar in the original sample. How Does A Benedict’s Test Work?

Benedict’s Solution is a method to determine glucose levels in people suffering from diabetes. It has copper sulphate in it, which reacts with sugars forming copper oxide. Copper oxide is reddish brown in color, therefore the redder the solution appears, the more copper oxide is present, which indicates higher levels of sugar in the solution. Qualitative or quantitative test for reducing sugars


Benedict’s solution reacts with reducing sugars on heating and reduces the Cu(II) ion to Cu(I) producing a precipitate of red copper oxide. The resulting color change depends on the type and concentration of sugar, so this test can be used semi-quantitatively to indicate approximate concentrations.

An alternative version of Benedict’s reagent for quantitative testing (QBS) contains potassium thyocyanate and does not form red copper oxide. Instead the presence of reducing sugar is measured by the loss of the blue colour of copper sulphate and a white precipitate is formed which will settle out or can be removed by filtration before measuring the colour of the filtrate.

Using a colorimeter you can obtain accurate, fully quantitative determinations of concentration down to 0.001M, (180/g of glucose/cm3). This is about 5 times lower than the concentrations detectable with test strips.

Lower concentrations can be detected rather more easily and in smaller volumes using DNSA reagent.



Benedicts reagent

Solution 1
Sodium citrate 86.5g
Sodium carbonate (anhydrous) 50g Dissolve in 400mls H2O

Solution 2
Copper sulphate.5H2O 8.7g
Dissolve in 50mls H2O.

Add 2 to 1 with rapid stirring then dilute to 500mls
Positive result on boiling with
reducing sugars

The stock solution does not require a hazard warning label.

Quantitative Benedicts reagent

Solution 1
Sodium citrate 100g
Sodium carbonate (anhydrous) 32.5g
Potassium thiocyanate 62.5g
Dissolve in 400mls H2O

Solution 2
Copper sulphate.5H2O 9g
Dissolve in 50mls H2O.
Add 2 to 1 with rapid stirring

Add 0.13g potassium hexacyanoferrate (II)
then dilute to 500mls

For colourimetric use dilute 35mls of this solution to 100mls with water.

The stock solution does not require a hazard warning label.



Add about 5cm3 of the reagent to a small amount of sample in a test tube. Stand the test tube in boiling water for a few minutes. A color change through green to yellow, brown and finally to red indicates the presence of reducing sugar.


Add 2cm3 of QBS to 4cm3 of sample in a test tube. Stand the test tube in boiling water for 5 minutes Allow the tubes to stand until the precipitate settles, or filter to remove the precipitate. Measure the absorbance using red light

To test for the presence of monosaccharides and reducing disaccharide sugars in food, the food sample is dissolved in water, and a small amount of Benedict’s reagent is added. During a water bath, which is usually 4–10 minutes, the solution should progress in the colors of blue (with no glucose present), green, yellow, orange, red, and then brick red or brown (with high glucose present). A colour change would signify the presence of glucose. The common disaccharides lactose and maltose are directly detected by Benedict’s reagent, because each contains a glucose with a free reducing aldehyde moiety, after isomerization.

Sucrose (table sugar) contains two sugars (fructose and glucose) joined by
their glycosidic bond in such a way as to prevent the glucose isomerizing to aldehyde, or the fructose to alpha-hydroxy-ketone form. Sucrose is thus a non-reducing sugar which does not react with Benedict’s reagent. Sucrose indirectly produces a positive result with Benedict’s reagent if heated with dilute hydrochloric acid prior to the test, although after this treatment it is no longer sucrose. The acidic conditions and heat break the glycosidic bond in sucrose through hydrolysis. The products of sucrose decomposition are glucose and fructose, both of which can be detected by Benedict’s reagent, as described above.

Starches do not react or react very poorly with Benedict’s reagent, due to the relatively small number of reducing sugar moieties, which occur only at the ends of carbohydrate chains. Inositol (myo-inositol) is another carbohydrate which produces a negative test. Benedict’s reagent can be used to test for the presence of glucose in urine. Glucose found to be present in urine is an indication of diabetes mellitus. Once a reducing sugar is detected in urine, further tests have to be undergone in order to ascertain which sugar is present. Only glucose is indicative of diabetes.

Experiment| Observation| Inference|
Substance in water + 3ml Benedict’s solution, then boil for few minutes and allow to cool.| Red ppt or green ppt or yellow ppt obtained| Reducing sugar e.g. Glucose is present| Substance in water + 3ml Benedict’s solution, then boil for few minutes and allow to cool.| Solution remains clear or is a little blue| Reducing sugar is not present|

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