Benedict’s reagent is used to detect reducing sugars, such as glucose and fructose, in food and urine. It works by reacting with copper sulfate and producing an insoluble reddish compound. The test is qualitative and can be used as a crude quantitative test. Another reagent, Benedict’s Quantitative Solution, can be used for more accurate measurements. The reagent is used in food testing and to detect glucose in urine, which can be a sign of diabetes. Sucrose, the most commonly added sugar in processed foods, cannot be detected by Benedict’s reagent, but can be detected by boiling it with dilute hydrochloric acid.
Benedict’s reagent is a solution of copper sulfate, sodium carbonate and sodium citrate in water. It is used to detect the presence of certain types of carbohydrates known as reducing sugars. These substances can undergo chemical reactions in which they give electrons to other compounds, resulting in the production of new substances, and thus react with Benedict’s reagent to produce an insoluble reddish compound. Glucose and fructose produce a positive reaction, but sucrose, table sugar, does not. The reagent is used in food testing and to detect glucose in urine, which can be a sign of diabetes.
Types of carbohydrates
Carbohydrates can be monosaccharides, which are simple molecules like glucose (C6H12O6); disaccharides, which consist of two monosaccharides linked together, for example sucrose; or polysaccharides, which are long chains of many monosaccharide units. Monosaccharides always have a carbonyl group – a carbon atom joined to an oxygen atom by a double bond – which reacts with Benedict’s reagent. Some disaccharides, such as maltose and lactose, have carbonyl groups, some do not; it depends on how the monosaccharide units are joined. In sucrose, one glucose and one fructose molecule are joined in such a way that their carbonyl groups break apart. Polysaccharides, such as starch, have very few of these groups and therefore produce little or no reaction.
How does it work?
The reducing sugars react with the copper sulfate in Benedict’s reagent, reducing it to copper oxide I, a reddish insoluble compound that forms a precipitate. The sodium carbonate is needed to make the solution alkaline, which is essential for reacting some types of carbohydrates, while the sodium citrate prevents the copper sulfate from reacting with the alkali. The solution is blue in color due to copper sulphate. The test is essentially qualitative, i.e. it simply serves to verify the presence or absence of a reducing sugar, and not to determine its quantity. However, it can be used as a crude quantitative test, as a greenish color indicates only a little reducing sugar; yellow, a little more; and red, a lot.
Another reagent, known as Benedict’s Quantitative Solution (QBS), can be used to determine, quite accurately, how much reducing sugar is present in a sample. It is similar to the normal reagent, but contains two additional chemicals. In this solution, a positive result is indicated by a white precipitate and some loss of the initial blue color. Color intensity indicates the amount of reducing sugar in the sample and can be measured using a device called a colorimeter.
Use in food testing
Foods can be tested for low sugars by crushing or grinding a small amount and adding it to some Benedict’s reagent in a test tube, then heating for several minutes. The color of the resulting solution indicates whether any of these compounds are present and gives an approximate idea of how much. This test will detect sugars commonly found in foods, such as glucose, fructose, maltose and lactose. However, it won’t detect sucrose, which is the type most commonly added to processed foods. Boiling sucrose with dilute hydrochloric acid will split it into glucose and fructose, which can then be detected.
Use in medicine
The presence of glucose in the urine can be a sign of diabetes. Testing a urine sample with Benedict’s reagent is an easy way to check for glucose in people suspected of having this disease. However, it’s not a definitive test, as other reducing sugars will produce the same reaction. If the urine test is positive, further tests will need to be done to confirm the condition. Pregnant women can be tested this way at regular intervals to detect gestational diabetes, which can appear during pregnancy in women with no history of the condition.
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