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{{short description|Chemical reagent}}
{{redirect|Benedict's|other uses|Benedict (disambiguation)}}
{{more citations needed|date=May 2015}}
{{ infobox chemical analysis
| name           = Benedict's test
| image          = Trommer's test.jpg
| caption        = Positive Benedict's test
| acronym        = 
| classification = Colorimetric method
| analytes       = Reducing sugars
| manufacturers  = 

| related        = 
| hyphenated     = 
}}

'''Benedict's reagent''' (often called '''Benedict's qualitative solution''' or '''Benedict's solution''') is a chemical [[reagent]] and complex mixture of [[sodium carbonate]], [[sodium citrate]], and [[copper(II) sulfate]] pentahydrate.<ref>{{cite journal |year=2002 |title=Benedict's Solution, a Reagent for Measuring Reducing Sugars: the Clinical Chemistry of Stanley R. Benedict |journal=J. Biol. Chem. |volume=277 |issue=16 |pages=10–11 |author1=Robert D. Simoni |author2=Robert L. Hill |author3=Martha Vaughan |doi=10.1016/S0021-9258(19)61050-1|doi-access=free }}</ref> It is often used in place of [[Fehling's solution]] to detect the presence of [[reducing sugar]]s and other reducing substances.<ref name="Collins">Collins Edexcel International GCSEBiology, Student Book ({{ISBN|978-0-00-745000-8}}) p.42-43</ref> Tests that use this reagent are called '''Benedict's tests'''. A positive result of Benedict's test is indicated by a color change from clear blue to brick-red with a precipitate.

Generally, Benedict's test detects the presence of [[aldehyde]] groups, [[acyloin|alpha-hydroxy-ketones]], and [[hemiacetal]]s, including those that occur in certain [[ketose]]s. In example, although the ketose [[fructose]] is not strictly a reducing sugar, it is an alpha-hydroxy-ketone which results to a positive test because the base component of Benedict converts it into [[aldose]]s [[glucose]] and [[mannose]]. Oxidizing the reducing sugar by the cupric (Cu<sup>2+</sup>) complex of the reagent produces a cuprous (Cu<sup>+</sup>), which [[precipitate]]s as insoluble red [[copper(I) oxide]] (Cu<sub>2</sub>O).<ref>{{Cite web|url=http://dept.harpercollege.edu/chemistry/chm/100/dgodambe/thedisk/carbo/bened/benedict.htm|title=Carbohydrates - Benedict's Test|website=dept.harpercollege.edu|access-date=2020-03-08}}</ref>

The test is named after American chemist [[Stanley Rossiter Benedict]].<ref name=benedict>{{cite journal | author = Benedict, S. R. | title = A Reagent For the Detection of Reducing Sugars | journal = J. Biol. Chem. | volume = 5 | issue = 6 | pages = 485–487 | date = 1 January 1909 | doi = 10.1016/S0021-9258(18)91645-5 | doi-access = free | url = https://www.jbc.org/article/S0021-9258(18)91645-5/pdf}}</ref>

== Composition and preparation ==
Benedict's reagent is a deep-blue aqueous solution. Each litre contains:<ref name=benedict></ref>
* 17.3 g copper sulfate
* 173 g sodium citrate
* 100 g anhydrous sodium carbonate or, equivalently, 270 g sodium carbonate decahydrate
Separate solutions of the reagents are made. The sodium carbonate and sodium citrate are mixed first, and then the copper sulfate is added slowly with constant stirring.

Sodium citrate acts as a complexing agent which keeps Cu<sup>2+</sup> in solution, since it would otherwise precipitate. Sodium carbonate serves to keep the solution alkaline. In the presence of mild reducing agents, the copper(II) ion is reduced to copper(I), which precipitates in the alkaline conditions as very conspicuous red copper(I) oxide.

== Organic analysis ==
To test for the presence of [[monosaccharide]]s 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 [[heated bath|water bath]], which is usually 4–10 minutes, the solution should progress through the colors of blue (with no reducing sugar present), orange, yellow, green, red, and then brick red precipitate or brown (if a high concentration of reducing sugar is present). A color change would signify the presence of a reducing sugar.<ref name="Collins"/>

{| class="wikitable" border="1"
|-
! Experiment
! Observation
! Inference
|-
| Substance in water + 3 mL Benedict's solution, then boil for few minutes and allow to cool.
| Red, green, or yellow precipitate is obtained
| [[Reducing sugar]], such as glucose, is present
|-
| Substance in water + 3 mL 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
|-
|}

The common disaccharides lactose and maltose are directly detected by Benedict's reagent because each contains a glucose with a free reducing aldehyde [[Moiety (chemistry)|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 undergoing [[isomerizing|isomerization]] to an aldehyde, or fructose to alpha-hydroxy-ketone form. Sucrose is thus a non-reducing sugar which does not react with Benedict's reagent. However, sucrose indirectly produces a positive result with Benedict's reagent if heated with dilute [[hydrochloric acid]] prior to the test, although it is modified during this treatment as 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.

[[Starch]]es do not react or react very poorly with Benedict's reagent because of lesser number of reducing sugar components that occur at the ends of the [[carbohydrate]] chains. Other carbohydrates which produce a negative result include [[inositol]].

Benedict's reagent can also be used to test for the presence of [[glucose]] in [[urine]], elevated levels of which is known as [[glucosuria]]. Glucosuria can be indicative of [[diabetes mellitus]], but Benedict's test is not recommended or used for diagnosis of the aforementioned condition. This is due to the possibility of a reaction in which the presence of other reducing substances such as [[ascorbic acid]], drugs ([[levodopa]], contrast used in radiological procedures) and [[homogentisic acid]] ([[alkaptonuria]]) creates a false positive.

As color of the obtained precipitate can be used to infer the quantity of sugar present in the solution, the test is semi-quantitative. A greenish precipitate indicates about 0.5 g% concentration; yellow precipitate indicates 1 g% concentration; orange indicates 1.5 g% concentration; and red indicates 2 g% or higher concentration.

== Quantitative reagent ==
'''Benedict's [[Quantitative analysis (chemistry)|quantitative]] reagent''' contains potassium thiocyanate and is used to quantitatively determine the concentration of reducing sugars.<ref name="Collins"/> This solution forms a [[copper thiocyanate]] precipitate which is white and can be used in [[titration]]. The titration should be repeated with 1% glucose solution instead of the sample for [[calibration]].

==Net reaction ==
The net reaction between an [[aldehyde]] (or an [[acyloin|alpha-hydroxy-ketone]]) and the copper(II) ions in Benedict's solution may be written as:
:{{chem2|RCHO + 2 Cu(2+) + 5 OH- → RCOO- + Cu2O + 3 H2O}}.

The hydroxide ions in the equation forms when sodium carbonate dissolves in water. With the citrate included, the reaction becomes:
:{{chem2|RCHO + 2 Cu(C6H5O7)- + 5 OH- → RCOO- + Cu2O + 2 C6H5O7(3-) + 3 H2O}}.<ref>{{Cite web|url=https://microbenotes.com/benedicts-test/|title = Benedict's Test- Objectives, Principle, Procedure, Results|date = 21 April 2021}}</ref>

==See also==

* [[Dextrose equivalent]]

Other oxidizing reagents
* [[Fehling's solution]]
* [[Tollens' reagent]]
Other reducing reagents
* [[Jones reductor]]
* [[Walden reductor]]

==References==
{{Reflist}}

{{Analytical reagents}}
{{Organic reactions}}

[[Category:Analytical reagents]]
[[Category:Biochemistry detection methods]]
[[Category:Carbohydrate methods]]
[[Category:Chemical tests]]
[[Category:Coordination complexes]]
[[Category:Copper compounds]]
[[Category:Oxidizing agents]]