If the color upon boiling is changed into green, then there would be 0.1 to 0.5 percent sugar in solution. Observe for color change in the solution of test tubes or precipitate formation.The solution is then heated in a boiling water bath for 3-5 minutes.2 ml (10 drops) of Benedict’s reagent (CuSO4) is placed in the test tube.Approximately 1 ml of sample is placed into a clean test tube.One litre of Benedict’s solution can be prepared from 100 g of anhydrous sodium carbonate, 173 g of sodium citrate and 17.3 g of copper(II) sulfate pentahydrate. Composition and Preparation of Benedict’s Solutionīenedict’s solution is a deep-blue alkaline solution used to test for the presence of the aldehyde functional group, – CHO.Ĭopper(II) sulfate pentahydrate = 17.3 gm Sugar needs to be decomposed into its components glucose and fructose then the glucose test would be positive but the starch test would still be negative. Table sugar (disaccharide) is a non-reducing sugar and does also not react with the iodine or with the Benedict Reagent. Sodium citrate complexes with the copper (II) ions so that they do not deteriorate to copper(I) ions during storage.Ĭomplex carbohydrates such as starches DO NOT react positive with the Benedict’s test unless they are broken down through heating or digestion (try chewing crackers and then doing the test). Sodium carbonate provides the alkaline conditions which are required for the redox reaction. Sometimes a brick red solid, copper oxide, precipitates out of the solution and collects at the bottom of the test tube. As the concentration of reducing sugar increases, the nearer the final color is to brick-red and the greater the precipitate formed. This accounts for the precipitate formed. The red copper(I) oxide formed is insoluble in water and is precipitated out of solution. The copper (II) ions in the Benedict’s solution are reduced to Copper (I) ions, which causes the color change. This reaction is caused by the reducing property of simple carbohydrates. When Benedict’s solution and simple carbohydrates are heated, the solution changes to orange red/ brick red. The titration should be repeated with 1% glucose solution instead of the sample for calibration Principle of Benedict’s Test This solution forms a copper thiocyanate precipitate which is white and can be used in a titration. Make sure to write any evidence of any evidence of a chemical reaction with sufficient detail to help you distinguish between similar precipitation reactions.Benedict’s quantitative reagent contains potassium thiocyanate and is used to determine how much reducing sugar is present. Heat (whether it was consumed or evolved) can also be an indicator that reaction occurred, but you may not be able to tell in these videos. You will know that a reaction occur if a precipitate, a gas, or color change occurred. Keep in mind that some reactions will not occur and you should write NR (for No Reaction). Indicate if a gel is produced or crystals form, if the solid was powdery, etc. Don't write “became cloudy” or “white solid”. Before proceeding with this or any other experiment students must sign the chemical lab safety form.įor each reaction in Part A and Part B record your observations, molecular equation, total ionic equation and net ionic equation. Make sure to write any evidence of a chemical reaction with sufficient detail to help you distinguish between similar precipitation reactions. Clean up all spills immediately! If contact with skin rinse with water for 15 minutes.Do non ingest any chemicals or inhale the vapors.Safety if you were to complete this lab in person: Compounds that do not dissolve in water remain a solid and indicated by "(s)". You might have heard that water is the universal solvent, however, water only dissolves substances that are hydrophilic (from the Greek "hydros" - water and "philia" - bonding or friendship). For example, NaCl(aq) present as individual ions Na + and Cl - dissolved in water. In chemistry aqueous solution indicated by adding "(aq)" to the reactant formula. Rain, vinegar, orange juice are all examples of aqueous solutions that you come across in your everyday life.
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