Fehling’s Test

Dr. Brown nods her head in agreement. She explains that the emulsion and Sudan III tests are used to test for the presence of fats, while the iodine test detects starch.

[Emulsion test: In this test, ethanol is added to the sample in question (e.g. olive oli). Cold water is then added, the solution is mixed vigorously and observed for any changes. The formation of small droplets or globules of fat in suspension indicates the presence of fats. These droplets give a milky appearance. The formation of droplets and globules is due to hydrophobic interactions with water. For more information click on the Emulsion test option.

Sudan III test: The experiment uses Sudan III, which is a red dye and olive oil as the fatty substance. Oil is added to water in a test tube. Sudan III is then added and the mixture is shaken. A positive test is indicated by the red-stained oil layer separating on the surface of the water, which remains uncolored. Fat globules are stained red and are less dense than water. For more information click on the Sudan III test option. 

Iodine test: A fresh solution of 1% starch is mixed with the iodine solution in a well plate or a white tile. A blue-black color indicates the presence of starch. |For more information click on the Iodine test option.] 

Dr. Brown asks you to explain how the Fehling test is carried out. 

You: Well, in the first step, you need to prepare Fehling’s solution by mixing equal volumes of Fehling’s 1 and 2.

Dr. Brown: That’s right. Do you recall what the two reagents are?

Monica: Solution 1 is made of an aqueous, blue solution of copper(II) sulfate (CuSO₄), while solution 2 is an aqueous solution of potassium sodium tartrate (Rochelle salt) and sodium hydroxide (NaOH).

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Dr. Brown: Well-done! What do you do next?

You: You add 2 mL of the urine sample to the test tube containing 2mL Fehling's reagent and heat the mixture in a boiling water bath for about 1 minute.

Monica: All you need to do then is observe any color change and the formation of a precipitate.

Dr. Brown: Excellent. Remember to always include a negative control; you can use sucrose in this case. Please let me see the results once you are done.

You also decide to watch a video before you carry out the test.