What indicator is used to detect compound?

Answer 1

The most common organic compounds found in living organisms are lipids, carbohydrates, proteins, and nucleic acids. Common foods, which often consist of plant materials or substances derived from animals, are also combinations of these organic compounds. Substance called indicators can be used to test for the presence of organic compounds. An indicator is a substance that changes color in the presence of a particular compound. In this investigation, you will use several indicators to test for the presence of lipids, carbohydrates, and proteins in various foods.

Problem: What are the major types of organic compounds in some common foods?

Pre-lab Discussion:

1. What is an indicator? How are indicators used in this experiment?
2. What is the purpose of using distilled water as one of your test substances?
3. What is the controlled variable for each group of testing?
4. What is the purpose of washing the equipment thoroughly?
5. What is the IV? DV?
6. You have added Sudan IV stain to each substance. What change indicates the presence of lipids?

Substances being tested:

1. honey solution
2. egg white solution
3. corn oil
4. lettuce solution
5. Gelatin solution
6. yolk solution
7. potato solution
8. apple juice
9. distilled water

Part A: Testing for Carbohydrates: Monosaccharides

Glucose, commonly called "blood sugar," and fructose, "fruit sugar," are two of the most common monosaccharides. Ribose, an important component of nucleic acids, is also a monosaccharide and will be discussed during our study of DNA and RNA. Monosaccharides can be identified in a substance using an indicator called Benedict's solution. When Benedict's is heated in the presence of a monosaccharide, the color changes from blue to green to yellow to reddish-orange, depending on the amount of monosaccharide present.

Procedure: You can start step 1. and then proceed to another test while the water heats up.

1. Turn on the hot plate to high and fill a beaker half full of tap water. Place the beaker on the hot plate. While this is heating, add 5mL of each sample to a test tube. Add 10 drops of Benedict's solution to each test tube.
2. Place the test tubes in the hot-water bath. Heat the test tubes for 3 to 5 minutes, or until the indicator changes color, if it changes color.
3. With a test tube holder, remove the tubes from the hot-water bath and place them back in the test tube rack.
4. Record any changes in your data table.

Part B: Testing for Carbohydrates: Polysaccharides

Monosaccharides may join together to form long chains called polysaccharides that may be either straight or branched. Starch is an example of a polysaccharide formed entirely of glucose monosaccharides. Polysaccharides can be tested for using the indicator Lugol's reagent (iodine/potassium iodine). Lugol's will change color from yellow-brown to blue-black-purple in the presence of starch.

Procedure:

1. Add one dropper full of each solution to a well in the well tray.
2. Add 2-3 drops of Lugol's to each sample.
3. Record any changes in your data table.

Part C: Testing for Lipids

Lipids can be generally categorized into two subgroups: saturated fats and unsaturated fats. In saturated fats, the carbon chains are bonded together with only single covalent bonds and the remaining electrons are bonded to hydrogen. This structure keeps the hydrocarbon tails relatively straight, so they readily align with each other, pack closely together, and thus tend to be solid at room temperature. Saturated fats are most common in animals. Unsaturated fats has double bonds between the carbons and therefore do not have the maximum number of links to hydrogen. These regions of double bonding allow kinks to form in the hydrocarbon tails, keeping them from packing closely together. For this reason, unsaturated fats are usually liquid at room temperature. Unsaturated fats are common in plants. Because fats are nonpolar, we will test for their presence using Sudan IV, a nonpolar dye that will readily mix with lipids but form small bubbles or separate with non-fats. Lipids can also be tested for using the "sack lunch" method: the lipid is rubbed into a brown bag. If a grease spot appears (the brown bag appears translucent), then a lipid is present in the food.

Procedure: Sudan IV

1. Add one dropper full of each solution to a clean well in the well tray.
2. Add 3-5 drops of Sudan IV to each sample.
3. Look for small bubbles or separation.
4. Record results in data table.

Procedure: Sack lunch

1. Add one dropper full of each solution to a small piece of brown bag.
2. Rub the solution until a "wet" spot appears on the paper. Rub off any excess with a paper towel.
3. Set the paper aside until the spot appears to dry, about 10-15 minutes.
4. Hold the brown paper up to a bright light.
5. Record observations in data table.

Part D: Testing for Proteins

Proteins are made up of one or more polypeptides, which are linear polymers of monomers called amino acids. Amino acids derive their name from the amino group and the carboxyl group (which is acidic). Polypeptides are formed when amino acids are joined together by peptide bonds between the amino group of one amino acid and the carboxyl group of another amino acid. Proteins can be tested for using Biuret's reagent, which reacts with the peptide bond between amino acids in the polypeptide. Biuret's reagent turns from blue to purple-violet if peptides bonds are present.

Procedure:

1. Add one dropper full of each solution to a clean well in the well tray.
2. Add 3-5 drops of Biuret's reagent to each sample.
3. Record any changes in color.

Answer 2

Spectroscopic techniques such as infrared spectroscopy, mass spectrometry, and nuclear magnetic resonance (NMR) spectroscopy are commonly used to detect compounds based on their unique fingerprint spectra. Additionally, other analytical techniques like chromatography and electrophoresis can also be employed to separate and identify compounds based on their physical and chemical properties.