A starch solution is primarily used as an indicator for the presence of iodine or as a substrate in enzyme activity tests. This mixture is technically a colloidal suspension, meaning the large starch particles are dispersed but not truly dissolved in the water. Starch polymers remain suspended rather than breaking down to the molecular level like salt or sugar. This unique structure allows the starch to form the characteristic deep blue-black complex when it interacts with the triiodide ion in the presence of iodine.
Essential Materials and Equipment
The preparation requires a few basic items. The primary component is a form of starch, such as cornstarch, potato starch, or purified soluble starch, which is often favored in a laboratory for consistent performance. Water is the solvent, and using distilled or deionized water is preferable to tap water; this avoids mineral ions that might interfere with sensitive chemical reactions, particularly in titrations.
For measuring, a precision scale is needed to weigh the starch, and a graduated cylinder or volumetric flask is used to measure the water. A simple saucepan on a stove or a beaker on a hot plate will serve as the heating apparatus. A glass stirring rod or spoon is necessary for continuous mixing, and a funnel lined with filter paper or cotton will be used to remove any undissolved clumps.
Step-by-Step Preparation of a Standard Solution
To prepare a standard 1% starch solution, you will need 1 gram of starch for every 100 milliliters of water. Begin by weighing the required amount of starch and mixing it with a small volume of cold distilled water to create a smooth paste. This initial step is important because adding the dry starch directly to hot water will cause the outer layer to gelatinize immediately, forming clumps that are difficult to break down.
Next, bring the remaining volume of distilled water to a rolling boil. Once boiling, carefully remove the hot water from the heat source and slowly pour the cold starch slurry into the hot water while stirring continuously. The mixing and high heat cause the starch granules to absorb water and swell, a process known as gelatinization, which is necessary to form the colloidal suspension.
The mixture should then be heated again and allowed to boil gently for approximately one to two minutes to ensure full gelatinization. After this brief boil, remove the mixture from the heat and allow it to cool. Once cooled, the solution should be filtered through a fine filter paper or cotton plug to remove any remaining particulate matter or undissolved clumps.
Concentration Adjustments and Proper Storage
A 1% solution is typically a strong indicator. To adjust the strength, a standard solution can simply be diluted by adding more distilled water. For instance, diluting the 1% solution with an equal volume of water will yield a 0.5% solution.
Starch solutions have a very short shelf life, often deteriorating within a day or two due to microbial growth. The complex carbohydrate chains of starch are an excellent food source for microorganisms, causing the solution to become cloudy and unusable. While preparing a fresh batch for each experiment is the best practice, storage can be extended slightly by refrigerating the solution in a sealed container for a few days.
The shelf life can be significantly extended by adding a small amount of a preservative, such as a few drops of toluene or a pinch of salicylic acid. These chemicals inhibit microbial action, allowing the solution to be stored for several weeks. If the solution ever appears cloudy, develops mold, or loses its ability to react correctly with iodine, it should be discarded and a new batch prepared.