What is the purpose of adding
zinc chloride in the preparation of starch iodide paste?
Please refer to the procedure below: -
Starch Iodide Paste- Heat 100mL of water in a 250mL beaker to boiling, add a solution of 0.75g of potassium iodide in 5mL of water, then add 2g of zinc chloride dissolved in 10mL of water, and, while the solution is boiling, add, with stirring, a smooth suspension of 5g of soluble starch in 30mL of cold water. Continue to boil for 2 mins, then cool.
ZnCl2 serves as a preservative. See
http://books.google.com/books?id=NFGSSSbaWjwC&pg=PA500&lpg=PA500&dq=zinc+chloride+starch+iodide+paste&source=bl&ots=V64YQciurs&sig=pwvOtYdmsSjIvK2ezPwOcsHaVWU&hl=en&ei=zbV2TdT_IqeC0QHIyd3nBg&sa=X&oi=book_result&ct=result&resnum=6&ved=0CDkQ6AEwBQ#v=onepage&q=zinc%20chloride%20starch%20iodide%20paste&f=false
The purpose of adding zinc chloride in the preparation of starch iodide paste is to improve the stability of the paste.
Here's how it works:
1. Start by heating 100 mL of water in a 250 mL beaker until it reaches boiling point. This provides a suitable environment for the subsequent reactions to occur.
2. Next, add a solution of 0.75 g of potassium iodide in 5 mL of water. This introduces the iodide ions to the mixture. Potassium iodide is commonly used as a source of iodide ions in chemical reactions.
3. Then, add 2 g of zinc chloride dissolved in 10 mL of water to the boiling solution. Zinc chloride acts as a catalyst, which helps in the formation of the starch-iodine complex.
4. While the solution is still boiling, add a smooth suspension of 5 g of soluble starch in 30 mL of cold water. Stir the mixture continuously to ensure even distribution of the starch particles.
5. Continue boiling the solution for 2 minutes. Boiling ensures that all the ingredients are thoroughly mixed and that the starch granules are completely dissolved.
6. Finally, allow the solution to cool. During cooling, the solution undergoes a gelatinization process, where the starch molecules join and form a gel-like consistency.
The addition of zinc chloride aids in the formation of a stable starch-iodine complex. The zinc ions act as a cross-linking agent between the starch molecules and iodine, resulting in a more durable and longer-lasting paste. This improved stability is important in applications such as medical procedures or chemical testing where the iodine concentration needs to be maintained over time.