Error in lab 22.1

[thompson]

On p. 386, the Substitutions and Modifications section is missing the following paragraph: "To prepare the Kastle-Meyer phenolphthalein solution, dissolve 0.1 g of phenolphthalein powder in 10.0 mL of 25% w/v aqueous sodium hydroxide in a test tube and add 0.1 g of mossy zinc and a boiling chip. Boil the solution very gently, adding water as necessary to maintain the volume, until the bright pink solution turns colorless or slightly yellowish. Allow it to cool, decant off the liquid, and dilute it to 100 mL with 70% ethanol. Store in a tightly-capped brown bottle."

[Mike]

In the preperation of the Kastle-Meyer phenolphthalein solution I found that after boiling the solution for 1 hour the bright pink color was hardly changing in color at all, I decided to add another 0.4 grams of mossy zinc to the solution and continued to add water to maintain the 10.0 ml fluid level, boiling for a additional hour the pink color did not seem so bright, the color transition seems very slow, I continued to boil the solution for a total of three hours, at this time the color has a very faint pink tint when the solution is hot, as the solution cools to room temperature the pink color is gone maybe a very slight yellow in color. I decided not to boil any further and decant off the liquid and dilute with 70% ethanol. Testing the solution on a sample as decribed on p. 386 does indeed make a color change to a bright pink color indicating a positive test for blood.

Does the preperation of the Kastle-Meyer phenolphthalein solution typically require a long boiling time?
I used 10.0 ml of distilled water with 2.5 grams of sodium hydroxide for the 25% w/v and 0.1 g of phenolphthalein.

[thompson]

I think I did that lab in late 2007, so it's been a couple of years. IIRC, I made up 100 mL rather than 10 mL (so I ended up with 1 L after dilution) and I refluxed the mixture on a hot plate, while I was doing other things. It may well have refluxed for a couple hours or more. I'm sorry, I don't remember. But, as you found, it indeed works.