Kastle-Meyer Reagent

[cafcom...@gmail.com]

I am currently making Kastle-Meyer reagent using 10ml 25% NaOH, 0.1g phenolphthalein, and 0.1 mossy zinc.  The solution has been boiling for about an hour and there is still no color change.  How long should I expect it to take and is there any downside to adding excess zinc?

[The Home Scientist]

I generally make up K-M reagent a liter at a time in a 2-liter flask on a hotplate. I use potassium hydroxide, but sodium hydroxide should also work fine. I bring the solution to a full boil and then turn it down to simmer. I'm usually doing something else in the lab while it refluxes, but I don't think it's ever taken more than maybe 15 or 20 minutes to turn colorless or a very pale straw yellow.

As long as you have sufficient zinc, the amount is not critical. I put somewhere around half a kilo to a kilo in the flask and run up a liter batch of KM. I then leave the zinc in the flask covered with water until the next time I need to make up a batch.

The chemistry involved is, first reacting the phenolphthalein with sufficient potassium hydroxide to solubilize it. (Phenolphthalein itself is very insoluble in water.) That bright magenta solution then reacts with the zinc to form phenolphthalin (the colorless reduced form of phenolphthalein.)

[labohio]

I just came in to post the same quandry as Mr. Hayes.  I have tried this same formula, as well as variations,  five times, and have boiled it in a water bath for up to four hours.  The tube never needs additional water, as it does not seem to really boil.  It seems to be darker when heated, and lightens up again when cool.  The only other place I found instructions was at about.com, and they were identical.  I will try potassium hydroxide and see if that makes a difference.  I will also try a boiling flask in a heating mantle.  Will post if I get results.

[The Home Scientist]

I'm not sure what the problem is. I've made up KM reagent many times, and it's always worked as expected. I make it up in 2-liter batches according to the following (from my lab notebook), but you should be able to reduce quantities to make up as much as you need:

"Transfer 1 L of DI water to a 2 L Erlenmeyer flask. Weigh out 400 g of potassium hydroxide and add it to the flask in small portions, swirling until the solid dissolves. (Caution: this process is very exothermic, and the resulting solution is extremely corrosive.) Weigh out 40 g of phenolphthalein powder, add it to the flask, and swirl until the powder dissolves. The solution turns bright pink. Weigh out 400 g of zinc powder and add it to the flask. Stopper the flask loosely to minimize evaporation. Place the flask on a hotplate, and heat the solution until it approaches a boil. Reduce the heat and allow the solution to simmer until the bright pink solution turns colorless (or a very pale straw yellow), which typically takes 10 or 15 minutes. Add distilled water as necessary during this process to replace water lost to evaporation and keep the volume of the solution near the original level. After the solution turns colorless, remove the flask from the heat and allow it to cool to room temperature. Make up the solution to 2 L using 70% ethanol. Decant the solution into a brown glass storage bottle. Add a few zinc granules to the storage bottle to prevent oxidation."

Incidentally, don't use any vessel you care about. Boiling 40% KOH etches glass badly. Also, 400 g of zinc is a large excess. You can salvage that extra zinc by washing and soaking it thoroughly with tap water until all pink color disappears and then washing with two or three changes of DI water. Note that wet zinc powder can catch fire spontaneously.

[labohio]

Okay, here's what I did, and finally got results:

Wednesday morning, I put on to boil the following-
   0.1g phenolphthlaein
   0.24g mossy zinc (two lumps) and a boiling chip
  30.0mL 25% potassiun hydroxide flakes
   This  was brought to boil in a 125mL Florence flask on a mantle at 80 on the rheostat.
   I let this go for just over an hour, with no noticeable change.  I took it off until the next day.

Thursday morning, I put it back on to "cook".  I checked it occasionally, but no change.  I deceded to boil it almost dry, and by the time I checked back, it was down to 5-10mL, and was a light yellow.  The zinc had become tiny black flecks on the bottom.
I let it cool and brought it back to 30mL, filtered it and drew off 10mL which I made up to 100mL with 70% ethanol.  As soon at the alcohol hit, a cloudy white, almost suspension, formed.  Several tests have given the desired results.  Alas, the photo I took is too large to post.

It seems the secret was, in this case, to let it boil down before adding additional water.

On Monday, July 21, 2014 11:09:36 AM UTC-4, labohio wrote:

[The Home Scientist]

The only differences other than scale between what you did and what I did/do is that I use 40% KOH, somewhat different ratios, and zinc dust rather than mossy zinc.

[j yeardly]

here is my recipe  I reflux it when boiling It takes about 1 hour normally

The reagent is prepared from:

Phenolphthalein     2g

Potassium hydroxide        20g

Zinc dust                         30g

Water                            100 ml

 

 

The mixture should be boiled until colourless, allowed to cool.

Add 100 ml ethanol and store at 4^OC over zinc dust.

The reagent is stable for several months.

[labohio]

Yes, Mr. Thompson, last night when I read your post, I realized the difference in KOH strength and use of zinc powder.   I will try again with the 40% KOH, and use more mossy zinc, as I find I do not have any zinc powder on hand.  I may try Mr. Yeardly's formula as well.  Thanks for the clarification.   

[j yeardly]

just noticed that the storage temp is wrong  should be 4 Celsius or 36 Farenhight or below  but not frozen

[labohio]

Mr. Hayes,

I FINALLY got some good results!  Being short on KOH, I used NaOH at the 40% Mr. Thompson mentioned.  To 30mL, I started with 1.5g phenolphthalein and 15.0g mossy zinc.  I had to stop after ~1 hr, and continued the next day.  At that time, I threw in another 5.0g mossy zinc.  I let it boil down more than half, and I finally got a clear solution.  I let it cool, brought it back to 30mL, and filtered it.  I drew off 20mL in a 200mL vol. flask, and brought it to level.  I let is partially settle, and filtered again.  Total boil time was 2 hr 50 min.  However the test results were as desired (See attached. photo).  I feel that the difference might be the use of powdered zinc vs. mossy.  Not a scientific conclusion, just a gut feeling.

On Thursday, June 5, 2014 3:19:30 PM UTC-4, Matthew Hayes wrote:

[labohio]

08 Aug 2014

I realized in re-reading my post on the success of Kastle-Meyer, I was unclear on the final volume contents.  I said I drew 20mL into a 200mL vol. flask and brought it to level.  That would be up to level with 70% ethanol.  Sorry!