Mixing Pyrocat-HD, white precipitate ?

[Cor]

I have been mixing Pyrocat-HD in glycol for quite some years now, and have come across a problem I haven't had before.

I faithfully follow Sandy King his instructions (http://www.pyrocat-hd.com/html/mixing.html).

The last 2 batches (small volume, only 50 ml, I do not use it that often any more) I had this white precipitate forming when I get to step 5:

"Add the bromide and sodium metabisulfite to about 50-75ml of hot distilled water at about 120F. Stir until completely dissolved, and then add this solution to the propylene glycol solution."

Before adding the bisulfite and bromide my solution is completely clear, but after adding a fine white precipitate is formed, it will not get into the solution, and settles to the bottom after 6 hours or so. I made a second batch, thinking that I had made a mistake, but the same phenomena occurred.

Have you also seen this problem ?

Would it be OK to mix the stock carefully (get the precipitate in solution as even as possible) and use it (I know less than optimal)

Best,

Cor

[BarryS]

I just mixed up a batch, but didn't have any precipitation issues. I use a magnetic stir plate, but with that small of a volume, you may be having trouble physically mixing the solution. How about doubling the volume of glycol and using 2X the amount to make your working solution? Warm up the solution and add an equal volume of warm glycol. You could also dilute in hot water since the KBr and meta bisulfite are less soluble in glycol. I'd avoid mixing any stock from a solution with a precipitate--sounds like a recipe for inconsistent results.

[Cor]

Hi Barry,

Thanks for the feedback, I agree that mixing back the precipitate is asking for inconsistencies. Mixing a small volume should be no different than mixing a big volume, provided that you measure your dry chemicals carefully, which I did. I do use a hotplate and a magnetic stirrer ( I actually use 'au bain Marie" method: a larger beaker with water with the mixing flask placed in it, I measure the temp. of the water.

Doubling the amount of Glycol sound like a good idea! I guess I have to add some extra water too ?

Thanks,

Cor

[Jim Noel]

I have a suspicion your water may be the culprit. Years ago our municipal water company changed their formula for purification which caused similar problems. My solution was to go to RO water for mixing all solutions.

[Cor]

I have a suspicion your water may be the culprit. Years ago our municipal water company changed their formula for purification which caused similar problems. My solution was to go to RO water for mixing all solutions.
Jim,

Good point, but I use very pure water (I have access to Millipore MQ water, used for laboratory purposes). I doubled the glycol, that didn't help, neither did adding some more water. I think that I'll try another batch of sodium metabisulfite, my main "suspect".


best,

Cor