From: Joe Smigiel (jsmigiel@kvcc.edu)
Date: 01/20/03-08:48:30 PM Z
Hello Neil,
I just mixed a batch of VDB today and noticed a similar event. The solutions were mixed separately with distilled water and then combined a bit at a time with rapid stirring. A+B no problem. As C (silver nitrate) was slowly added, the combined solution formed a greenish cloud which dissolved rapidly in solution. The combined solution kept clearing until almost all of part C was added. With about 25ml to go (solution total of 500ml) the cloud formed again and remained. I did a test print and it apparently did not matter if the solution was a bit cloudy. The solution printed well.
I wouldn't refer to the mixture as having a precipitate (which I think of as something crystalline falling out of solution and settling at the bottom) but rather it was more like milk, i.e., an emulsion with something very fine suspended in solution, almost looking like air clouding the solution. (Then again, I'm not a chemist so my terminology might be askew.)
I suspect the stuff clouding the emulsion is due to excess silver nitrate-maybe it is very fine silver dispersed in the solution and bouyed up by all that citrate. I've read somewhere that old VDB solutions that have some silver precipitating or plating out in the bottle can be refreshed by adding a bit more tartaric acid to the bottle in order to redissolve the silver. Maybe that would clear your final mix.
In any event, I'll check the look of my solution tomorrow and if I observe any change, report back.
Joe
>>> neil@miller.gioserve.com 01/20/03 05:02 AM >>>
...
One thing that continues to plague me is mixing the solutions without forming a precipitate. I had three attempts over the weekend. All three had this in common - each part (a, b and c) mixed with distilled water. 'A' added to 'B' and mixed thoroughly. 'C' added to this mixture drop-by-drop. The first attempt was at room temp, not very warm at this time of year. As in the other two attempts, I dropped the 'C' solution through a syringe drop by painful drop. A bright green milky solution formed, the precipitate gradually settling a little. 2nd attempt - all solutions heated to 30 degrees centigrade - still a precipitate formed, but much less this time. 3rd attempt - all solutions at 55 degrees, mixing vessel kept in water bath to maintain temp. No precipitate at all, but next morning a very slight precipitate had settled at the bottom of the brown glass bottle. This is the solution I have used to coat the next batch of test paper. Do you think that the other solutions will be OK if I filter them?
Many thanks once again,
Neil.
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