From: Sandy King (sanking@clemson.edu)
Date: 01/20/03-03:45:19 PM Z
Judy Seigel wrote:
>On Mon, 20 Jan 2003, Neil Miller wrote:
>
>> 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?
>
>
>Neil, I've never seen a precipitate when mixing the AB& C for VDB... it's
>not normal to the process. The one time we saw anything at all suspicious
>it turned out that the "distilled" water was a fake... just tap water
>according to the chem lab, & it got cloudy right off when we mixed in the
>silver nitrate.
I also have never had a precipitate form when mixing the ABC vandyke
solutions (that is ferric ammonium citrate, tartaric acid and silver
nitrate), and I mix the three solutions together quickly and
virtually all at once, not like the drop by drop method you describe.
On the other hand I have had precipitates form when experimenting
with other acids (oxalic, for exampel) and with other forms of
citrates (sodium ferric citrate), and of it has always taken place
when mixintg in the silver nitrate solution. Therefore, like Judy, I
suspect something is amiss with one or more of your chemicals.
Sandy King
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