The main reason is it's easy. I've been premixing stop bath for a few years, and it's just easier than mixing from concentrate each time, not to mention that the smell of the concentrate is kind of strong.
Originally Posted by ann
As a further test I mixed up a liter in a separate container, and while it's still slightly yellowish, it's definitely less yellow than it was when I mixed it up. So it appears that the problem is with my local water supply rather than my storage bottles. Wow, I knew that my local water was slightly alkaline, I just didn't know how bad it was! I wonder if there are other chemicals in the water that are causing this? Of course the quick fix for this is to mix up the stop bath using distilled water (which is exactly what I'm going to do now) but I'm kind of curious what's in my local water!
I'm curious as well to know what's in your water although I can't imagine it'd be too alkaline or it'd feel slimy and you'd end up with lots of people in hospital. Most acid/alkaline reactions are fairly quick (throw some bicarb into vinegar/acetic acid and you'll know what I mean) so there might be some kind of weird endothermic reaction going on which is breaking down the acid (or indicator). You could stick some in the fridge and see if that slows down the colour change. We really need a chemist as although I walk past chemistry labs nearly every day, apart from photography, I haven't done any real chemistry for 15 years.
At this point, a call to Kodak might be helpful. Have not worked with their customer service in years, but at one time they were pretty helpfull. Also, there are places that will test water (and local water supplies are tested - that's how they get to put those little signs that say something like Superior Water Supply, etc - they should be able to give you a break down of what IS in the water). Keep us posted, as this is very interesting and NOT the norm.
thanks for the reply,
Don;t worry that much about this, the indicator is becoming clear, which might as well be normal. The indicator is bromocresol purple, which changes color with pH as follow:
5.2 -> yellow
6.8 -> purple
If your water was alkaline it should have turned PURPLE, not clear.
I don;t believe the acid is gone bad, but maybe the indicator is being affected by something, so I'd try to see if the acid is bad or if the indicator is gone:
Put a teasppon of baking soda in a glass and add small qttys of your stop bath.
If the stop is good, it'll fizz and if the indicator is good it'll become dark
Depending on how you are diluting it you'll need some more or less to stop the fizzing.
But at 1+63 (kodak reccoemnded) there should be 1.35% acetic acid, so it should take about 250 ml of your solution to make 1 tsp of baking soda stop fizzzing
(5g of soda are neutralized by 3.57g of acetic acid)
If you diluted less then it should take proportionally less.
Originally Posted by MikeS
Mama took my APX away.....
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Perhaps a Kobayashi Maru is called for.
My suggestion would be to fill your stop tray with tap water each session, then add 1 teaspoon of citric acid for each liter of solution. Little or no smell. Lemonade has more of a smell. Very effective. If you do TONS of printing in one session, rub your fingers together in the solution and check to see if there's still a bite to it. If it starts to feel a bit slimy, then it's time to add a bit more citric acid.
As I got it, citric acid comes as a powder. I got it from my local organic grocer's bulk spice section for a few cents an ounce. I got two or three dollars worth, and it was more than I needed to last me until I switched to a completely alkaline tray line, at which point I didn't use a stop anymore.
There's lots of discussion of citric acid in the archives. This thread is probably as good a place to start as any.