Stop bath, pH and indicators

<p>Some time ago, when I couldn't get hold of indicating stop, I decided to make my own. I bought a litre of 80% acetic acid to dilute 1+39 for working strength solution. Into the 80% acetic acid I dissolved a pinch of bromo-cresol purple which changes from yellow to violet over the pH range 5.2 - 6.8.<br>

Recently, I got caught out when my stop bath had gone weak (and my fixer was nearing its end) and I got dichroic fog on my negs .<br>

Now, bromo-cresol purple changes to violet just before the stop turns neutral but stop bath should really be pH 5.5 or lower. So, I am about to make up a new batch but this time I am going to use bromo-cresol green which changes from yellow to blue over the range 3.6 - 5.2 so it should turn blue while the stop is still 'safe'.<br>

Am I right to assume that commercial indicating stop uses bromo-cresol green? And what might be the result of mixing bromo-cresol green with bromo-cresol purple? That is, would the mixed indicators turn green in the range 5.2 to 6.8 before turning completely blue above 6.8?<br>

TIA.</p>

<p>Kodak and others have published stop bath formula:<br>

SB-1 for Paper<br>

Water 750ml<br>

Acetic acid 80% 17ml<br>

Add water to top off to 1000ml</p>

<p>SB-5 for Film <br>

Water 500ml<br>

Acetic acid 80% 11ml<br>

Sodium sulfite anhydrous 45g<br>

Add water to top off to 1000ml</p>

<p>The most common indicator used are bromocresol purple.<br>

The indicator:<br>

Bromcresol purple, 10% solution, 9.25 g<br>

Sodium hydroxide, 1.0 g<br>

Distilled water to make 250.0 ml<br>

Add this concentrated indicator solution to a working strength fresh stop bath solution. Continue adding until the solution changes from clear to distinctly yellow. The yellow tinted stop bath will appear clear under safelight conditions. The solution will change to purple when the stop bath is exhausted. </p>

<p>I just use white vinegar, about 8 or 10 parts water, one part vinegar. It won't give you a chemical burn in the nose or elsewhere if you mishandle it either, unlike Kodak Stop Bath.</p>

<p>Alan, Steve, thanks for that.</p>

<p>I've just been given a job lot of white pickling vinegar which I'm going to dilute and use as stop, and I've just ordered some bromocresol green to run some trials.</p>

<p>Thanks again.</p>

 

<p>When Clayton Chemical manufactures both ODORLESS STOPBATH with INDICATOR, SUPER STRENGTH STOP BATH with INDICATOR and AMMONIA FUME ABSORBER, we have always used Bromo-cresol Purple. I have never experienced indicator failure. Maybe there was not enough indicator in the solution. When we do testing in the lab, the pH change point is instant and very appearant.<br>

LOWELL HUFF</p>