On several of these forums I have seen people refer to Bromophenol Blue and Bromothymol Blue interchangeably as though one is the same as the other. I'm as about as far from being a chemist as you can get but my little bit of searching shows Bromophenol Blue has a useful range as an indicator lying between pH 3.0 and pH 4.6. It changes from yellow at pH 3.0 to purple at pH 4.6. Bromothymol Blue has a range of pH 6.0 to pH 7.6 with it being yellow below pH 6.0 and blue above ph 7.6. While the structural formula for each is generally similar Bromothymol Blue has some additional variations. I have been supplied with Bromothymol Blue (in dry form) instead of my requested dry Bromophenol Blue, so before I make a fool of myself by asking my supplier to correct his mistake(?) could somebody please help with answers to these questions :
1) Can these really be considered the same indicator?
2) If the soap test calls for about pH 4.5, as I think I can recall, is it possible to use Bromothymol Blue which appears so far away from that figure? If so, how would I approach this?
3) Could Bromothymol Blue be a useful indicator for determining acceptable acidity of the finished bd product?
4) Any other points I might have missed and a knowledgable person could help by covering?

As I need to action any return fairly soon a quick reply would really be appreciated.

Thanks in anticipation.

You need bromphenol blue because of the range of the indicator. Here is a great web site that should help you out.indicators As you are adding acid, you are freeing the Na+ or K+ from the FAA. When all of the FAA is free from the Na+ or K+, the pH will be around 4.5 and this is why you should use bromophenal blue instead of bromothymol blue. If you used the bromothymol blue, you would not use as much acid because your color change would be at a higher pH which would give you a lower soap value. Since bromothymol blue's indicator range is between ph 6-7.6 and a pH of 7 is the goal of finished BD, then it would be a suitable indicator. Here is a link to the catalyst and soap test written by Dr. Gerpen. It explains the soap test in more detail. catalyst and soap test

Thank you Dieseldan for your response to my bromo--- blue questions and the useful links provided. My apologies for the delay. This has been my first opportunity to acknowledge. You have confirmed what I suspected and I've now arranged bromophenol blue for the soap test. Although bromothymol blue will indicate a narrow range around the ideal pH 7 for finished BD, would a swimming pool test kit(as a cheap alternative to a pH meter) be more effective? This would permit the reading of a wider range of pH which might exist in the wash water.

I've used universal indicator for this process and it was a pain in the butt= there was a yellow and an orange color which indicated close to the pH we wanted, and they were hard to distinguish.

Girl Mark thank you for your response.
I am reluctant to ask the questions that have invariably been answered (in some form) somewhere on this forum countless times in the past. However it is often extremely difficult to precisely identify what is current best practice - even after hours of searching.
I accept some on this forum believe that testing pH of final wash is pointless. Are you saying that you do test the pH of finished BD (wash water) and --
1) universal indicator is simply difficult to use effectively, or 2) that a narrow range indicator such as Bromothymol blue is preferable to the universal indicator, or 3)you have tried testing with a universal indicator in the past, it was a pain in the butt and the whole exercise is indeed a waste of time.
I think it was Neutral who said it was sufficient to simply leave the final wash slightly cloudy to ensure the end result was slightly alkaline rather than acid. If that is of any importance at all(and I accept that it is with regard potential corrosion problems) isn't it better to have some definitive measure of what the pH of the water is rather than just guess?
Thanks for your help.