Measuring pH with photons

I was busy browsing when I came across a project called An Open Source Seawater Carbonate Analyzer for the Citizen Scientist. They were discussing building a pH analyzer for seawater using a spectrometer, and linked to a paper called “Measuring Protons with Photons: A Hand-Held, Spectrophotometric pH Analyzer for Ocean Acidification Research, Community Science and Education”. In the paper they showed this image:

![pHyter, a spectrophotometric pH analyzer](upload://9hGiY8OnUzCfD9YzYSFxiTQktXT.jpeg)

This made me wonder if we could use this method to measure pH in the Pioreactor? That also reminded me that IO Rodeo has an open-source colorimeter that they've [used to measure pH](

@“Gerrit”#p123 Wow, so this doesn’t require any reagents? Just two LED’s a photodiode and a thermistor & from that it tells you pH, temperature & salinity? Sounds amazing - the only thing I can get my head round is how it measures temperature… I can’t see how it could possibly get a take on either of the others just by looking at colour, particularly if the solution was already coloured (as with our beta carotene) or worse still if the colour changed with time (as with any tinted microbial growth). Sceptical as I am about it working for seawater, I’d fall off my chair if it worked with our variables. That said it would be so easy to implement with a Pioreactor that we can’t ignore it.

I haven't heard the paper for tonight's Journal Club yet - should we make it this? Oddly the journal club has disappeared from my google calendar, but it's still in Teams.

Also, I see you posted this 5 days ago - sorry I missed it (I'm sure i've been in the forum since then) do you know how you can set up a notification for any new posts other than having to first discover then subscribe to each? Discourse happily gives a weekly overview of all activity - which is how I get most of my Pioreactor news.

Hi, I just has a quick look at the paper and they are using meta-Cresol Purple (mCP) as an indicator. This will react and change colour depending on the pH. They take a background of the initial sample and then proceed to add the indicator and measure the absorbance to figure out the pH.

@“SandrineH”#p125 of course - hence the bottle of mCPurple in the photo…

In that case, any suggestions for a paper we could quickly read for tonight's Journal club anyone ( @"Gerrit"#4, @"SandrineH"#13, @"CamDavidsonPilon"#5, @"danwchan"#6, @"NarcisP"#9, @"Hoff"#11, @"RobJ"#8 )? If we don't have a paper we can read in time, but we'd still like to meet, we could discuss AI tools that may help with research, like [Semantic Scholar]( and [Research Rabbit](

Aaah, I missed the indicator, that makes more sense.

Unfortunately I won't be able to do Journal Club tonight. Let's move it to next week, then we also have some time to decide on and read a paper.

@“Gerrit”#p127 OK, I’ve adjusted the Teams invite.

Also @SandrineH & anyone else who may fancy joining us: I realise 9pm UTC may not be the best time for Europe - so if that's a problem please vote on when we should meet in this rather cool open source alternative to doodle poll: - I'll let someone else start a new thread on what we discuss on the 29th.

Back on topic: mCP has an infinite shelf life, so this could still be a valid way of checking pH in a Pioreactor at the start (if you have two vials) and end of an experiment..? Probably cheaper & possibly more accurate than the [DF Robot]( I bought.

Hi, I was wondering around what pH in the Pioreactor is or expect it to be? This indicator, mCP, has a pKa of around 8. So the indicator is best used in the pH range around this value since that is where the colour change will happen. It might be a cheap option for ocean water but maybe there is a different indicator that can be used for the pH range that the Pioreactor works at?

Quite right, and I don’t know as I haven’t set up my pH probe yet. Universal Indicator Solution could make for some interesting school experiments, but we’d need an indicator well suited to our actual range of interest if we wanted accurate results. Perhaps long-term, if we find that the culture passing a certain pH limit indicated a potentially hazardous situation, colorimetric pH could be useful.


@“Martin”#p124 do you know how you can set up a notification for any new posts other than having to first discover then subscribe to each? Discourse happily gives a weekly overview of all activity - which is how I get most of my Pioreactor news.

Not related to the discussion, but I wanted to respond to this question earlier. While a weekly digest/overview for Flarum is [still in development](, I enabled the "FoF Follow Tags" extension. This extension allows you to follow tags, so you could click on the Discussions tag and then click the star to be notified of all discussions. Hope this works for now!