May be this can help …
Prices are not very hobbists-aware … but interesting for many applications.
(This is not scam, spam or advertisement … just an info ) :naughty:
Interesting content but ouch on the prices! 
One of these days pH sensors will become reasonably priced – then I’ll make me a pool monitor.
You can buy a pH sensor probe for about $10 on eBay. The challenge then is the amplifier to read the signal accurately - Love Electronics had one but are gone, so you’d have to DIY at this stage.
I looked at making a gadgeteer module that you could plug the Vernier analog sensors into. But finding the BT through hole sockets is impossible 
I’ve been trying to use the one from Love Electronics but with no luck. Module code has a bunch of bugs and also needs the AD convertor accurate recalibration. Not acceptable as the complete solution.