Citric acid pH down stopped being effective

[Secretchief]

Quick question, I have a bottle of the flairform pH down, I didn't bother reading the label and turns out it's citric acid based.
Usually I need about 20/25 drops to bring my mix down to 5.9/6, today it's just not working, I measured some of the pH down directly and it is at 6.0, I have a bluelab pH pen but it's about a 5 weeks since calibration. Should I be trusting the meter? Seems to be reading tap water correctly and pre ph mix is about right, can a pH down just rise like that? I won't be using CA again but not sure if I should dump the mix. Bit broke otherwise I would just go get calibration solution

 

[HGO]

if the ph pen is calibrated it should have a little tick next to the ph symbol on the screen
I would save up for the 4 & 7 calibrating solutions but until then maybe remix a new batch and check the ph again
what size is the res to need 25 drops?

 

[Secretchief]

Yea the tick went about a week ago, still I know people use them for months so I am surprised.
It's a tiny res, just mixing individual feeds so 4L total.

 

[R3za92]

Citric has poor buffering qualities compared to phosphoric and nitric acids.
Citric allows the res to drift far quicker than nitric or phosphoric. Sulfur if also works but is harder to find in aus

Personally I use phosphoric as it’s a stronger acid but safer to store than nitric.

A guide to different pH down options in hydroponics – Science in Hydroponics

scienceinhydroponics.com

 

[Indy]

+1 with the phosphoric acid.

I found the ph rarely fluctuated in the rez over time. Sometimes it'd be 2-3 weeks in between rez changes with DWC, and would get topped up with ph balanced water from a plastic rubbish bin (with aerator) i'd always have prepared nearby.

 

[Secretchief]

So the update is my PH pen was faulty, it's a bluelab and only 4 months old, guys at the shop tested it and swapped on the spot.
Apparently they had a few faulty units from bluelab lately as they were overloaded during covid. Worth keeping in mind if you have bought anything from them in the last 12 months.

I picked up some phosphoric acid and will be using that now. I have always had issues with pH going right up (6.5/7 from 6 within hours, depending on mix) but it's Coco and shit is healthy so I wasn't worried (I also get mag deficiency if I water below 6 for to long, so it's probably not that high for the plant), hopefully this will be different.