Very very low PH and Redox Potential

[MasterBlaster]

Hello ,


I got a GHL profilux and set it up yesterday. All good until I noticed the measurements the PH and Redox are way way low ( PH ~ 6.0 - 6.2 and Redox between 65 and 80). I immediately assumed I did not calibrate them correctly and carefully calibrated both sensors again. I even tested the PH sensor in the RODI water where I washed the sensors of the calibration fluid . The PH probe recorded 7.2 which is about correct considering the temperature of the water being 22 . But the PH number shown for the saltwater in the tank is 6.2-6.0 so I am assuming this is correct. I don't understand why I have such a low PH and redox and what I am doing wrong .

I don't have mechanical filtration and I just started using carbon. The tank is 4 months old and I don't have problems with most corals ( i did lose a SPS frag). I am using a Nyos 120 skimmer . My nitrates are usually around 1, PO4 is very low (don't have a way to say for certain but for sure under 0.1 , probably under 0.03). My tank is about 240L , I used a lot of sand and rock(live rock as well as dry rock) about 30kg rock and 10 kg sand. I used Aquaforest cement to glue some of the rocks and that cement started to break down. I also use Sangokai basic. My last water change was about 1 week ago with 100L of water made via Tropic Marin Pro. I did feed allot but never noticed the nitrate rising more than 3 ppm.

I tested using the PH test from Tropic Marin and I get the same color I usually got in the past . It's hard to match it to any color on the chart but In the past I assumed it is closest to 7.7-8.0.



Thanks ,

Mihai

[tropicreef]

I immediately assumed I did not calibrate them correctly and carefully calibrated both sensors again.

With which standard buffer solutions you calibrate the pH sensor? The general process for calibration is to use at least two known data points to adjust the pH values associated with the mV output. It is best to choose solution standards that represent the range of measurement expected during sensor use. For example, one solution standard with pH value slightly lower than the minimum expected measurement and another with pH value slightly higher than the maximum expected measurement. This will allow the user to calibrate as accurately as possible. It is common to use pH 7.00 as one buffer solution and then pH 4.01 or pH 10.00 as the other. For seawater you take the higher one (pH 9 or 10). Temperature must also be considered. It is important to calibrate the sensor in solution close to 25°C.You need to know the temperature of the buffer solutions. It is best to place the buffer solution in the aquarium a while before, so that the buffer solution takes the temperature of the aquarium.
Once buffers are used for calibration, they are assumed contaminated and should not be used again. Reusing buffers can lead to slow pH electrode response or the inability to calibrate. The cause of calibration failure is difficult to determine if the pH buffers have already been used. Used buffer solutions can be kept for rinsing the calibration container and the electrode between calibration points.
Expired buffer solutions should not be used and buffer bottles should not be left open. Carbon dioxide in the air can change the pH of basic buffer solutions, so basic buffer bottles should only briefly be opened. Use opened containers of buffer as soon as possible.


How old is the pH sensor?
How long need the ghl profilux computer to set the ADC parameters?

[MasterBlaster]

Thanks for the input tropicreef

The calibration :

I used the standard 7 PH and 9 PH that came with Profilux marine version . The Profilux is new with 2019 firmware so I am assuming it was produced in 2019.The probe should be new as well.The calibration solution was left a few hours in the sump to get to 25 degrees C (the temperature of the tank). The probe tip was in a storage solution. I removed the probe , washed the tip in rodi water and then dried it as best as I could (paper towel ) and then I blowed air on tip to evaporate some of the water. Then proceeded to calibrate via instructions. After insertion in each solution I dipped again in rodi and cleaned. I also tested the PH of rodi water at about 22 degrees and the value seemed correct , for this reason I am assuming it works correctly. Plus the redox is in the 50-80 range , which I know is linked with ph.

On your question about how fast it takes to calibrate first try a few minutes for each calibration fluid , second time it took longer , maybe 5-7 minutes.
The only thing I did which might not be good is that I shaken a bit the probe in the calibration fluid so that the calibration fluid properly get to where the tip of the instrument is. Also in the sump where the probes are placed there are micro bubbles (skimmer chamber). I do not have chambers without some amount of micro bubbles.

Yesterday I added a Knepo filter bed(400l/h) with 50 g carbon (the one that needs to be taken out in 3-4 days) to absorb any organics and maybe raise the redox. After less than 24 h I see a very small increase in both PH and redox. I also added some PH/KH buffer (not allot) . My observation it is that first the PH dropped a bit. Now it is 6.1. There seems to not be any drop at night which I think is as expected as the tank has very good aeration and I have a light in the "live rock refugium".