Strange PH readings

[Barbara Turner]

I'm playing with a data logger and PH sensor and getting some strange results, I wondering if anyone has seen anything similar.

Each blue dot is based on a 100 readings that are then averaged. I take a new set of readings every 5 seconds.
The data below looks at a period of 5 hours.

The PH sensor was borrowed from a kedida CT-6020A

The water I'm measuring is RO water that's sat on my desk for 2 weeks in a glass tub.
I've checked the data logger against a calibrated fluke multi meter and it seems to be behaving itself.

I'm guessing at some point the sensor will level out.

I need to try and work out if it's the data logger or the sensor now..

 

[ceg4048]

Hi,
RO water does not have a pH.
pH is a ratio of acidic (H+) and basic (OH-) components of the water. If there aren't enough of these components in the water the the electrode simply reports spurious readings. RO water would have an equal balance of H+ and OH- so they cancel each other out.

You need an excess of acid or of a base in the water to make a meaningful reading.

Buffer the water to an alkalinity of about 4 dKH using any bicarbonate or carbonate product you have in your cupboard and then take a pH reading. Arm & Hammer Baking soda for example or any such, or calcium carbonate if you have will work.

Cheers,

 

[tiger15]

RO water opens to the atmosphere will absorb CO2, and will show acidic pH.

 

[ceg4048]

RO water opens to the atmosphere will absorb CO2, and will show acidic pH.

Which is all very fine and well, except that the data shown on the graph provided by the OP clearly shows alkaline values.
The point I'm making is that there is very little value in reading the pH of RO water. The data will be meaningless.
The probes and reagents are always confused by this type of water.
It is also a fact that you should not store probes such as conductivity probes in RO water because the lack of free ions in the water will deplete the ionic strength of the probes and will ruin them.

This is an important point because we use the change in pH readings when we inject CO2 to reflect the level of CO2 dissolved in the water. When the water has no alkalinity the pH cannot be determined with any accuracy and therefore the CO2 content will be unknown. This is also why dropcheckers have RO or distilled water adjusted to an alkalinity of about 4 or more.

Cheers,

 

[Barbara Turner]

My big concern was change. I wasn't too worried about the actual values.

I need to recalibrate it against my buffer solutions at some point, but until I can get stable values I wasn't that worried about the actual values. I need to know how long it takes to settle to get a stable value.

[

RO water opens to the atmosphere will absorb CO2, and will show acidic pH.

After 2 weeks in the open I would expect it to be reasonably stable when it comes to co2 content. If it was straight out the tap / filter I might have be able to explain it.



If you look carefully you even see some diatoms in the bottom.

QUOTE="ceg4048, post: 529435, member: 36"]pH is a ratio of acidic (H+) and basic (OH-) components of the water. If there aren't enough of these components in the water the the electrode simply reports spurious readings. RO water would have an equal balance of H+ and OH- so they cancel each other out.

You need an excess of acid or of a base in the water to make a meaningful reading.

Buffer the water to an alkalinity of about 4 dKH using any bicarbonate or carbonate product you have in your cupboard and then take a pH reading.[/QUOTE]

I've just dropped it into some buffer solution so it's now sat in solution arround 5, be interesting to see the data.

This is slightly worrying if all sensors are like this.

I'm in a hard water area so after a water change I'm at a pH of arround 7.5 this drops to 6.3. so operating right in this area. I'm also using about 90% RO water.

 

[tiger15]

Which is all very fine and well, except that the data shown on the graph provided by the OP clearly shows alkaline values.

Yes, I missed that. It appears the pH meter is far off calibration, or mal functioning.

Rain water has a pH of around 5 due to saturation of CO2 falling through the sky, and near zero kH and gH

RO water sitting around will be similar to rain water due to absorption of atmospheric CO2. I tested it many times with my pH meter as I used distilled water to rinse the probe, and it always showed acidic pH.

What bother me is that pH meter may underestimate pH if some portion of CO2 is undissolved, or in free phase. I compare my pH meter readings with API test kit readings at the peak of CO2 injection. The test kit readings showed 0.2 to 0.4 lower pH than the meter. I attribute it to micro CO2 bubbles (free phase) that is not registered with the meter, but picked up by the liquid test kit. This can be translated into 10 ppm difference in CO2, tipping over the safety edge if pH meter is used to control CO2 injection.

 

[alto]

I need to try and work out if it's the data logger or the sensor now..

What Frank Paparone, Technical Support Leader, OAKTON Instruments has to say on the subject of measuring the pH of Pure Water

I’d guess the sensor may not be suitable for the measurement you’re attempting

As to why alkaline rather than acidic value - there may be some “break through” during the RO process (not atypical, especially as membrane ages, or lower grade membrane)

The classic method to obtain a “stable” pH measure of very soft water is the addition of high grade potassium chloride (KCl) not the addition of any “buffer”

 

[Barbara Turner]

Hey all
Sorry about the delay getting back to everyone, sometimes work gets in the way of fun.
I think the Sensor is faulty, I've attached some more graph's below.

Until I get this fixed it's really not even worth trying to take measurements of my tank.

I read online about rejuvenating PH sensors (instructions below)
I need to get hold of some potash and give it ago, I'm also going to order a better PH sensor from poland.

"
1) Let the pH Electrode soak for 4-8 hours in an HCl solution between 0.1 and 1.0 M.
2) While the probe is soaking, prepare a batch of new storage solution. The recipe is: 10 grams of solid KCl dissolved in 100 mL of pH 4 buffer. Any pH 4 buffer mixture works well, but use only KCl for the salt - other salts do not work. (NOTE: Use this solution to store your pH probe when it is not in use.)
3) After the HCl soak, rinse off the probe and place it in 50-75 mL of the storage solution you made in step C. Soak the probe tip in this storage solution for 1-3 hours.
3) Rinse the electrode and give it another try.
"

We have found that many old electrodes can be revived in this manner. Generally, storing them in a storage solution (buffer pH 4 + KCl) will keep them in good condition.

 

[alto]

kedida CT-6020A

This is a rather inexpensive system - I suspect your expectations are just not consistent with the quality of pH senser supplied

The method of cleaning/rejuvenating pH sensors also causes damage to the membrane, it’s useful for a short term “fix” while a new probe is ordered

If you’re looking for pH measurement of, eg, 7.1 +/- 0.1 precision/accuracy this probe should deliver that

Note that while the pH sensor has a Resolution of 0.01 pH, Accuracy is 0.02 pH, and there are no specifications for Precision and Drift
Drift is always a challenge for pH sensors

Look for a calomel pH electrode - it will sooner meet your expectation
eg, this one from Hanna Instruments is a reasonable price
(though you’ll need to contact tech support for performance specifications as they’ve chosen to not list them in the online catalog )