Doing pH profile - keeps dropping when stirring probe

[Iwagumi_Scaper]

Hi all,

I've borrowed a pH meter from one of the labs at university to do my pH profile but I'm noticing that when I'm stirring the probe as per the instructions to wait for the "ready" symbol, the pH seems to just keep dropping. Am I stirring too much? Has anything experienced this with their own pH meters?

My pre co2 measurement dropped right down to about 5.95 before it somewhat stabilised. Would it be an idea to stir say 10 times and then use whatever reading it settles at in order to be consistent?

Cheers

 

[ian_m]

Stirring in tank or in sampling cup ?

If sampling cup you could be degassing the CO2 by stirring.

Try repeating in tank but with all tank electrics disconnected & completely unplugged ?

pH probes are notoriously sensitive to leaking electric fields.

 

[Iwagumi_Scaper]

Stirring in tank or in sampling cup ?

In tank, Ian. I'll try with the filter switched off, thanks.

 

[Iwagumi_Scaper]

Still seems to drop quite a bit, but if I keep stirring it eventually settles... more or less. 5.8pH just before co2 on seems quite low to me... then again the tank is still fairly new so maybe the soil is still pulling it down a fair bit.

 

[ian_m]

more or less. 5.8pH just before co2 on seems quite low to me

This does seem exceptionally low before CO2 on.

What is your approximate KH (hardness) as most probes won't read reliably at low KH values, below 4-8KH depending on probe.

Is probe calibrated ?

 

[dw1305]

Hi all,

Still seems to drop quite a bit, but if I keep stirring it eventually settles... more or less. 5.8pH just before co2

I agree with @ian_m that is a low pH reading, and pH meters really need buffering before you use them (on pH4 & pH7 buffers).

Usually if a pH meter takes a long time to stabilise it is because there aren't many ions in the water (they work by exchanging K+ ions). If you have a conductivity (TDS) meter then a conductivity reading would be useful.

You can add some more ions to your water sample (not in the tank) from a neutral salt (formed from the reaction of a strong acid and strong base), the usual one people use is NaCl or KCl, you would need to get some from the lab. because "table salt" or "low sodium salt" has magnesium carbonate added.

cheers Darrel

 

[Iwagumi_Scaper]

What is your approximate KH (hardness) as most probes won't read reliably at low KH values, below 4-8KH depending on probe.

I use RO and mix my water to a KH of 4-5 but it seems to get pulled down a degree or two in the tank and sits at about 3-4.

If you have a conductivity (TDS) meter then a conductivity reading would be useful.

TDS is 250-300 at water change - higher at the moment due to ferts

Is probe calibrated ?

It was calibrated in the labs before I took it away with me but I'll re-calibrate it tomorrow before I do a 2nd profile. I'm doing it more to see what the pH drop is rather than get accurate measurements. Doesn't "new" ADA aquasoil tend to pull the pH down a bit lower than is expected to begin with? I'm sure I've read that it can be as low as 5.3 for a few weeks and gradually increases to about 6.4.

Cheers,

Chris

 

[ian_m]

I use RO and mix my water to a KH of 4-5 but it seems to get pulled down a degree or two in the tank and sits at about 3-4.

At these low KH levels pH is hard to measure reliably and repeatedly, as Darrel points out. Most probes I have seen require KH of significantly above 4KH to get a reading. This would explain you funny readings.

I thought aquasoil leaches ammonia in its first couple of weeks of life. This would significantly raise the pH, so again something funny somewhere.

Why are you complicating issues using RO as opposed to tap water ?

Are you dechlorinating your RO ? As most RO units do not 100% guarantee to remove chlorine or the even worse chloramine from water supplies.

 

[Iwagumi_Scaper]

I thought aquasoil leaches ammonia in its first couple of weeks of life. This would significantly raise the pH, so again something funny somewhere.

Strangely enough my Ammonia has been very low. My API test kit has been showing 0-0.25ppm. Tank is cycled according to tests (I always use Seachem stability to get things going) and shrimp have been happy in there for a couple of weeks now with successful molts and showing no unusual behaviour.

Why are you complicating issues using RO as opposed to tap water ?

My tap water parameters vary wildly week to week and after speaking with a well known and reputable aqua-scaping company they recommended I aim for a KH of between 3-5 so I've been using RO for consistency. It's what I use in all my tanks so it's not really complicating anything for me. Completely normal My tanks are small so it's no hassle.

Are you dechlorinating your RO

Yea I use prime with every water change for good measure.

 

[dw1305]

Hi all,

TDS is 250-300 at water change - higher at the moment due to ferts

That should be fine, the meter will eventually stabilise.

I thought aquasoil leaches ammonia in its first couple of weeks of life. This would significantly raise the pH, so again something funny somewhere.

I think the Aquasoil might also exchange H+ ions for Ca++ ions etc in the water column. It would depend upon the composition of the solution they soak the Aquasoil in before they sell it.

I'm fairly sure that when they dig up the base material (volcanic clay) all the exchange sites will be filled with H+ ions.

Ammonia is a base, but a thought occurs that if the ammonium source, used by ADA, was ammonium sulphate ((NH4)2SO4) then it would serve a couple of purposes, it would reduce pH, which would help keep the ammonia as the less toxic ammonium ion, and it would be a very cheap compound, maximising profits.

Ammonium is both soluble and easily exchanged (it is a monovalent cation).

cheers Darrel

 

[Iwagumi_Scaper]

That should be fine, the meter will eventually stabilise.

That's good to know. If I may Darrel, would it be OK to pop you another PM this evening to have a look at the results of my pH profile? I create enough new topics as it is and I feel bad about it

Cheers,

Chris

 

[dw1305]

Hi all,

would it be OK to pop you another PM this evening to have a look at the results of my pH profile? I create enough new topics as it is and I feel bad about it

You can do, but just paste the values in this thread, a lot of us find these sorts of thing very interesting.

cheers Darrel

 

[Iwagumi_Scaper]

You can do, but just paste the values in this thread, a lot of us find these sorts of thing very interesting.

Here's what I've got:

Co2 on at 10:00
Lights on at 12:00

Co2 off at 17:00

 

[ian_m]

Looks good, dropping 0.7 in 2 hours and steady after that.

Maybe try to get a bigger pH drop by either or a combination of:
- Increased bubble rate
- Increased/decreased any surface agitation.

By varying the angle of my spray bar, altering the surface agitation and can vary my drop checker between blue/green right to green/yellow and this changes the CO2 surface losses.

Try reducing surface agitation first, leaving bubble rate as is.

 

[Iwagumi_Scaper]

Try reducing surface agitation first, leaving bubble rate as is.

This should be nice and straight forward since I have a bit of adjustment on the flow rate of my filter I'll give it a try.

What are the reasons and/or pros of having a pH drop of precisely 1pH? What's the disadvantage to mine being just 0.7?

Cheers,

Chris

 

[ian_m]

This should be nice and straight forward since I have a bit of adjustment on the flow rate of my filter I'll give it a try

Moving the height of the lily pipe is the normal way people change the surface agitation.

What are the reasons and/or pros of having a pH drop of precisely 1pH? What's the disadvantage to mine being just 0.7?

A pH drop of 1 unit in buffered water corresponds to about 30ppm CO2 concentration. 30ppm (probably higher) is about the maximum fish can easily tolerate, though if more O2 (aeration) is added can go significantly higher.

Formula roughly is CO2 (ppm) = 3 x KH x 10 ^ (7-pH). For carbonate buffered water.

Thus for KH of say 4 and a measured pH of 6.6 will give a ppm of 3 x 4 x 10^0.4 -> 30ppm.

For a drop of 0.7, pH 6.9 -> 15ppm, so half the amount CO2 and maybe the indication of the start of poor CO2 for plants.

Obviously, this is for carbonate buffered distilled water, not fish tank water where many interfering ions will be present. But gives you the idea. Generally a pH drop of one will be less than 30ppm in real world tank, which is handy to not gas your fish.

 

[Iwagumi_Scaper]

Formula roughly is CO2 (ppm) = 3 x KH x 10 ^ (7-pH). For carbonate buffered water.

Is this the pH before co2 comes on? So for me 3x3x10^(7-5.8) = 142.6ppm? :/ Or if let's say my pH was 6.4 before co2 it would = 35.8ppm? I'm wondering if it's worth me fiddling around with it, because my current drop of 0.7 is almost there and my drop checker is green with a slight hint of yellow by the time co2 turns off.

 

[ian_m]

Is this the pH before co2 comes on? So for me 3x3x10^(7-5.8) = 142.6ppm?

As I said, be careful, this is for carbonate buffered water not your tank water.

Something else is causing your abnormal pH reading.

I suspect if you harden your water to say 8dKH, you will more than likely get sensible readings of CO2.

As Darrel states, in this post
https://www.ukaps.org/forum/threads/tds-and-remineralising-ro-water.53891/#post-533029
pH values become wildly varying and heading towards meaningless at low dKH values, which could explain your abnormal pH readings.

 

[tiger15]

Have you tried checking with a pH liquid indicator to compare the readings with the probe.

What appears to me is that due to low pH of your tank water, a substantial portion of CO2 is free gas. PH probe measures H+ from ionized CO2 or carbonic acid. When you shake the probe in the tank water, you are forcing more CO2 to ionized into H+ and HCO3- and thereby reading lower pH from higher H+ . When you use a pH liquid indicator, it reacts with both free and ionized COs in a test tube avoiding instability you see in the probe.

The proportion of free to ionized CO2 is pH dependent. At low pH end as in your case, free CO2 gas dominate and the stirring instability effect is greater. Here is a link that shows the relationship.

http://www.soest.hawaii.edu/oceanography/courses/OCN623/Spring2012/CO2pH.pdf

 

[Iwagumi_Scaper]

Have you tried checking with a pH liquid indicator to compare the readings with the probe.

I used my API master tetst kit and it showed it being 6.0... could well be trying to show lower but that's as low as the kit will read. So do you think the probe is being fairly accurate in that case?