# pH probe calibration, question



## Rasmusm (3 Jan 2012)

Hi

Okay I've been running my tank for about a year now. 530l heavely plantet tank.
Ive got the GHL profilux II aquarium computer and a GHL pH probe (should be good quality)

Last I calibrated it was 19dec, now I just calibrated it again 3 january aprox 15days between.
I noticed something. I run the pH from 6.7 down to 6.65 and then raises again to 6.7 and co2 kicks in to bring it back down.

But thing I noticed after calibration, took it out of the tank when pH was at 6.70, calibrated and then back in the tank. pH showed 6.49 so about 0.2 pH difference? Is this normal hmm I dont remember the difference beeing that high. Some times there can go a month to 1½ between I calibrate and never seen this much difference.

Is it my calibration fluid thats gone bad? pH4.01 + 7.01
How often do you calibrate it?
Is this normal behavior of an pH probe?

I relly like some thoughts about this :/

Happy new year 
Best regards Rasmus
Denmark


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## Rasmusm (3 Jan 2012)

Infact if probe is bad this might solve my BBA problems 
Another thing. I should mention is, one month ago i send in my temp probe for warranty. I noticed a crack in it when cleaning it. Could this have caused any electrical interferens with my pH probe?

I know you might say, dont run a pH controller


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## mdhardy01 (3 Jan 2012)

Very few use ph probes to switch co2 on as they cause unstable co2 and therefor cause bba
Matt


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## Rasmusm (4 Jan 2012)

I know, but thats no answer to my questions


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## GHNelson (4 Jan 2012)

Hi
Your probe needs time to settle to take a proper reading.
Did you do a water change.
hoggie


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## Rasmusm (4 Jan 2012)

Nah didnt do any water change, I just find it strange that it shows that much difference between a calibration. Does that happen to you aswell?


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## GHNelson (4 Jan 2012)

mdhardy01 said:
			
		

> Very few use ph probes to switch co2 on as they cause unstable co2 and therefor cause bba
> Matt
> 
> Hi Matt
> ...


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## GHNelson (4 Jan 2012)

Rasmusm said:
			
		

> Nah didnt do any water change, I just find it strange that it shows that much difference between a calibration. Does that happen to you aswell?


I haven't used my controller in months as I'm about to decorate and get a new aquarium.
I don't use any Co2 in that aquarium at the moment....just keep the light low and feed the plants as normal EI. 
Did you clean the probe?
hoggie


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## GHNelson (4 Jan 2012)

Hi Rasmusm
Nice to see someone using a Ph controller for a change.
Ph probes have a shelf life of about 1 year to 24 months it depends on what water its testing.
So yours could need replacing soon.
hoggie


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## Rasmusm (4 Jan 2012)

I've used ph controllers with great succes, I believe that the 0.05pH hysteresis is not gonna cause any bba problems. 
But I've heard of probes having a lifetime of 12-24months. Maybe I should order some new calibration fluid.

How do you clean your probe? Mine might be dirty,


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## GHNelson (4 Jan 2012)

Hi 
This is from the Dennerle manual :arrow: Should the head of the sensor be soiled, the globular measuring tip can be cleaned carefully with a soft cloth - dab only, do not rub!
Afterwords place the sensor tip into KCL solution for about 12 hours. Then rinse
and calibrate. Should this not be sufficient, replace it with a new sensor.

Do you re-use the calibration fluid?
hoggie


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## Rasmusm (4 Jan 2012)

What is KCL ?

Yea I reuse the calibration fluid, usualy fill up the soft rubber tip the probes come with to calibrate in.
I also heard that calibration fluid max last for 12 months then its time for replace.

Btw. this is the probe I have: http://www.ghl-profilux.co.uk/acatalog/GHL-Profilux-PH-Electrode.html


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## mdhardy01 (4 Jan 2012)

hogan53 said:
			
		

> mdhardy01 said:
> 
> 
> 
> ...


hi hoggie
I know that some members have had good results from ph controled co2 but was always led to believe ( Clive ) that ph is fairly irrelevant whereas keeping a steady and even supply of co2 is the main objective. 
PH can be altered by many factors in a tank ( fish waste,wood,soils etc ) so ph controlled co2 is very difficult to control as we are unsure what is causing the ph to change
In the past I've kept bare discus tanks no substrate plants or wood and have still noticed ph drops and rises during a day
Matt 


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## GHNelson (4 Jan 2012)

Rasmusm said:
			
		

> What is KCL ?
> 
> Yea I reuse the calibration fluid, usualy fill up the soft rubber tip the probes come with to calibrate in.
> I also heard that calibration fluid max last for 12 months then its time for replace.
> ...


Hi 
The above probe is similar to the Dennerle.
You shouldn't re-use the calibration fluid...the normal instructions are pour some into a separate vessel to cover the probe tip up to 2cm and discard after calibration.
Use distilled water between each calibration fluid ie 7-4.
hoggie


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## GHNelson (4 Jan 2012)

Hi Matt
Yea I get your jist.
There is a link between Carbonate Hardness Co2 content and Ph.
This from the manual
Depending on the carbonate hardness, select pH values only which are
well-tolerated by the fish. DENNERLE recommends a CO2 content of 30 - 40 mg/l. Refer
to the accompanying tabulation “Connection between pH value, CO2 content and car
bonate hardness”, as well as to the references in the DENNERLE brochures, the
“Instructions for use” and in specialized books.
The bubble output is set correctly, as soon as the pH value matches the carbonate hardness**
listed in the tabulation. To avoid overdosing, it is important to set the bubble out
put not too high, especially at the beginning.

So its really up to the user to judge how much Co2 he wants to pump into the tank.
A ph probe is just a fancy timer really.
You could set it up so it will pump Co2 till you get the desired Co2 content say Ph 7 with a Kh of 10 this will give you 35mg Litre Co2 in your aquarium.
When this rises by .01 say through turbulance or water changes the solenoid will open and fire up the Co2.to the preset Ph.
Cheers
hoggie


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## GHNelson (4 Jan 2012)

Hi
Got of the beaten track a tad...when i set-up my new aquarium.
I will go full blown Dennerle system undergravel heating the lot not because I got loads of money but because i have it all lying in a cuboard doing nothing.
Heres a picture :arrow: 



Saying that I will add a couple of drop checkers to see how much Co2 it takes to get them lime green.
Cheers
hoggie


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## dw1305 (4 Jan 2012)

Hi all,
This is quite a complicated question, *personally I'd replace the probe and the buffer solutions*, but the question really needs dealing with in bits. 

*The pH meter*
What people often don't realise is that you can't just dip a pH meter into a tank and get a meaningful reading, they are quite complex bits of kit. As Hogan53 suggested earlier they need to be left to equilibrate before reading, and if you have low conductivity water this may take a long time. If you have low conductivity water combined with little carbonate buffering, the pH will be inherently unstable and will fluctuate wildly with every addition, however small, of acid or base (This is the RO water scenario). pH only tells us the ratio of acid and bases, not the quantity.

*pH range*
pH will always be more prone to vary between pH6 and PH8, as we are only talking change from 10-6 to 10-8 in the H+ ion (and 10-8 to 10-6 in OH- ion) concentration. But if we go from pH6 to pH5 we have gone from 10-6 to 10-5 H+ (and 10-8 to 10-9 OH- ion conc.)

*Buffer calibration solutions.*
These are the 2 x buffers for pH4 and pH7 (possibly coloured red and yellow). These should be fine, they are what they say, "buffers", they are often an mix of 0.2M Na2HPO4 & 0.1M Citric Acid, by varying the proportion of these in the buffer you can cover pH3 to pH8. You need to replace them if you think they have been diluted with water, but within reason they should always read "pH4" and "pH7".

*Reference electrode.*
The reference electrode membrane is the most likely point of failure. These are usually combined glass and "Calomel" reference electrodes  "_is a reference electrode based on the reaction between elemental mercury and mercury(I) chloride. The aqueous phase in contact with the mercury and the mercury(I) chloride (Hg2Cl2, "calomel") is a saturated solution of potassium chloride (KCl) in water_ <http://en.wikipedia.org/wiki/Saturated_calomel_electrode>." This is where the KCl comes in, you need  to store the electrode in dilute KCl, and if it dries out, you may be able to resuscitate it with a stronger KCl solution. My suspicion would be that storing it in the buffer has damaged the electrode.

*pH the measure*
Water is subject to a self-ionization process ~   H2O is in equilibrium with H+ + OH-. pH is a bit of a strange measurement, it is a measure of the ration of H+ ions to OH- ions expressed as the "_negative decimal logarithm of the hydrogen ion activity in a solution_" in the case of pH7 it is 0.0000001M H+
(1 x 10-7 H+ ions) and also 0.0000001M OH- ions. This range of pH0 - pH14 is the possible H+ ions from 1.0M (pH0) through to 0.0000000000000001M (pH14). The dissociation constant, KW, has a value of about 10-14, so, in neutral solution of a salt, both the hydrogen ion concentration and hydroxide ion concentration are about 10-7 Mol l-1. 

You can derive pOH from pH as well: -  [OH-] = KW /[H+]  where KW is the self-ionisation constant of water. 
Taking log 10  ~  pOH = pKW - pH. So, at 25oC, pOH ˜ 14 - pH.
*
Measuring CO2*
We can use a drop checker or pH controller to estimate/control dissolved CO2, because water absorbs carbon dioxide of which a small proportion is then slowly converted into carbonic acid, which dissociates to liberate hydrogen ions:

    CO2 + H2O is in equilibrium with H2CO3 is in equilibrium with HCO3- + H+ 

You can alter this equilbrium the other way by adding carbonate (disassociated as HCO3-) ions, and this is the buffering we measure as dKH (as described in Hogan's last post). 

I can't see why a pH controller should be less good than a drop checker, although I can see that the drop checker with "Bromothymol blue" pH indicator would be more robust, if not as "accurate".

cheers Darrel


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## GHNelson (4 Jan 2012)

Hi Darrel
Thanks for your input...I just though it was time to voice my opinion regarding Ph controllers.
They seem to get a bit of a hard time.
I have had mine a long time as you can see its the green type..cant remember how long.
As you say they are a complex piece of electronic kit and should be treated with care.
Cheers for that  
hoggie


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## Rasmusm (4 Jan 2012)

Thanks for your input 

Well I mean I fill up the rubber tip probes come with, with the desire pH solution. And after calibration I throw it away.
I will try and clean it a bit.

Ok so just now i recalibrated the probe. Let it stay in ph4.01 water for 10min before calibration, same with ph 7.01 for 10 min before calibrating.

Back in the tank, after 10 min ph showed 6.58.. I took it out for calibrating at 6.70, if there should be any difference pH should read higher than 6.70 as there has not been injected any for 30-40mins
humm ?


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## GHNelson (4 Jan 2012)

Well I mean I fill up the rubber tip probes come with....?

I'm not sure what you mean....from hoggie

Is this the rubber tip that protects the probe in transit?
The probe should be submerged up to 2cm above the glass like monitor.
You should have 3 separate vessels for adding the Ph calibration fluids.
1 for say 4
1 for say 7
1 for distilled water to clean each calibration.
hoggie


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## GHNelson (4 Jan 2012)

Hi
I'm not that familiar with your Ph Controller.
But I'm sure that if you haven't re-calibrated your probe properly you will get discrepancies..with the readings.
If you take a probe out of the aquarium the Ph will rise on the monitor i think.

You should be calibrating the probe without returning it to the aquarium till the process has been completed.

The other scenario is that you cleaned the tip of the probe and it is recording a true reading.
Remember .02 swing should not cause major problems or cause algae.
hoggie


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## Rasmusm (5 Jan 2012)

Thanks it was 0.2pH swing bit more than .02 

okay calibrated last night, tested in 7.01 + 4.01 solution, ph showed 7.01 and 4.01 so that was ok.
Now this morning I tested in only 7.01 water, it showed 7.06-7.07pH

I belive its gona bad?


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## GHNelson (5 Jan 2012)

Hi Rasmusm
Purchase new calibration fluid and get some distilled water...from the garage its cheaper.
hoggie


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## Rasmusm (5 Jan 2012)

hogan53 said:
			
		

> Hi Rasmusm
> Purchase new calibration fluid and get some distilled water...from the garage its cheaper.
> hoggie



I dont belive its the water thats gone bad.
Cause if the calibration water isnt exactly 7.01 lets say its 7.05 infact, the probe should read 7.01 in it no matter what..
On the profillux controller you type in the 2 pH fluids you calibrate with, like calibration fluid 1 = 4.01 calibration fluid 2 = 7.01 then it says put your electrode in 4.01 10mins later it will do 3 beep's and the screens says put it into 7.01 and press enter, 10mins later beep and save the settings.

But offcause I will buy some new fluid and test


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## dw1305 (5 Jan 2012)

Hi all,


> You should have 3 separate vessels for adding the pH calibration fluids.
> 1 for pH4
> 1 for pH7
> 1 for distilled water to clean each calibration.


That is the way to do it, rinse the probe off with the distilled H2O between each reading taken in tank or in the buffer.


> On the profillux controller you type in the 2 pH fluids you calibrate with, like calibration fluid 1 = 4.01 calibration fluid 2 = 7.01 then it says put your electrode in 4.01 10mins later it will do 3 beep's and the screens says put it into 7.01 and press enter, 10mins later beep and save the settings.


That sounds fine as a calibration technique, I'd leave the probe in for 10 minutes before taking the reading. This is the method we recommend the students for our standard "Jenway 370" pH meters: 





> When you want to use the meter, you need to turn it on (probe in the de-ionized water) and wait for at least 5 minutes. Ideally you want to have the tank water, buffers and de-ionized all about the same temperature, even if the meter has temperature compensation. You dip the meter in the pH4 buffer, wait for at least a minute and adjust the meter to read pH4, you may find that you can store this reading as "cal. 1", you then need to sluice the probe in the de-ionized and then put it in the pH7 buffer, you will probably find that you won't get a reading of exactly pH7, you don't need to worry about this, as long as it is reasonably close (pH6.9 - pH7.1 range), this is due to the nature of the pH scale.
> If the meter reads along way out at pH7 you need to go through the pH4 buffering again. Once you have readings of pH4 and pH7, you can test your tank water, the nearer the water is to pH7 the longer the meter will take to give a stable reading.



Around pH7 you will always get some movement, the difference between pH7.01 and pH7.05 is small in H+ ions, we are back to the log10 pH scale and this may just be a temperature difference. We have about 50 pH meters in the lab from £100 dip meters up to bench meters costing several thousand£, which are regularly serviced and calibrated before every use, and very few of them will read exactly the same around pH7, or read much different at pH5 etc.

I'm not sure what option you get with your pH controller, but the Hach "IQ" pH meters use an "ISFET" chip (rather than a glass membrane electrode) and are more robust (stainless steel or graphite probe) and  requires far less maintenance than traditional pH probes. They are more expensive (about £250 upwards).

cheers Darrel


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## GHNelson (6 Jan 2012)

Rasmusm said:
			
		

> hogan53 said:
> 
> 
> 
> ...



I dont belive its the water thats gone bad.
Cause if the calibration water isnt exactly 7.01 lets say its 7.05 infact, the probe should read 7.01 in it no matter what. :arrow: 


Hi Ramusm
Your ph7 fluid has been contaminated by the Ph4 fluid that's why you need 3 separate vessels to calibrate the probe correctly.

Clean and rinse the probe with distilled water...discard this water.

Add distilled water to a clean vessel add the probe, when the reading is stable....transfer the probe to a clean vessel with the calibration ph7 fluid, when stable remove and briefly rinse in distilled water.

Do the same with ph4 fluid.......rinse briefly with distilled water after calibration.
Transfer the probe to the aquarium, you may have to wait for the probe to take a proper reading.
Throw away all the fluids you submerged the probe in.
For quick and accurate results the fluids should be between 20-25 Celsius as Darrel stated  
Cheers
hoggie


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