# Calibration of pH meter



## Soilwork (23 Nov 2016)

I know distillation is a superior method of water purification.  The instructions state that distilled is to be used.  I have not been able to find distilled water anywhere.  Only RO or De-ionised.  I have free access to RO water. Its made in a lab where i work.  Would this be suitable or would it effect the accuracy of the meter in a way that would deem any measured results useless?

Thanks.


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## rebel (23 Nov 2016)

De-ionised would be good enough I think.


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## kadoxu (23 Nov 2016)

Have you tried ebay?
http://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=distilled+water&_sop=15


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## Soilwork (23 Nov 2016)

kadoxu said:


> Have you tried ebay?
> http://www.ebay.co.uk/sch/i.html?_from=R40&_sacat=0&_nkw=distilled+water&_sop=15


No I hadn't thought of ordering it online.  Thanks!


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## dw1305 (23 Nov 2016)

Hi all, 





rebel said:


> De-ionised would be good enough I think.


De-ionised water should be purer than distilled. Distilled water was produced from a steam still, and I'd be very surprised if any lab. still has one.

You should be able to get a read-out from the DI unit of the conductivity, or resistance, of the water. You usually get a conductivity value (~0.42 microS) for (type III)  water and a value in micro-Ohms for ultra-pure water (types I & II). 





Soilwork said:


> I have free access to RO water. Its made in a lab where i work.


 There should be a conductivity meter to check the RO water, it should read less than 5 microS.

Both RO and DI use reverse osmosis, DI has an extra de-ionising resin filter. 

cheers Darrel


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## Soilwork (23 Nov 2016)

Thanks Darrel if the water is just pushed through an RO membrane what that take care of all ions too? Would pure RO water be a suitable calibration tool for the purpose of ph meters?


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## dw1305 (23 Nov 2016)

Hi all,





Soilwork said:


> if the water is just pushed through an RO membrane what that take care of all ions too?


Yes, the conductivity is a measure of the amount of ions.

The RO membrane is semi-permeable, basically it excludes any solutes on size but lets the solvent (H2O) through, and you need the high pressure on the supply side to drive the reverse osmosis process.

Better membranes have small pores, but you need higher pressure to force the permeate through the pores and they are more prone to fouling etc.






You can use RO membranes to desalinate sea water, but you need very high pressure and you will have to process a large volume of water to get a relatively small volume of desalinated water.  





Soilwork said:


> Would pure RO water be a suitable calibration tool for the purpose of ph meters?


You need RO to make up the buffers, but pH isn't a useful measurement in pure water.

The <"pH buffers">  (for calibration) are quite specialised solutions so most labs buy them, rather than making them.

cheers Darrel


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## KipperSarnie (23 Nov 2016)

Even with 2 diaphragms my RO water varies in PH every time I test it, today's water changes the RO water readings were PH 7.2 KH 3 & GH 4.
I use a meter from ePorter via (Amazon) & 2 PH calibration fluids.


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## rebel (24 Nov 2016)

KipperSarnie said:


> Even with 2 diaphragms my RO water varies in PH every time I test it, today's water changes the RO water readings were PH 7.2 KH 3 & GH 4.
> I use a meter from ePorter via (Amazon) & 2 PH calibration fluids.


Hiya, ph measurement in RO water would be meaningless. The explanation is technical but it's tricky to measure pH in pure water.


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## zozo (24 Nov 2016)

As far as i know it is adviced to use pure water to rinse the probe after use, to wash off the mineralized waterfilm from the glass probe. Obviuosly if you would never do this eventualy it will form a layer of salty residue on it. In terrestrial horticulture this is rather more frequently adviced because the ferts contents in the water is very much higher than the way we use ferts in an aquarium. Depending on the plant and substarte used the fert sollution sometimes can be up to 2.5 mS which is rather high and can leave quite some residue behind when dried up. These are values we never would reach in the aquarium. So the ph meter will get much less dirty in our water than it would do in ferts sollution for terrestrial plants.

I never heard of using pure water, ro or distilled or what so ever for calibration purpose.. Only for rinsing the meter before putting it in the calibration solution or in between if you do 2 point calibration and shake of excess water. This to prevent cross contamination of the calibration fluids. 

I think it doesn't realy matter if you use distilled ore ionised water for that, both are so low in mineral content what ever it would leave behind when it dries up is neglectible. Next to that the ph penn probe should never dry up in the first place, it need to be constantly wet or at least moist.. This is also to prevent dry residue from forming and clogging the probe ofcourse as long it is moist it will not realy stick and wash off more easily.

But as said, the miniral contents in our tank water is relatively low and maintenance description of the meter is more aimed towards a worse case scenario where mineral values are rather high like the + 2,5 mS. So i guess the discription most likely points to cleaning with distilled water.. 

At least that was always my basic understanding of how to maintain a ph meter and ensure it's longevity..


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## rebel (24 Nov 2016)

I store my ph probe in the 4.5ph calibration solution. I think they suggest keeping it moist but never with distilled water. 

@zozo your suggestions make sense to me.


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## zozo (24 Nov 2016)

rebel said:


> I store my ph probe in the 4.5ph calibration solution. I think they suggest keeping it moist but never with distilled water.
> 
> @zozo your suggestions make sense to me.



Regarding my personal experience it kinda depends on what pH meter you buy, some even offer a special storing solution.. I still got one in the draw i bought maybe 20 years ago which are still around today which is probably dry to the bone right now havent used it for many years.



These have no rubber seal in the storage cap covering the probe so it's not airtight, i remember these come from the factory with a little piece of moist cotton in the cap and are recomended to store this way. Disadvantage is even when stored like this you need to regularly think of dripping some distiled water on the cotton or else they still dry out and need a refill for the probes solution after a while. Which actualy is the main purpose of keeping them moist, preventig the internal probe solution from drying out. These designs with a o-ring closing the cap, recomend a storing solution or as you do a calibration solution. This differs from one to another.

But take for example a permanent pH meter, i guess they kinda use the same glass probe as a pocket design, at least i do not realy see any difference here and both function with the excact same propperties.. And this probe is permanently stored in the fluid it needs to measure and it aint realy a problem, these are general only regularly rinsed and cleaned when calibrated. When i calibrate my permanent pH probe, i clean the outside plastic shaft with a cloth and put the probe in distilled water for a while before calibrating and put it back again for a number of days in the tankwater. Till now say after almost 2 years actualy never noticed any residue on the probe, still do it this way to be sure it's cleaned regularly. But actualy this kinda makes the story of using a special storing solution contradictive..  But it probably has some to it regarding the probes lifetime, most permanent BNC probes are recomended to be replaced once a year and pocket probes which get stored do not have this recomendation and can't be replaced, but usualy can be refilled.

Tho this recomedation of replacing bnc probes is also vague and just a recomendation if used in a worse case scenario and in very high concentrated solutions. It doesn't mean they stop functioning after x amount of time... I still got a 15 year old hana bnc probe and it still functions perfectly and stored properly and hana makes indeed very good quality products but still recomend to replace it each year.

Bottom line is in our aquariumwater pH probes have much less to suffer if you compare it with using them for growing tomatoes etc..


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## Soilwork (24 Nov 2016)

Thanks to all.  Much appreciated.  I should have been clearer.

The distilled/RO water is to be placed in a container (250ml) then you add the buffer 6.86 and tweak the pot to this value.  I then rinse the probe in RO then check the reading with the ph4 buffer sachet.  I've had my meter for a good while and have never kept it moist or calibrated it before as I didn't know how important this was.  

Seemed fairly accurate during calibration.


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## zozo (25 Nov 2016)

Just for the record and maybe you like to know.. But it actualy is a bit nitpicking and it probably wont influence the reading that much or not at all. Since this pH measuring also just is an approximate vallue and the same with thermometers, if you take 3 different meters you likely will have 3 different readings and impossible to know which one is most accurate. So it al aint realy of such importancy..

But when it comes to calibration the meter should be calibrated closest to the target sample one wants to measure. That's why there are several calibration buffer solution available.4, 7, 9.6, 10 etc. Now the manufacterer makes an assumption about what you want to use it for, since it is a ph meter the assumption is you want to measure acidity.
Most are therefor used in the horticultur where fert sulotions generaly are made acidic depending on what you want to grow down to pH 5 can be recomended. According to this assumption they recomend buffer solution 4 and 7..

Now these meter have a degree deviation this deviation is lineaire the further away we are from the sample the more this devation can be. So to be most accurate should be calibrated near the expected sample reading. Do we want to make the sample pH 5, we need to calibrate first with pH7 as zero point calibration and secondly with pH 4 to be closest to the sample. Do we want a sample from pH 8 then we should calibrate secondly with a buffer of pH 9.6 or pH10 to get most accurate reading.

In the aquarium hobby almost all want a nutral as possible sample, even if we use co2 we stay much closer to pH 7 than we would be to pH 4.  So logicaly interpreted the pH 4 calibration makes no sence for us, we calbrate to 4 and stay always at least 2 to 2,5 units away from it. Actualy to be most accurate we should calibrate secondly with a 9.6 buffer to be closer to the sample.. My personal pH in the high tech is pH 6.8

But as said this is nitpicking..  I guess the final truth is, we are wasting money on pH 4 calibration buffer..


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## Soilwork (25 Nov 2016)

Thanks zozo.  I've just ordered a drop checker.  Easier


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