# 4DKH Fluid PH



## AverageWhiteBloke (2 May 2010)

Am I wrong in thinking that 4dKH fluid is always a PH of 7? I have a digital PH tester pen and wondered if I can use a 4dKH fluid to calibrate it.


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## Colinlp (2 May 2010)

I would imagine that the ph would depend on how much CO2 was dissolved in it changes colour in a drop checker, if you know what I mean


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## ceg4048 (2 May 2010)

Hi,
   If RO water was used to make the 4dKH water then it has no pH theoretically, yet if measured with a pH test kit will show 7.0 typically, if measured straight from the container. Normally the probes require at least two points for calibration though...

Cheers,


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## AverageWhiteBloke (2 May 2010)

yet if measured with a pH test kit will show 7.0
I was going to make some from distilled water from halfords for my DC. So out of the bottle it should measure 7.0 right? 
I appreciate what your saying about 2 ref points I have seen them for sale. Not sure if its worth investing in though for the use I would get.  I have a DC in to monitor levels just trying to get some use out of equipment I bought years ag.


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## ceg4048 (3 May 2010)

Well, distilled water is not pH controlled, and while it might be free of Calcium and Magnesium it may still be high in the components that cause pH. It's probably close enough to 7 for a liquid test kit but perhaps not enough for an electronic meter. If you don't want your meter to tell lies then it's probably worth buying the calibration solutions, as annoying as that might be.

Cheers,


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## dw1305 (4 May 2010)

Hi all,
You do need to buy or make standard buffering solutions for your pH meter, usually pH4 and pH7:
For pH7 buffer "Add 29.1 ml of 0.1 molar NaOH to 50 ml 0.1 molar potassium dihydrogen phosphate."
or "dissolve 1.20g of sodium dihydrogen phosphate and 0.885g of disodium hydrogen phosphate in 1 litre  distilled water." For pH4 "Add 0.1 ml of 0.1 molar NaOH to 50 ml of 0.1 molar potassium hydrogen phthalate." So probably easiest to buy the solutions made up.

In de-ionized water pH doesn't really  have any meaning as pure H2O has no buffering, any addition of acid or alkali will cause the pH to drop or rise to extreme values. In this case it is the buffer that counts,  via James' C Planted tank "6.0g of Sodium Bicarbonate and add to 5 litres of deionised water to make a 40dKH solution. Mix 10ml of the 40dKH solution with 90ml of deionised water to give you 1 litre of 4dKH reference solution." Sodium bicarbonate is a base, so that this solution would be alkaline, you also add the pH indicator "Bromothymol blue", which is yellow below pH6, blue above pH 7.6. 

http://en.wikipedia.org/wiki/Bromothymol_blue






The drop checkers solutions pH range around pH7 happens because you have "buffered solution", in this case the acid and "conjugated base" are carbonic acid and bicarbonate, and the reactions are: 

NaHCO3 = Na(+1) + HCO3(-1) dissolved in water
HCO3(-1) + H2O = H2CO3 + OH(-1) dissolved in water
H2CO3 = H2O + CO2 dissolved in water

Both of these reactions are at equilibrium and reversible. If you add components to one side of the reaction, you drive it in the other direction, so if you add CO2 to an aquarium the pH will fall and the pH indicator will change colour. This is also why hard water resists pH changes much better than soft water, and we re-mineralise R.O water to 4dKH to achieve a much larger reserve of buffering (or carbonate hardness - KH).

cheers Darrel


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## AverageWhiteBloke (8 May 2010)

Interesting stuff! I made up a 4dKH solution using De-ionised water from an API purifier. I checked it with a KH test kit to within 1/2 a degree and used it to calibrate the electronic pen to ph 7.0. Using the DC and some PH test strips compared the results and all came within .2 PH so that will do me just as a quick reference point.

The pen isn't the most accurate so I don't think its worth buying or looking at Darrels post making any calibration fluid, I remember from my Discus days using the pen a lot and getting two different results from the same batch of water. I just don't like to see equipment going to waste.


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## dw1305 (9 May 2010)

Hi all,
The pH meters do need calibrating or they "drift", I don't know how much by as we tend to calibrate them every time we use them. The 2 problems we have in the lab with the students is them turning the meter on and using it straight away (they need ~10 minutes to warm up), and failing to take the cap off the electrode!

I think the only problem with using the 4kdH bromothymol "green" to calibrate to pH 7  is that the  carbonic acid and bicarbonate reactions are at equilibrium and reversible, and the colour change is fairly slow, so that the green colour may relate to the situation 20 minutes before you took the pH reading, rather than at the time you took it.

I don't personally worry too much about the pH levels, particulalry if you don't add CO2 the pH will be changing  all the time as the carbonic acid and bicarbonate level equilibrium changes (dependent upon the degree of dissolved CO2  and the respiration/photosynthesis balance). The less buffering you have the bigger the swings in pH will be (in my tank at about 150 microS, pH will be well above  pH7 during active photosynthesis and about pH6 before the lights come on).

I do occasionally check the conductivity, because I know most of my cations will be calcium from calcium carbonate dust in the rainwater, and this is a "proxy", an indirect measurement of buffering . 

cheers Darrel


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## AverageWhiteBloke (9 May 2010)

> (they need ~10 minutes to warm up)


I did not know that nice tip   I have also started calibrating before every test getting better results.


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