Alkalinity and KH measurement

[Cornelius]

I sometimes read that the standard KH testst in our tanks can't really be reliable. That would be because our the standard test kits don't measure the KH but only alikinetics
It seems that the presence of other substances in our water can influence the KH. Such as phosphate, nitrate, silicate, ammonia, certain types of wood.
But to what extent are these substances decisive for a deviation from our test kits? I myself think that it is negligible, but what do you guys think about this?

(I am asking this question in relation to adjusting the CO2 by a prope - pH-KH determination and the credibility to achieve a set amount of CO2)

cheers, Cor

 

[LMuhlen]

Our tests add acids to the sample until the pH drops a set amount. So anything that prevents the pH from dropping will increase the value, therefore the test measures alkalinity. It is expected that carbonate species will be the most relevant source of alkalinity, so it is a reasonable way to estimate the KH.

Regarding the quest to achieve a set amount of CO2, I wouldn't worry too much as all we have are references from other aquarists, and they all have the same issues with precision of CO2 measuring. It doesn't help a lot to have the most precise CO2 measurement and compare it to the less precise measurement of others as a reference.

It is kind of like the discussion about filter real flow rate and the 10x reference. Most people use the manufacturer flow rate info to compare to the 10x turnover mark, so if you decide to get a very precise measurement, it won't make sense to use it to compare your tank to other's.

 

[dw1305]

Hi all,

That would be because our the standard test kits don't measure the KH but only alikinetics

Yes, they are semi-titrimetric tests that measure alkalinity.

It seems that the presence of other substances in our water can influence the KH. Such as phosphate, nitrate, silicate, ammonia, certain types of wood.
But to what extent are these substances decisive for a deviation from our test kits? I myself think that it is negligible, but what do you guys think about this?

"Negligible" is my thought as well. Most <"alkalinity"> will come from dissolved limestone (CaCO3) and that means that <"alkalinity and dKH"> are pretty much one and the same for people in N. Europe. Have a look at <"Confusion concerning 0dKH and nitrification"> and @hax47 post <"Confusion concerning 0dKH and nitrification">, they give values for some of the other basic anions.

I am asking this question in relation to adjusting the CO2 by a probe

I'm not a CO2 user, but if I was? I'd be keener <"on using a drop-checker">. I know @LMuhlen isn't a <"drop checker fan">*, so they may be able to add some practical experience of using a pH meter.

*Edit <"Scrub that">

cheers Darrel

 

[LMuhlen]

I know @LMuhlen isn't a <"drop checker fan">, so they may be able to add some practical experience of using a pH meter.

Ehhh... I still like the drop checker, I just try to play the devil's advocate sometimes. I don't even have a pH meter. Maybe I'm just a pessimist, I don't trust anything to give me a precise result, I just try to get used to the uncertainty.

 

[Cornelius]

I think a drop checker is not meant to measure CO2, and certainly not to adjust the CO2.
It is meant to get an indication of a possible deviation of the CO2 concentration. It is not called a drop 'checker' for reason. I'm convinst the best way for us hobbyists to adjust the Co2 is still by using a pH meter.

 

[martinjcxn]

I think a drop checker is not meant to measure CO2, and certainly not to adjust the CO2.
It is meant to get an indication of a possible deviation of the CO2 concentration. It is not called a drop 'checker' for reason. I'm convinst the best way for us hobbyists to adjust the Co2 is still by using a pH meter.

I went deep into this rabbit hole when I restarted with the hobby 2 years ago. I may have misunderstood the principles so am happy to be corrected:

There are a couple of issues at play with trying to determine CO2 concentrations. Namely that you need to know your kh (and case in point being the discussion above about potential unreliability in the kh testing), and that most retail ph pens aren’t great at precision ph measurement in soft water (like when you have aqua soil). Things get even trickier when your dKh heads down towards zero - as neither your pen is reliable (without tricks like adding pure salt to increase conductivity) nor is the ph/kh/co2 chart accurate at that level.

Assuming though that you have overcome these issues. You can then use the co2/ph/kh chart to extrapolate your approximate CO2 injection level by the ph drop (or even the formula underlying the chart if you’re attuned to mathematics and chemistry)- but then your actual CO2 level would also be dependent on the equilibrium CO2 level pre injection thanks to the gas air exchange principles. This in itself is dependent on altitude and some other variables.

Thus, I’ve gone back to the drop checker for simplicity.

My approach now is to turn up the CO2 until the drop checker goes bright yellow and the fish try escape, and then dial it back a few bubbles.

I jest, of course, but very much remain interested in being able to exactly measure CO2 levels.

 

[Cornelius]

When using a pH pen (such as the 1 point pH drop method) the KH is not relevant.
for example; for a tank with 'Easy Plants' I use a drop of 0.8. For Medium Plants a pH drop of 1.0 and for a High Energy tank a pH drop of 1.2
Works as a charm, and I have not come across any better method yet.

 

[hax47]

It seems that the presence of other substances in our water can influence the KH. Such as phosphate, nitrate, silicate, ammonia, certain types of wood.

Any buffer system will affect the alkalinity if the pH is within ~1 pH of the buffer's pKa (the same rule does not apply to bicarbonate buffer because of the gas nature of the "acid" part of the buffer). From your examples, phosphate would make a good buffer between ~6.2 and 8.2.
For a comparison, 0.357 mmol/l bicarbonate makes 1 dKH alkalinity. The same concentration of phosphate (HPO4--) would be ~34 mg/l. That is not a usual aquarium phosphate concentration, and at a pH of 7.21, half of the phosphate would be already protonated, and would not contribute to the alkalinity. At lower pH, even less of the phosphate would contribute to the measured value.
Humic and fulvic acids are also a theoretical option, I do not know the pKa of these compounds (probably there is a range of different acids, with different pKa-s. However, I have tried to dissolve humic acids in water, and a very low amount already stained the water.
So, to answer your question, at higher KH values the effect of other substances is probably negligible. At lower KH values (below ~1 dKH), the measured alkalinity could be inaccurate, partly because of the relatively higher impact of other buffers, and partly because the accuracy (resolution) of the usual KH tests is not very good.

 

[Andy Pierce]

I think a drop checker is not meant to measure CO2

Understandable point of view, but this is in fact just what a drop checker does.

 

[Cornelius]

a DC shows that there is a certain concentration of CO2 present, but not the exact amount of ppm.
Because your DC gets a green color at a CO2 concentration of 15ppm but you also have a green dropchecker at a concentration of 25ppm CO2

 

[martinjcxn]

When using a pH pen (such as the 1 point pH drop method) the KH is not relevant.
for example; for a tank with 'Easy Plants' I use a drop of 0.8. For Medium Plants a pH drop of 1.0 and for a High Energy tank a pH drop of 1.2
Works as a charm, and I have not come across any better method yet.

This is a good rule of thumb to work, but a ph drop of 1 would certainly mean different levels of injected CO2 in ppm at different levels of kh (and pre-injection ph levels).

 

[Bradders]

There seems to be a massive market for C02 measuring equipment that is both accurate and cost effective! I had a look at one of CO2 measuring units and was shocked to see $2,500 was not uncommon in the price range!

 

[hax47]

There seems to be a massive market for C02 measuring equipment that is both accurate and cost effective! I had a look at one of CO2 measuring units and was shocked to see $2,500 was not uncommon in the price range!

Alternatively, there is the $30-50 DIY option, it needs some wiring and programming skills though.

 

[martinjcxn]

Alternatively, there is the $30-50 DIY option, it needs some wiring and programming skills though.

This is an amazing experiment. I’ve been playing around with ESP32s for tank automation (water changes, cooling etc) and am very keen on seeing if I can incorporate this somewhere into my sensor suite.


Sent from my iPhone using Tapatalk

 

[Andy Pierce]

a DC shows that there is a certain concentration of CO2 present, but not the exact amount of ppm.
Because your DC gets a green color at a CO2 concentration of 15ppm but you also have a green dropchecker at a concentration of 25ppm CO2

It is fair to point out that a drop checker does not come with a numerical readout for how green the greenness is, but there are easy ways to take this measurement digitally if that's important to you. The problem with numerical readouts, and this is particularly true for pH amongst the digital measurements commonly made in the hobby, is that it's easy to generate a number but much harder to make sure that number is an accurate representation of the actual data you're looking for.

there is the $30-50 DIY option

This is the best solution I've ever seen, essentially putting an air-based digital CO2 spectrometric detector into the air gap of a drop checker. @hax47 you should put a drop checker in there too and take some colourimetric readings!

 

[hax47]

@hax47 you should put a drop checker in there too and take some colourimetric readings!

Do you mean in the sensor chamber or the aquarium? It doesn't matter I guess. Technically doable, but I am not sure that the DC could keep up with the rapid CO2 changes in my tanks. In the DC, there is an additional equilibration between the air and the still DC solution which slows down the process. It would probably show a slow moving average value, not the actual one.
What I was thinking instead, was putting an industrial pH electrode into the water (available for ~$65) and monitoring the pH for a longer period. The price is semi-prohibitory for equipment with no practical use besides the satisfaction of my curiosity, but it could be also used to check the reaction constants we use for CO2/KH/pH calculations. In this case, my sensor would be lagging a bit compared to the pH (the sensor values are also moving averages compared to pH-based readings) but if I monitor continuously, I could compensate for that with calculations. Regular pH measurement is not an attractive option, I would need to measure it too many times a day.

 

[hax47]

This is an amazing experiment. I’ve been playing around with ESP32s for tank automation (water changes, cooling etc) and am very keen on seeing if I can incorporate this somewhere into my sensor suite.

I can share the code and wiring if you need it.

 

[Andy Pierce]

Do you mean in the sensor chamber or the aquarium? It doesn't matter I guess. Technically doable, but I am not sure that the DC could keep up with the rapid CO2 changes in my tanks.

In the sensor chamber I was thinking. We hear a lot about how the drop checker lags behind the actual CO2 levels, but I've never seen that (lack of) responsiveness accurately quantified.

 

[hax47]

In the sensor chamber I was thinking. We hear a lot about how the drop checker lags behind the actual CO2 levels, but I've never seen that (lack of) responsiveness accurately quantified.

Good point. I think I could put the DC holder outside of the chamber to be visible, with air connection to the chamber.
The sensor is on a dark-start, cycling aquarium now, but I can pour a bottle of soda water to get some CO2 action in there. I'll schedule it for a weekend so I could take multiple DC readings during the day.

 

[dw1305]

Hi all,

a DC shows that there is a certain concentration of CO2 present, but not the exact amount of ppm.
Because your DC gets a green color at a CO2 concentration of 15ppm but you also have a green dropchecker at a concentration of 25ppm CO2

I understand where you are coming from, but they will be different in shade. The green colour comes from the mix of protonated (yellow) and unprotonated (blue) <"bromothymol blue"> molecules.

You definitely can differentiate differing shades of green <"The scientific background to the "Leaf Colour Chart"">, although this may not apply to those who <"are red / green colourblind">.

It is fair to point out that a drop checker does not come with a numerical readout for how green the greenness is, but there are easy ways to take this measurement digitally if that's important to you.

That would be my argument as well, via this very useful chart <"in your link">. Protonated on the left, and unprotonated on the right of the chart.

The problem with numerical readouts, and this is particularly true for pH amongst the digital measurements commonly made in the hobby, is that it's easy to generate a number but much harder to make sure that number is an accurate representation of the actual data you're looking for.

That is it, and it really matters with CO2 because you have every chance of asphyxiating your fish - <"CO2 Disaster">. I'm not a CO2 user, so I have no practical experience of adding CO2, and no "skin in the game" but my issue is that a pH meter could <"be wildly wrong">, but a drop checker will always give you a <"ball-park figure">, the only proviso being that <"it isn't reading in real time">.

cheers Darrel