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pH pens and low KH

Sacha

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Been having a few issues trying to get an accurate pH profile so I'll try and make this as clear as possible...

I have a cheap pH pen from eBay. From what I can tell, it works extremely well. My reason for saying this is that, even after 3 months of sitting unused and dry, it still gets the pH reading perfect when I put it into buffer solution. I put it into buffer solution 7, and the pH pen reads 7 or 7.1. I put it into buffer solution 4, and the pen reads pH 4 or 4.1. So it clearly holds its calibration.

However when I tested the tank water with it this morning, it read 6.6. I find this hard to believe. The reason being that when I tested it, the Co2 had been off for 15 hours, and there is enough surface agitation to drive off any residual Co2 during that time. The KH of the tank water is 2.2. Based on the KH of 2.2, a pH of 6.6 would mean a Co2 concentration of around 17-20 PPM. That clearly isn't right, considering all the Co2 should have gassed off during the night. The concentration should definitely be below 10, meaning a pH of at least 6.8 or 6.9.

Secondly, I have taken pH profiles in the past with this pen. According to the pen, my pH took about 7 hours to drop from 7.2 to 6.4. That is clearly awful. However, the fish started to suffer at the end of the photoperiod, so I was left with no choice but to reduce the Co2 injection, and accept an absolutely terrible pH profile, with what I assume to be poor Co2 dissolution.

Now I'm starting to think that maybe the pH profile is bogus, and the pen is inaccurate.

So I suppose my question is this: is it possible that the pen is giving incorrect readings in the tank, despite the fact that it is extremely accurate at measuring the pH 4 and pH 7 buffer solutions? Will the pH 4 buffer solution have a higher KH than the tank? Is the pen unable to accurately measure pH of water with such low KH?

Or is it just that my pH really is 6.6? And if so, why is the Co2 concentration so high after a night of no Co2 and moderate surface agitation?

If the pen is giving me correct readings, then I have a serious problem with my Co2 injection, because I can't even achieve a full pH drop after 7 hours of injection.

If the pen is unable to accurately measure pH in such soft water, would a more expensive pen like this one be able to give me accurate and reliable pH readings?

I hope this all makes sense and I'm very grateful for any responses.[DOUBLEPOST=1407329293][/DOUBLEPOST]Also, let's say my pH is really 6.6. If I wanted to achieve a full pH drop before lights on, that would put the Co2 concentration well above 100 PPM.

I've never really understood this "full pH drop" gospel. In most tanks with a KH above 2, a full pH drop would put the Co2 well above 50 PPM and into lethal amounts. So why do we aim to have a full pH drop when that means having lethal doses of Co2?
 
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GlassWalker

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I remember reading on another forum that Seneye units had difficulty reading pH with low KH. I think in that case, it was around 4 where it started to have problems. I have to wonder if something similar is going on here.

I also have a cheap pH pen from ebay. That also calibrates ok, but I don't trust the readings it gives. I do have one more observation, which is to leave it submersed in the liquid to be measured for some time, at least a minute. I don't know if it is temperature or some other effect, but it does take a while to stabilize.

As hard as they may be to read, if you have one, a liquid based pH test may be useful as a 2nd opinion?
 

Sacha

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That was my very long rant about Seneye, about 2 years ago it started. Took such a long time for me to get a straight answer out of them!
 

GlassWalker

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Ok, so you've been though this before :) I debated moving my Seneye into my to be CO2'd tank but then remembered that so thought it wasn't worth it. I'm running on the low side of 4 KH so I guess it would be unreliable in that condition. It can stay in my marine tank where I struggle to keep KH above 7.

I do have and use a TMC pH controller now. Well, I use it as a meter, not a controller. These aren't cheap new but seem to go for peanuts used, which is how I got mine. As their common application in with marine aquariums, the probe condition is important to look out for which may be part of that depreciation. When I got mine, the cables and connectors had corroded badly. I replaced them and it seems to work fine after a calibration. Probes are also expensive to replace, so it was a bit of a gamble. It has two point calibration, where you adjust offset then slope. I found I could do either 4 and 7, or 7 and 10, but not all 3 at the same time. Error was up to 0.15 units for the point not calibrated so not that bad anyway.

Having written all that, I've not verified it yet at low KH. I was using it to observe tank pH prior to adding CO2. I estimated levels were about 5ppm based on reported pH and measured KH, so if that is about right for a non-CO2 tank it can't be far out.
 

Sacha

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My pH pen is now telling me the pH is 6.1. If that's true, then the Co2 concentration in the tank is around 50 PPM. Can that be correct, considering the fish aren't struggling?
 

Iain Sutherland

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With low kh you shouldn't have an issue dropping your ph. My tank sits at kh3-4 ish and ph drops from 6.6 - 3.5ish within 30 mins of the co2 coming on.

That's not to say my co2 is great in 30 mins, it still runs for 3 hrs prior to lights as the plants tell me that's what I need and the fauna are happy.

I don't understand the science side of it but I'm a sceptic to the charts as in theory all the fish should be dead and not sparring happily.

I would trust what's in your tank more than the pen and charts for sure.
 

Sacha

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The fauna are happy at Co2 levels well in excess of 200 PPM?
 

Andy Thurston

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Tom barr states kh/ph/co2 charts will always underestimate co2 levels in the tank. Theres an explaination in this thread
http://www.barrreport.com/showthread.php/12022-CO2-pH-KH-table[DOUBLEPOST=1407350094][/DOUBLEPOST]And don't trust that Chinese ph pen, i've got one and its rubbish compared to the hannah one i broke being heavy handed
 

Sacha

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Hmm interesting. The Hanna one I linked to- will that be reliable in low KH water?

According to my pen, I have only achieved a 0.7 point drop today, from 6.6 to 5.9. Also it's taken about 7 hours to drop that far. What could cause the pH drop to be so slow?
 

Andy Thurston

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The pen you linked is the one Iain uses. You could always buffer your water with extra kh so you know your pen is giving a good reading. Charts and test kits are inaccurate i only trust my tds meter for water quality, i use my ph pen to time my co2 period and find the lowest ph value, the drop checker is a quick day to day indicator. with a 1.4ph drop my fish were happy and drop checker was green yellow about 3hours after lights on(crappy delay on colour change)
 

X3NiTH

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From observation using a Weipro pH controller the probe has difficulty getting a reading in 0KH and TDS 0. It's fine with 0KH when there's a TDS reading. Your KH of 2.2 may not be accurate as you see that you do not correspond to the chart. From my own experience I deduced my tested KH not to be accurate by looking where I was on the charts in relation to my pH drop and what the drop checker was telling me, my measured 2.5 was in reality 0.5 or less according to the charts, my pH drop from start point at lights on to getting the drop checker green still correlated with the charts but I had to shift my results to make them fit. Very confusing, so I stopped looking at the charts and instead concentrated on the drop checker that had a known KH and just concentrated on making sure it went green. Through adjusting a few things to ensure enough oxygen was in the water before adding any co2 and that I get the one point+ drop in pH by lights on my plants and fish are fine and the drop checker is green/yellow. This all sounds easy but to get the tank where it needs to be I'm now chewing through a soda stream bottle in 15 days for a 28L volume than 30 days previously just by ensuring the increase in oxygenation of the water to stop the fish gasping.
 

Spnl

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KH has no effect on performance of the pH meter , but TDS (or conductivity) does.
You probably notice that the reading stabilises much faster in the calibration buffers than in water, because the buffers are strong solutions of ions.
It is usually said that at TDS of less than about 40 or so pH pens don't work, but mine is not very reliable even at around 90 or 100ppm, the reading depends a lot on how much the water is stirred. I can get an accurate reading but only with care.
PH pens are high maintenance, and as "test kits" go, they don't get much flakier. There is an excellent post on Apistogramma by "Regani" on how to maintain them properly.

I have to say I think a pH of 3.5 due to bubbling CO2 into the tank is not possible, you would need to increase the pressure to >1 atmosphere of CO2 to achieve that.
The pH/KH/CO2 charts are ok for natural water, but because they assume there are no sources of alkali other than KH and the only acid is from CO2, they give meaningless values for fish tanks (or even tap water!).

You read a lot of strange stuff about pH on various fish forums, I wonder how much of it is down to erroneous pH pen readings.
If the results look unbelievable I always follow up with a chemical test kit, they are not perfect either but at least you have two independent measurements.
Personally I don't understand why people don't use drop checkers, which seem to be out of favour for some reason.
 

Sacha

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I appreciate all the replies.

But the problem still stands. Every single pH profiling thread on this site lists pH and KH readings which, according to the charts, equate to lethal amounts of Co2. Does that mean that the charts are horrifically inaccurate? Or that everyone's pH meters are broken?

Like I said, in most tanks, a "full pH drop before lights on" means a lethal dose of Co2. So I don't know why we are encouraged to aim for that.
 

ceg4048

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No Sacha, your interpretation of the information provided to you is horrifically inaccurate. I have never instructed anyone to use the data in the charts to determine CO2 concentration levels. In fact I always tell people to NOT use the chart to determine absolute values of the CO2 concentration level. What we are using the charts for is to determine the differences. Mathematically, by using differences in the chart readings we are essentially subtracting the error associated with reading the charts as a single point calculation.

When we use the expression "ph drop" we are talking about the pH difference.
See the post http://www.ukaps.org/forum/threads/reliability-of-drop-checkers.19499/page-2#post-198360

Cheers,
 

Sacha

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I fully understand all of that.

Using the charts as an extremely rough guide, it's clear to me that in most water (above KH 2.5), a full pH drop before lights on means we have injected a dangerous amount of Co2.

Can't you see that? In most water, a full pH drop means we have injected well over 30 PPM.
 

dw1305

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Hi all,
KH has no effect on performance of the pH meter , but TDS (or conductivity) does.
You probably notice that the reading stabilises much faster in the calibration buffers than in water, because the buffers are strong solutions of ions. It is usually said that at TDS of less than about 40 or so pH pens don't work, but mine is not very reliable even at around 90 or 100ppm, the reading depends a lot on how much the water is stirred. I can get an accurate reading but only with care. PH pens are high maintenance, and as "test kits" go, they don't get much flakier. There is an excellent post on Apistogramma by "Regani" on how to maintain them properly.
That's the one, "Regani" is excellent on water chemistry <http://www.apistogramma.com/forum/threads/a-note-on-ph-meters.14870/>, I've learnt a lot from him.

You can add a salt like NaCl (has to be NaCl, "table salt" has magnesium carbonate (MgCO3) added to it) to the water sample, this doesn't effect the pH but makes the reading stabilise much more quickly. Have a look at this one (again via the excellent Apistogramma forum) <http://www.apistogramma.com/forum/threads/ok.12048/>.
Every single pH profiling thread on this site lists pH and KH readings which, according to the charts, equate to lethal amounts of Co2. Does that mean that the charts are horrifically inaccurate? Or that everyone's pH meters are broken?
The pH/KH/CO2 charts are ok for natural water, but because they assume there are no sources of alkali other than KH and the only acid is from CO2, they give meaningless values for fish tanks (or even tap water!).
This is the reason in "Spnl"'s very informative post.

Additionally I'd be very surprised if many people pH meters are accurate in softer water, pH is quite a complicated measurement and a pH value on its own doesn't tell you very much. That is one of the problems that occurs when people extrapolate from pH readings in buffered systems (like the Rift Lakes or Marine) to soft water systems. You get this a lot on some Cichlids forums, where certain aspects of keeping Apistogramma etc. and Lake Tanganyika cichlids coincide (diet, low BOD water, stocking etc), but they come from water at either end of the carbonate hardness spectrum and often successful Tanganyikan cichlid keepers attribute problems in soft water to "lack of pH stability".

In buffered systems pH will be fairly stable and changes in pH indicate significant changes in water chemistry, but in systems with few salts, and little buffering, pH is a "moveable feast".

cheers Darrel
 
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ceg4048

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Can't you see that? In most water, a full pH drop means we have injected well over 30 PPM.

No, you are missing a couple of key points. The charts are a very rough guide because they assume that the KH of the water is due to Carbonates - and that's not really true for most tap water. Your KH test kit does NOT measure KH, it measure alkalinity which is affected by a lot more ions than Carbonate. So right off the bat there is n overestimate of CO2 concentration on the charts. You will never have the opposite situation where the charts underestimates the CO2 concentration level. The second issue is that the pH readings are corrupted by acids in the water so the pH reading again, always overestimates the CO2 concentration.

So, both parameters are overestimates and that's why you're mistakenly assuming that we're adding well over 30 ppm. The 1 unit pH drop for medium to medium-high KH readings is about as good an estimate as we can get because no one knows exactly how much the non-Carbonate ions are contributing to the KH reading but generally that difference accounts for the error.


Cheers,
 

dw1305

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Hi all,
First of all I should say I've never added CO2 and have no intention of ever adding it, but because of the combination of 4dKH/bromothymol blue fluid and air gap, a drop checker will give you the actual CO2 concentration as shown on the CO2/dKH/pH chart (making the assumption that the chart itself is accurate).

Bromothymol blue is just a narrow range pH indicator.
223579_orig.jpg

Air gap
Because of the air gap you are only then looking at the HCO3 ~ CO2 (as H2CO3) equilibrium, and other acids, sources of alkalinity etc in the tank water are irrelevant. The downside is the time taken to equilibrate, which means that the CO2 level shown is not the current level.

DIY 4dKH solution

If you wanted to be absolutely sure you could make up your own drop checker fluid from NaHCO3 (12g NaHCO3 in 1litre of RO, diluted 1:100 in RO for the drop checker, will be ~ 4dKH) and bromothymol blue pH indicator (you can buy bromothymol blue indicator for £5 from "Better Equiped" or similar).

cheers Darrel
 
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