Tropical Fish Forum
Tropical Fish Keeping Help and Advice => Fishtank Filtration and Cycling => Topic started by: Andy The Minion on May 01, 2016, 08:24:04 PM
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Dear All,
I was wondering if there is a chemist in the house?
I would like to create a calibration curve for a pH test kit so I can use a Colourimeter and a wet test kit to accurately measure pH.
I have both pH4.0 and 7.0 buffer solutions that I used to calibrate a pH probe until it started playing up.
Is it possible to mix these two in different proportions to create intermediate pH values rather than assume the colour response of the test kit is linear?
The plan would be to have 4.5, 5.0, 5.5, 6.0 and 6.5 then carry out a pH test and measure there light absorption with the colourimeter to make a colour - pH curve for a particular test kit to replace the stupid colour chart
Regards,
Andy the minion
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I was wondering if there is a chemist in the house?
Me :) Though my degree is rather old now, and I've forgotten most of what I knew back in the 1970s.
The problem with pH is the scale is logarithmic not linear. pH is the reciprocal of the logarithm of the concentration of hydrogen ions. So pH 7.0 is 10 x pH 6.0.
To be honest, anyone who doesn't like the colour charts would be better off buying a pH meter. It would probably be cheaper than a colorimeter.
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Thank you Sue,
Yes its true a pH probe is cheap but mine failed and I already have a low cost colourimeter ($85 see post on improving water tests) that I use for other parameters and it produces really repeatable, accurate results for NH3, NO2, NO3, PO4 so I wanted to complete the set.
I was aware of the log relationship, so it could be that the volumes of the two buffers would make mixing them with sufficient accuracy impractical.
Maybe I should ask how to make pH buffers from first principles?
Regards,
Andy the minion
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Buffers from first principles here (http://delloyd.50megs.com/moreinfo/buffers2.html). Good luck ;)
I really like what you're doing with this sort of stuff, Andy. Keep it coming.
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I was wondering if there is a chemist in the house?
The problem with pH is the scale is logarithmic not linear. pH is the reciprocal of the logarithm of the concentration of hydrogen ions. So pH 7.0 is 10 x pH 6.0.
:yikes: I didnt understand a word of that!
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I use a pH meter that cost just a few pounds from EBay. Last year, I worked with a consultancy that used one that they had bought for £200+ from a scientific instrument supplier. Our results were exactly the same.
Trying to make a whole range of buffer solutions and using a colourimeter seems to me like a lot of effort when a cheap pH meter gives you a direct readout simply by dipping it in the tank.
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Indeed. And by the time you've bought all the chemicals and balances etc to make your own buffers, two things will have happened:
1) It'll have cost as much as a really good pH meter
2) The police will have battered down your door and pinned you to the floor 'cos it'll seem like you're manufacturing drugs. :))
Just a thought. ;D
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Thanks for the heads up Colin, sounds like you have connections in the sleazy world of class A street buffers Trust me, I won’t be upsetting you in a hurry. :)
Perhaps I have been unlucky but I thought had a really good pH meter from Extech, it had a good write-up, very good spec, replaceable probes and could accept other probe types to measure ORP as well. So my logic was that it should have out-lasted the probes that don't live forever, I wouldn’t have to buy test kits and it would more importantly be more accurate as I was injecting CO2.
Within a week of getting it the pH reading started constantly cycling 0.3pH in the calibration buffer. That was in Jan of this year, I'm still trying to get my money back. Grrrrr !
If anybody is thinking of buying stuff from Armco or Air concern JUST DONT !! Their tech and customer support royally stinks and I have 59 (and counting) Emails to prove it.
I'm not sure about the costs of the materials, you may be correct but in many cases a kg of lab quality chemical (Colin, that's 'gear' in your circles) is very cheap. Plus half the fun for me is the learning process, and yes I am aware that this is weird.
The way I justify it to myself is as follows.
A pH probe always has to be calibrated at two points and fairly regularly - at least monthly therefore buffer is a consumable item at about £20/litre and you need two buffer solutions. My calibration curve would be a one time job and once I have it the software just loads it into the colourimeter each time I do a test.
The only thing a colourimeter does is to replace the human eye when it comes to reading the colours. Once it knows the relationship between the sample colour and pH from the calibration curve it is many times more accurate than the eye and the stupid colour cards that test kits provide. [The scraping sound you hear is me getting my test kit colour card soap box out - sorry]
I now get repeatability between NO3 tests (different water samples from the same tank) within 2 or 3ppm. I absolutely know it’s not important for NO3 to be that accuracy, but it’s the test I do most regularly so I have the most experience and can quote numbers. This compares to ‘Hey Pet, what do you think this is 40 or 80ppm?’ with a card.
It always seemed strange to me that in the case of Ammonia the rule is ALWAYS ZERO yet my kit starts reading at 0.25ppm and goes up to 8ppm ! - where 0.5ppm is said to causes stress and harms fish. On some of the cards it’s hard to see the difference between two colours or as you open a new pack you notice a significant difference between the two. This just seems wrong to me and devalues the test kits.
My last Ammonia test was 0.07ppm and I would be confident that if I retested the same water I would be within a couple of hundredths of a ppm of this reading, with this resolution I can spot a problem well before it affects the poor blighters in the tank.
Yes I hear you shout, but this would read zero on a card. True I would shout back, but only if the card colour match is correct, and then we all go off in a big huff :)
Sorry this went off the pH topic a tad.
Regards, Andy the weird minion.
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Well, you can get a pH meter for about £6 from EBay. I use one for pond surveys for my work. I reuse the buffer solution for multiple calibrations, it only needs a small amount anyway.
I admit I haven't tested my water in my tanks for months. Why bother to keep testing unless there's reason to believe there's something wrong? A tank that is correctly set up should stabilise after a while. I think it's possible to become obsessed with test readings. In any case a high degree of accuracy isn't required for an aquarium, fish all have a more or less broad range of tolerance to water parameters, which is what they would get in nature.
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Andy, just a thought I should've thunk a while ago.
A colourimeter is for testing density of colour (which is great for nitrites, nitrates etc) or for colour end points (like GH and KH), but pH is a colour change rather than a gradually increasing density of one colour. So, my postulate would be; is a colourimeter a suitable instrument for measuring pH?
Answers on a postcard to the usual address, please. :)
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ColinB,
I don't think it will be a problem mine has three colours of LED and I can select just one or a combination then pick the best curve during the calibration, it doesn't have to be linear fit either. If it can cope with the ammonia and nitrate colour charts I attached it should cope with other colour shifts.
By the way thanks for the buffer link, I had found and then lost that site some time ago when looking for the pH probe. I see caustic soda and citric acid feature quite a lot and I have both for general household descaling and cleaning.
I'm off to see how accurate the scales need to be to give an accurate mixture, I can see a hack approaching so I don't have to create a bucket full of mixture.
Regards,
Andy the minion
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I'm kind of confused why you'd need such accurate readings, unless you're keeping an especially delicate fish.
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Fiona, I don't think I do need the accuracy this method gives, not for the fish at least. I started using the method because it takes the guess work out of the test.
However it allows me to see the rate of increase of any of the parameters very easily because of the repeatability and resolution. I didn't realise it at the time but I can now tell how the external filter is doing. As the muck build up in it the rate of Nitrates produced between water changes increases because the muck is trapped by the filter and start breaking down. I have a well stocked tank but keep the Nitrates down around 10-15ppm, if by the end of the week I see the level has increase by much over 6ppm I know its time to heave the external filter out - otherwise I let it run without disturbance for 10 to 12 weeks.
The other advantage is that I could detect very small amounts of the dangerous waste well before they become a problem.
The test doesn't take any more time or effort than normal test - the meter just reads the colour and its displays the exact value in ppm.
I know a lot of people wont ever get why I bother about it, but water is THE critical thing in the hobby and I started it because the colour charts are just so wrong and then discovered the added benefits afterwards.
Ill stop going on about it - it know will just get annoying - if it didn't already :)
Regards,
Andy the minion
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ColinB,
I was just looking at your link again
http://delloyd.50megs.com/moreinfo/buffers2.html
Then I noticed the company name in the top line. Oh my! I'm looking as shocked as your avatar.
Perhaps they have a range of suppositories ?
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:rotfl:
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mmmm - a very poor choice of product name. Puts a whole new meaning on 'product placement'! :P
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Heeheehee :rotfl: