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Water Chemistry According To My Seneye

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Offline Helen

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Water chemistry according to my Seneye
« on: October 12, 2017, 10:59:28 PM »
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So I've set up my Seneye again and bought some slides. It is now giving me readings for pH, NH3 and NH4+.

After 24hrs of readings, the NH3 is stable at 0.016ppm. my Seneye is telling me this is good.

My pH seems to have fluctuated between 8.25 and 7.96 (considerably higher than I expected) The noticeable, although gradual dip being at 7am.

The NH4+ graph is essentially the inverse of the pH with a peak at 7am of 19.98ppb (yes, definitely b). From what I was reading last night, and my chemical engineer husband tried to explain to me in sentences I could understand, is that the pH and temperature affect how much of the N molecules are ammonium and how much are toxic ammonia.

In guessing that the reason I've not seen a corresponding change to the NH3, is because it is measured in ppm, not ppb or log.

What could be causing the pH to change?

By the way, the temperature seems to have been stable, fluctuating around 25.5 degrees (+/- 0.2). Also slightly warmer than I'd realised from my analogue thermometer (which I read as just below 25 degrees).

Also, my light controller failed last night, so the LEDs didn't come on till I turned them on about 6:15pm.

Oh and the Seneye is also telling me I have a dissolved oxygen content of 8.1. Which is also apparently ok.

I think my tank is ok?

Offline Andy The Minion

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Re: Water chemistry according to my Seneye
« Reply #1 on: October 13, 2017, 06:44:05 AM »
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@Helen Your husband (or significant engineer) is correct, the Senseye will in all probability be measuring total Ammonia, NH4 plus the toxic NH3, the amount of each is not stable and can fluctuate as a result of both pH and temperature.
Firstly, yes your water is fine. 'Zero ammonia' that is always quoted is not actually possible, it's just 'zero on the test strip/ texture' and you have a system that is more precise.
What supprises me was the low NH4 reading, without checking I was expecting this to be NH3 levels but I'm only a Mechanical engineer so Im probably wrong.
This aside I expect what you are seeing with the fluctuation is the effect of the light cycle on your plants, I'm guessing they come on sometime around 7am and the plants start consuming CO2 in the water which will start to increase the pH. Even if the total ammonia remains constant the toxic NH3? will increase as the pH increases. The plants will also consume Ammonia with the lights on so I was expecting the total reading to decrease slightly but maybe the reduction is too small to see

Offline Sue

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Re: Water chemistry according to my Seneye
« Reply #2 on: October 13, 2017, 09:29:27 AM »
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I agree with Andy. There is always some ammonia in the tank, it is just too low to register on our testers. With Seneye being able to measure tiny amounts, it can see this trace amount.
The 'ammonia' will be NH3 + NH4 together, like with test kits. NH3 in water is in an equilibrium, which I can't type properly as I am missing symbols on my keyboard. (I'll have to use the symbol for reversible equation instead of equilibrium)

NH3 + H+ <-> NH4+

Ammonia and hydrogen ions are constantly joining together and separating, and the amount that are in the NH4+ form at any given instant is temperature and pH dependent. The more acidic the water, the more H+ there is - this is the definition of an acid. With an equilibrium, the more there is on the left side of the equation, the more it is pushed over the right side. So increase H+ on the left (ie making it acidic) and the more it is pushed over to the right, into the less toxic NH4+
In basic water - pH over 7 - the amount of H+ in the left is lower, so the equilibrium is displaced towards the left, so there is more NH3 and less NH4- at higher pH.

I can always complicate things further by saying where the H+ comes from but I'll leave that for now  ;D

pH varies during the day particularly in a planted tank. Plants take up oxygen and make carbon dioxide 24/7 in respiration just like we do. But they also take up CO2 during daylight during photosynthesis and use this CO2 to make carbohydrates. The waste product of photosynthesis is oxygen.
The net effect is CO2 made all the time but CO2 taken up during daylight. There is more CO2 in the water at night than there is during the day. When CO2 dissolves in water it forms carbonic acid. So tank water is more acidic in the dark that it is in the light - the pH is lower at night than during the day. At 7 am the pH should be lowest because all the fish and plants have been 'breathing out' carbon dioxide during the night so there is more of it in the water at the end of the night than at any other time. As soon as it starts to get light the plants start to photosynthesise and remove some of the CO2 so the pH starts to rise.

Offline Helen

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Re: Water chemistry according to my Seneye
« Reply #3 on: October 14, 2017, 12:22:30 AM »
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Thanks @Sue. Really interesting explanation.

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