Date: 10 January 1999
From: Daniel Gruenberg <jifasdeg@gol.com>
To: AQUA-L@killick.ifmt.nf.ca
QUESTION:
I am trying to design an experiment comparing growth rates in seawater with the only variable being pH. My idea is to fix pH controllers with HCl/NaOH injection on the filters of the tanks and setting the control values at three different levels. Obviously, the problem with different levels of free ammonia will be unavoidable (?). Is there any other (better) way to do this?
Daniel Gruenberg
Chief International Liaison,
R&D Team Leader
Japan International Food & Aquaculture Society (JIFAS)
9-5-26-606 Akasaka, Minato-ku, Tokyo 107 Japan
Tel: +81 3 3470-4302
Fax: +81 3 3470-3995
E-Mail: jifasdeg@gol.com
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COMMENTS 1:
Yes, your pH will influence the ammonia levels, but it will depend greatly on your system (flow though or re-circulating) bioload, temperature and efficiency of your biofilter, if one is being used.
Maintaining the ammonia and nitrite levels as close to 0 as possible
should avoid any toxic problems regardless of the pH. From your subject I'm assuming that you will be monitoring alkalinity as well, so the addition of regular water quality monitoring (ammonia, nitrite and nitrate) will alert you of any problems.
Trevor Keough
Ocean Sciences Centre
Memorial University of Newfoundland
e-mail: tkeough@morgan.ucs.mun.ca
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COMMENTS 2:
I suggest you try not to use strong acids/alkali such as HCl/NaOH, if
possible, try using combinations of weaker acids/alkali's to buffer the
water to a particular pH (lime/gypsum/acetic acid, sodium acetate, potassium dihydrogen phosphate etc). This will give you a greater margin for error. I found with adding strong acids the pH changes tended to be too dramatic and hard to control accurately (even when dilute solutions were used). Buffering would be easiest if you are working with a recirculation system. I've not tried it with seawater though.
I also found it important to allow some time for the pH of the water to
stabilise after you have added the acid/alkali, a stabilisation/mixing tank should be used after dosing and before the water enters your culture tank. The larger the volume the better, as changes in pH will be slower and easier to control.
There are a number of pH/ORP controllers available commercially (quite cheaply) which can be connected to variable speed volumetric dosing pumps (LMI have agood range) and used to control accurately the pH as you suggest. I have found these to be very accurate but the probes do need regular cleaning, calibration and checking though. A back up/alarm would be essential to avoid overdosing.
David Brown
e-mail: dbrown@mozart.inet.co.th