Seachem Equilibrium and effect on PH and dKH - The Planted Tank Forum
 
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post #1 of 5 (permalink) Old 03-18-2015, 06:43 PM Thread Starter
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Seachem Equilibrium and effect on PH and dKH

I am confused about chemistry and wonder if someone can help me understand.

I am using RODI water and Equilibrium plus Alkaline Buffer to mix water. I always end up too hard (dGH) and too alkaline (ph) as well as slightly high on dKH.

Some of those fit -- too much dKH means higher ph, I get that, but it's not enough.

First some assumptions -- without CO2, with decent aeration, I assume I'm getting CO2 at about 3ppm, and if I look it up on various charts, that yields a dKH vs ph relationship. So for example, I was aiming at dKH=4, which should yield a PH of 7.6.

When I aim for four, I actually end up (according to API test) with a dKH=5, and a ph of about 8, as opposed to 7.7. Not far off, but why not closer?

So I did some testing... first, the density of the Alkaline Buffer (basically baking soda) seems to vary a lot. I got .19g/tsp a few months ago, today I get 0.16g/tsp, and Seachem's site says 0.14g/tsp. Seachem says go by weight not volume, but that's touch for small amounts, and this density variation could easily explain my dKH variation. Let's consider that "solved". But why is my PH higher than that implies?

So I did a test of Equilibrium - I mixed 4 gallons of RODI water, and started it mixing with a pump in a bucket.

With no additions, I measured PH < 6, and dKH and dGH as unreadable (first drop changed colors). As expected.

I added 1.2tsp of Equilibrium, mixed for about 30 minutes or so (big pond pump, small bucket, lots of aeration), and tested again. dGH was a bit high (7.5, should be 6), dKH seemed unchanged (not accurate enough to read if it changed, but very low), and ph a whopping 8.4.

Now I realize that lack of buffering makes PH more variable, but 8.4?

So I added back (as best as I could measure) the Alkaline buffer at about an 1/8th teaspoon, which should give dKH=2. It measured about dKH=3.5, and came in at PH=8.

Beside wondering why I still end up high on dKH, even allowing for dKH=4 the PH should be in the 7.6 range, not 8.

If I start WITHOUT the equilibrium, and use just Alkaline buffer, I still end up a bit high on dKH, but the PH seems to track the charts based on measured dKH.

It would appear that something in Equilibrium mixed to a moderate amount is giving (at least on the API high-range PH test) a PH indication about .3 to .4 too high (which is half as acidic of course). But not showing dKH change, at least that I can measure.

Since dKH in the test kit is (I think?) basically an acid titration, not sure how to interpret that.

Mostly this is just curiosity -- I can keep cutting back on Alkaline Buffer and/or Equilibrium until I get what I want, regardless of what the recipe calls for, but I would like to understand how this comes to be. In particular, how with something that doesn't show on a (admittedly very imprecise) dKH test, give such a dramatic swing on measured PH.

Am I just using too blunt of instruments for what I'm trying to measure?

Linwood

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post #2 of 5 (permalink) Old 03-18-2015, 07:06 PM
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My thoughts were identical with your last statement. I wonder if your test kit is in full calibration. We can only be as accurate as the instruments we are using.

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post #3 of 5 (permalink) Old 03-18-2015, 10:28 PM
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A basic problem with mixing things into the water to get the KH, GH and pH we want is that we are then forever stuck with trying to match those parameters when we do water changes. It is far better to use the tap water as it is, if possible. Of course that isn't always possible, but why introduce this problem if it is possible?

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post #4 of 5 (permalink) Old 03-19-2015, 02:26 AM
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Hoppy, I'm starting to agree with that! I was using RO to try and lower pH but it's really not working all that well.

today I put 4T of equilibrium in (mixed first) and my tank is still a little cloudy and still no gH reading. I thought it would at least bring it to 3. I almost hate to put that in the water. Even with the RO, my PH is up at 7.8 at least. I have trouble with those 3 orangy/brown shades.


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post #5 of 5 (permalink) Old 03-19-2015, 05:08 AM Thread Starter
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Quote:
Originally Posted by Hoppy View Post
A basic problem with mixing things into the water to get the KH, GH and pH we want is that we are then forever stuck with trying to match those parameters when we do water changes. It is far better to use the tap water as it is, if possible. Of course that isn't always possible, but why introduce this problem if it is possible?
I think saying that tap water is far better is a bit of an over-generalization, not everyone's tap water may be suitable; not everyone's personal predilection may not be to accept tap water as it comes out.

I'm also not sure what you mean by "trying to match... when we do water changes". If you mean "it's too hard to mix water the same way each time"... well, not for someone who can use a teaspoon and measure volume of water.

If you mean that the tank is different at the time of water change, and we need to adjust the input water to somehow match it... well, if true, that would be true for tap as well. My plan for RODI water is to simply mix it the same way each time, and use it. If the tank drifts (let's say it becomes a bit more acidic and consumes KH over time), then when I do water changes its PH will raise a bit each time, and reach some steady state of down/up. That's exactly the same thing that will happen with tap water.

My questions here are not so much about making it the same each time, but making it match a specific value, and why it is not. It's understanding the chemistry. I could do just the same as it sounds like you do with tap, and just use whatever I get once I mix it.

I do freely admit that there is no good reason to try for a specific value in what I am doing. But you know what -- I also try to park in the middle of a parking space, if I'm shooting basketball I try not to have the ball touch the rim, only net, and if I'm docking a boat I want it to just barely kiss the dock, not bump into it, even if that's what fenders are for.

After all... if I just wanted to see any old fish in a tank I could get a screen saver.

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Quote:
Originally Posted by natebuchholz View Post
My thoughts were identical with your last statement. I wonder if your test kit is in full calibration. We can only be as accurate as the instruments we are using.
Yeah... I can see that it is fairly precise (i.e. consistent), but especially for ph I do not have anything good to calibrate against for accuracy. I keep thinking about getting one of those handheld PH meters, or maybe a sensor to hook to the controller I have doing lights, but I worry that may just be yet another different number and then I need a tie breaker.

The thing that bothers me is the interaction with Equilibrium. Without it, KH and PH seem to track (in theory you should be able to measure one and know the other, right?) It's equilibrium that seems to be throwing me off - it is changing the PH I measure without changing the KH I measure (I say it purposely that way -- maybe whatever the PH test is, is effected by fresh Equilibrium in solution).

I'm putting the RODI away now, next time I get it out, maybe I'll do all the tests again from scratch, see if I get the same results again. And maybe pick up a dissolved CO2 test if they are cheap -- maybe I'm off on aeration and assuming about 3ppm co2. Heck, maybe Equilibrium somehow drives out CO2? Consumes it? I also admit I did not leave things for many hours to be sure I am in equilibrium (small "e") -- maybe it takes time for some of these things to settle down.

Just hoping for a chemist to stop by.. ...

Linwood

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