Reef Chemistry Cheat Sheet

[sihaya]

Like most of you probably, when I first got into this hobby, the chemistry all seemed a bit daunting. And for awhile I had some trouble keeping everything straight. I also had a lot of questions no one at the LFS felt like answering (apparently). Now that I'm oh so much older and wiser, I wanted to share my "cheat sheet" with you all... and include some answers to those questions I couldn't get at my LFS.

Broken down to basics, we're just talking about charged particles (ions) in water. The ones we care a lot about are H+(hydrogen), Na+(sodium), Ca2+(calcium), OH-(hydroxide ion), Cl-(chloride), CO3^2- (carbonate), HCO3^-1 (bicarbonate). The positively charged particles match up with negatively charged particles to make different chemicals. It might sound a little complicated, but don't over think it. It's kinda like putting magnetics together...

This chart shows how the different combinations of positive and negative ions give us our basic inorganic additives:

So where do these ions come from? Well, check it out...

1. What's the difference between pH and alkalinity?

pH is short for "power of Hydrogen." It's the concentration of H+ in a solution (aka acidity).

Alkalinity is a more complicated beast. In function, it's a buffer. Buffers inhibits sudden changes in pH over a certain range. The ability of the buffer to prevent sudden changes in pH is called a "buffer capacity." As we usually use the term in the hobby, alkalinity is the "buffer capacity" of our water. Bicarbonate and carbonate are the major contributors to alkalinity. They're not the only contributors, but in our tanks, they're by far the most important. (The contribution of borate is an interesting topic, but I don't want to give you all a headache. If you're curious, here's RHF great article on that: Boron in a Reef Tank (and its effect on pH buffering))

2. Is aragonite a good Ca/alkalinity suppliment?

No. The answer as to why is a bit complicated. But basically, there's already so much Ca and CO3 in the water already, that additional CaCO3 isn't going to dissolve unless you put it in more acidic conditions. This *can* happen in an aragonite sand bed. But if and when it does, it's not a good sign.

3. Why does calcium go down when alkalinity goes up and vice versa?

It's a complicated thing I could never hope to explain better than Mr. Farley has here: Calcium and Alkalinity Balance Issues

4. What do the alkalinity tests actually test for?

Most don't test for any one compound. Most of them are titration kits that give you an idea of buffer capacity by finding out how much acid it takes to break the buffer (the point at which the buffer can no longer inhibit the change in pH). For more info on all that: What is Alkalinity

5. Where do dissolved silicates come from?

Lots of places (like tap water)... but not from silica sand.

I hope this can be of some help to anyone starting out.

 

[4251cpd]

Nice job padawan. And welcome, if no one has already rolled out the red carpet for ya. Feel free as you did to bring up stuff, as everyone is willing to learn. Mike.

 

[Boomer]

Well I like your sheet but CO2 has no effect on Alk as indicated by the sheet, unless we are dealing with something like calium reactors or kalk. If you blow CO2 into water it will not change the Alk but will lower the pH no matter how high the Alk is.

Also on Kalk you left out buffer, kinda, as it adds to Alk.

Ca(OH)2 + CO2 ==> Ca++ + 2HCO3- (2-bicarbonate ions=buffer)

C(O3)2- (carbonate), H(CO3)-

It is better to write these as CO3-- and HCO3- or CO3^-2, CO3-2 and HCO3^-1 or HCO3-1. Your method makes it look like the 03 has a charge of -2 and it is the whole ion that has a charge of -2 or the CO3 has a charge of -1 in the HCO3, again it is the whole ion.


You needs to be careful with the word Acidity, which is the opposite measurement of Alkalinity. It is better to just say more acid or alkaline (high pH)

On your pH you should have linked Randy's other article.

The Relationship Between Alkalinity and pH.
http://www.advancedaquarist.com/issues/may2002/chem.htm

Buffers inhibits sudden changes in pH over a certain range

But don't forget there are buffers that also lower pH/Alk, like sodium biphosphate. We are using Alk buffers and not acidity(Acy) buffers.Think pH buffers of 4 pH, like those for calibrating meters.


Alkalinity is really the ability to neutralize an acid, often called ANC (acid Neutralizing Capacity). Buffering Capacity is the ability to resist downward or up ward trends in pH. Alkalinity is a measurement of Buffering Capacity but it is not Buffering Capacity. It is Tricky subject. Bc is measured in meq / pH unit.

All in all it is a good post

Where did those charts come from I've never seen them. They are not Randy's

 

[sihaya]

Thank you Boomer. And thanks for the added detail. I understand all you're saying. I was only trying to keep things simple here in the beginner's forum ;-)

With regard to Alk and pH, as Holmes-Farley explains in this article (http://www.advancedaquarist.com/issues/feb2002/chemistry.htm), the term "alkalinity" can be used in many different ways. And to a lessor extent, so can "acidity." But again, just to keep things simple, I used the Arrhenius definition. Indeed, there are more "modern" definitions that don't at all consider H+ to define "acidity." In any case, I'm not sure why you'd say alkalinity and acidity are "opposite" measurements. I suppose they could be thought of that way if you thought of alkalinity as pOH and acidity as pH. Not sure about that one...

And yes, I know there are all kinds of buffers. As I understand it, a buffer is classically any weak acid and its conjugate base. So, theoretically, I'd think there would be as many potential buffers as there are weak acids. However, I honestly haven't thought much about what other buffers there are in sea water. I just thought I'd focus on the most important one here. Thanks for the clarification.

The charts are mine. I made them just to keep track of things. The info in them was pulled mostly from Farley's articles though (that and my college chem lab book and wikipedia).


Thanks again

 

[4251cpd]

Boomer, speak ingles por favor. You make this seem too much like work and not a hobby :lol: , you da man.

 

[sihaya]

Nice job padawan. And welcome, if no one has already rolled out the red carpet for ya. Feel free as you did to bring up stuff, as everyone is willing to learn. Mike.

Well thank you! Nice to see so many smart, welcoming and helpful people around here.

 

[sihaya]

Boomer - btw, do you by any chance know a Jesse from Akron, OH? I thought he told me he knew you once (I could be confused though). I used to work for Jesse... scrubbing algae and salt creep. Fun stuff. lol

 

[4251cpd]

Well thank you! Nice to see so many smart, welcoming and helpful people around here.

You'll never see smart and myself ever in the same sentence. You must share with the rest of the class on your own set up.

 

[sihaya]

You'll never see smart and myself ever in the same sentence.

Oh, I was talking about Boomer there, not you. Sorry. .... j/k!

My set up has been through hell recently, but it's surviving. I just replaced the main tank (yes, the actual tank) because it was leaking. It had just been recovering from a pesticide posioning too. It's just been chaos lately. But it's a 65g display tank with a 29g reverse photoperiod refugium and it's the love of my life.

 

[4251cpd]

Never claimed to be smart, just saucy. When it gets situated you are required to take pics and have them critiqued by Krish. No nudies.

 

[sihaya]

lol Saucy! I love that word! I haven't heard it in awhile. Thanks.

I will take pictures once it all gets settled again. I'll tell you one thing, my glass has never been cleaner! lol ::sigh::

 

[Boomer]

Jesse from Akron, OH?

Does not ring a bell. Could also be on of my two evil twins, Bomber and BoomerD both great guys and posters.

I would not want to take your scheme and over complicate it, if you use it elsewhere or on other posts I would leave as is, with a couple of very samll changes.


1. It is better to write these as CO3-- and HCO3- or CO3^-2, CO3-2 and HCO3-1 or HCO3^-1.

2. CO2 has no effect on Alk as indicated by the sheet With CO2 there people are going to get the idea CO2 can change the ALk. Remove Alk from list

3. Kalkwaaser; Add Alk to list

You could also add Sodium Carbonate/Washing Soda / Baked Baking Soda to the chart. You already have it in the top table.

Also I think it would be better if your links read as follows, so one knows what they are clickin' on

The Relationship Between Alkalinity and pH.
http://www.advancedaquarist.com/issues/may2002/chem.htm

What is Alkalinity
http://www.advancedaquarist.com/iss...2/chemistry.htm

Boron in a Reef Tank (and its effect on pH buffering)
http://www.advancedaquarist.com/issues/dec2002/chem.htm


Calcium and Alkalinity Balance Issues
http://reefkeeping.com/issues/2002-...ature/index.htm

 

[sihaya]

Alright, alright, since you're the resident chem pro I changed it. ;-)

 

[Boomer]

I like it and more than likely will be using it Sara

 

[sihaya]

awesome, thanks again

 

[Boomer]

oop's I found a typo Sara SIO3, no such thing, other than in metasilicates. It is SIO2 = quartz. You can also add pH to baking soda on the list. You should also add this to the pH section. As it talks about CO2 and its affect

The Relationship Between Alkalinity and pH.
http://www.advancedaquarist.com/issues/may2002/chem.htm



I'm not sure why you'd say alkalinity and acidity are "opposite" measurements. I suppose they could be thought of that way if you thought of alkalinity as pOH and acidity as pH. Not sure about that one...

Acidity or Acy is called BNC (Base Neutralizing Capacity). There are test kits for it Alkalinity deals with ions that affect or can raise the pH or Alk. Acidity deals with "ions" that deal with lowering the pH and acidity, i.e.. H+, H2HCO3, CO2. There is a more or less a direct relationship between the two. We could say that Alk and Acy = true Buffering Capacity ( Bc or Bi). In general as Alk increases so does Acy. Just like when Alk increases so does CO2 when you add a carbonate buffer. Acy/Acidciy is discussed in almost any water or aquatic water chem book.

In short terms for a pure carbonate system

Akalinity (TA) = [HCO3-] + 2[CO3--] + [OH-] - [H+]

Acidity (Tacy) = [H+] + HCO3- + 2[H2CO3]- [OH-]

HCO3- is an amphoteric product, meaning it can act as a base and acid. For example; at a pH of say 10 it is an acid and at a pH of say 4 it is a base.

In most peoples minds no need to test or talk about Acidity, just like in most peoples minds there is no sense in testing for or talking about pOH

 

[sihaya]

Oh, duh... that SO3 is a typo. My bad. Thanks. Let me fix that...

 

[sihaya]

Well, I learned something new the hardway the other day. Apparently, new aragonite is loaded with phosphates that like to grow brown snot in my tank. lol

 

[steve-s]

Apparently, new aragonite is loaded with phosphates that like to grow brown snot in my tank. lol

Actually I think you'll find that is primarily from a silicate problem although the P will contribute. The same would be true of crushed coral although it would be heavier laden with P than with Si.


Cheers
Steve

 

[sihaya]

Where would they come from? I use R/O water.