Word for the Day - Alkalinity

[ellebelle]

Alkalinity: a measure that indicates that amount of hydrogen ions it will take to lower your pH to 4.2 through the creation of carbonic acid (carbonic acid = dissolved carbon dioxide).

Most test kits measure the total alkalinity of your water, which includes predominantly the amount of carbonate, bicarbonate, but also the amount of some other compounds (including phosphate, if it high).

What does this mean? We want a certain amount of carbonate and bicarbonate for our corals, so we use total alkalinity as a measure for this. The more carbonate and bicarbonate present in the water, the higher the alkalinity of the sample. Adding too much carbonate or bicarbonate (in the form of two part of kalk) will drive your pH up because it is increasing the alkalinity of the water (i.e. it for every new bicarbonate molecule dumped in, it will take one more H+ ion to make carbonic acid and bring the pH down)

If you have high phosphates, most alkalinity test kits will not necessarily be an accurate reading of how much carbonate or bicarbonate you have in the system as phosphate in high number can affect the total alkalinity of a sample.

(If I have screwed something up, please feel free to correct me. I am not a biochemist. )

 

[Harif87]

Isnt alkalinity also the measure of how basic a solution is? I know the term is used in relation to carbonate hardness because crabonates resist changes to a lower pH. But any in that case any buffer can be spoken about in terms of "alkalinity", is that how the term is used? Ive always been under the impression that alkalinity only referred to bi/carbonates .....

I find the whole alk topic so confusing! The more i know the more questions come up lol But ive never heard it used in terms of being a measure that indicates how much H+ is needed through the generation of carbonic acid. As far as my knowledge goes (and it might not be that far :p) it should be the amount the solution can buffer against any acid. That is to say that its a measure of how well it can accept protons -be it from any source- and still resist pH change.
Do you mean o bring the example of carbonic acid because its the most common proton donor in our aquariums?

This is a great word of the day

 

[ellebelle]

Being "alkaline" and being "basic" are slightly different. You can raise the pH of a solution by adding more carbon dioxide and make it more basic, but it won't necessarily change the alkalinity.

There are actually a whole bunch of different compounds that contribute to the total alkalinity of our aquariums (borate, silicate, etc.), but bicarbonate is the major contributor (somewhere around 90%). But if you have an unnaturally high concentration of one of the other compounds, then you can have a high total alkalinity without necessarily having a high amount of bicarbonate.

I used the carbonic acid example because that's the typical reaction in the marine aquarium

H+ (hydrogen ions) + HCO3- (bicarbonate) --> H2CO3 (carbonic acid)

Since everything I've read has to do with aquariums, this is what I've seen. But I imagine if you have some other substance that "buffers" pH, it could result in a different acid.

 

[Deanos]

Alkalinity

 

[Harif87]

You can raise the pH of a solution by adding more carbon dioxide and make it more basic, but it won't necessarily change the alkalinity.

Dont you mean that by adding CO2 you would decrease your pH making it more acidic?

But I imagine if you have some other substance that "buffers" pH, it could result in a different acid.

There are many types of buffers, each buffering to its specific region. A buffer is made simply of acid/base conjugate pairs where both an acid and base are present. Depending on the conjugate pairs found in the buffer it will buffer a certain region. So its not that any other type of buffer would result in a lower pH

(did i misunderstand what you meant to say in that scentence? Sorry if i did :spin: )

 

[piranhapat]

What a fool I was for not testing my water before I started adding two parts. I started a sps tank about 4 months ago. I just put in SPS. I started to add a little of two parts (EVS). Not much. I saw SPS under stress by not opening up like I started. So I did a check of all the Water and my Alkalinity was 12.0. My question even if a I started adding in equal parts and every thing was good. Why Alkalinity went high. PH WAS ABOUT 8.1 CAL 400. I STOP DOSING FOR NOW. SPS ARE STARTING TO OPEN AGAIN. Alkalinity going down. Did it matter if the salt that am using which is Salinity by seachem. Had All bells and whistles. That I didn't need to add unitl I tested the tank water. Does bio pellets have any thing to do with Alkalinity. I started tank with it so no change in that. I'm running two reactors with bio pellets.

 

[FaviaFreak]

http://www.wetwebmedia.com/alkalinity.htm