now i'm confused....

[Mr Burns]

what is the difference between pH and acidity/alkalinity of water? i was reading an article last (quoted below), and the biggest line that threw me was "if you want to lower pH, be prepared to lower alkalinity as well." that confused me because i thought pH was a measure of acidity/alkalinity in the water (H+ and OH- ions.) so i read it again today during class and it seemed like it was referring to alkalinity as the buffering capacity of water (mineral content like gH and kH), and that if you don't lower that buffering capacity first, you won't be able to effectively lower your pH.

so what's going on here?
am i making a mountain out of a mole hill?
i'm all mixed up!

I would like to strongly suggest, however, that you not do lower pH by putting something into the water, with the exceptions of peat extract and CO2. And if you use CO2, be very careful and be prepared for fish losses if your CO2 injection breaks down. The reason for this is simple; by lowering pH without simultaneously buffering the water to the lower value, you are depressing pH below its natural level. This creates an inherently unstable pH. When the pH (inevitably) returns to its natural level, your fish will be stressed by the changes in water chemistry.

So if you want to lower pH, be prepared to lower alkalinity as well. But be forewarned that this is not without its own potential dangers. The lower water’s alkalinity is, the less stable its pH becomes. This is because fish produce acidic wastes that can dangerously lower pH in low-alkalinity tanks if these wastes are allowed to build up. You must monitor pH more closely and water changes become extremely important if you lower your tank’s alkalinity. If you find that the pH of your water drops continuously, you must reduce the fish load, do more or larger water changes, clean the filters more often, and consider raising the alkalinity to a somewhat higher level

http://www.aquaticquotient.com/aq_articles/data/103/page0.php?page=0&cat_id=3&article_id=103

 

[Endgame319]

I think the article is confusing alkalinity with buffering capacity. pH does measure whether a not a substance is acidic or akaline (or basic)

 

[BrianNY]

Exactly Endgame. However KH and GH have an effect on the buffering capacity. Harder water will tend to be alkaline, while softer water will tend to be acidic. I think that article is trying to say there is no point in trying to lower the PH unless you lower the hardness as well.

 

[Mr Burns]

so i was right? the guy is just confusing alkalinity with buffering capacity?

brian, i know what you're saying. here in cincinnati, we have very hard tap water with a high pH, which is consistent with your comment. the diy co2 has lowered my pH to around 6.4, something that has NEVER happened in my tank before. i'm actually kinda happy that i can have hard water and a lower pH...but at the same time, i'm doing a 10% every two days to keep the buffering capacity up.

i'm thankful to have a python

 

[Allivymar]

Here's a nice simple explanation of pH, gH and kH: http://faq.thekrib.com/begin-chem.html

 

[QTOFFER]

I can understand the confusion - it's due to some unfortunate overlapping nomenclature - darn chemists!

Some of this you already know... I shamelessly cut and pasted most of it from the sites listed below.
Sorry for the extremely long-winded post, but I hope this clears up the confusion - it did for me!


pH is a negative logarithmic measure of hydrogen ion concentration. Mathematically, it is expressed as pH=-log[H+] where log is a base-10 logarithm and [H+] is the concentration of hydrogen ions in moles per liter of solution.
A solution containing a high concentration of H+ has a low pH and is called acidic while a solution containing a low concentration of H+ has a high pH and is called basic or alkaline.
To avoid confusion, use the term basic instead of alkaline when referring to pH!



Alkalinity (kH) is a measure of the buffering capacity of water, or its ability to resist change in pH. It is expressed in units of milligrams per liter (mg/l) of CaCO3 (calcium carbonate). The primary buffering compounds in water are bicarbonate (HCO3-), and carbonate (CO3--). Occasionally hydroxide (OH-), borates, silicates, phosphates, ammonium, sulfides, and organic compounds (e.g. acetates) contribute to alkalinity.
Water with high alkalinity is able to resist major shifts in pH. As increasing amounts of acid are added to a water body, the pH of the water decreases, and the buffering capacity of the water is consumed. If natural buffering materials are present, pH will drop slowly to around 6; then a rapid pH drop occurs as the bicarbonate buffering capacity (CO3-- and HCO3-) is used up. At pH 5.5, only very weak buffering ability remains, and the pH drops further with additional acid. A solution having a pH below 4.5 contains no alkalinity, because there are no CO3-- or HCO3- ions left.



Hardness is a measure of dissolved compounds in water. Most test kits measure hardness in German degrees, which is equal to 17.9 mg/L or ppm.  
Carbonate hardness (kH or alkalinity) refers to the hardness derived mainly from carbonate and bicarbonate ions and directly reflects the pH buffering capacity of the water.  
General hardness (GH) refers to the dissolved concentration primarily of magnesium and calcium ions.
(To make matters even more confusing, magnesium and calcium are members of a group of elements called the alkaline earth elements!)
Other ions can contribute to water hardness but they are usually insignificant and difficult to measure.  When fish are said to prefer ``soft'' or ``hard'' water, it is gH, not kH that is being referred to.  

sources:
http://www.drhelm.com/aquarium/chemistry.html
http://www.chelonia.org/Articles/waterchemistry.htm
http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/what-is-pH.shtml
http://bcn.boulder.co.us/basin/data/NUTRIENTS/info/Alk.html.

 

[Mr Burns]

excellent link, allivymar! that clears it all up.

 

[Mr Burns]

D'OH! double post...