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Mg and pH

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wrightme43

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Colin, I have a very vauge understanding of pH, something about hydroxol ions and what not. Could you take the time to explain it, and what in the world magneisum has to do with unstable pH. Thank you in advance. Steve
 

cwcross

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Aug 18, 2004
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Hi Wrightme43. Let me answer the second question first as it is easier. The solubilities of all salts in water change with the pH. If you have an unstable pH, funny things can happen if it varies very much. However, many salts (magnesium carbonate) for instance can participate in a buffering mechanism. The magnesium itself won't participate but its associated anion might.

First question now. as you might have read in the phosphate thread, dynanic equilibrium is a central concept to nearly all apsects of chemistry. pH is the same. Imagine some water in a cup. Most people know that the chemical structure of water is H2O! However, most people don't know that water is really a salt. Water can and will dissociate into H+ and OH- cations and anions, respectively. In any glass of perfectly pure water a very tiny fraction of the water will always be dissociated into H+ (protons) and OH- (hydroxyl anions) at any given time. Furthermore, the ratio of H+ to OH- ions is fixed at a value called the "dissociation consant" or more generally the "equilibrium constant". Lets just pretend that the equilibrium constant is 1x10-14 (that is a 1 with 13 zeros in front of it...very very small). I will call this value K, just so I don't have to throw numbers around. Anway...in pure water... [H+]*[OH-]/[H2O]=K. Where [H+] is the concentration of protons in the bulk water and similarly for the [OH-] and [H2O]. It doesn't matter how much water we have the product of proton conentration and hydroxyl concentration divided by water concentration will always be equal to K. pH is defined as the pH=-log[H+], that is to say that pH is dictated by the proton concentration. Well, if water as always perfectly pure, the pH would always be the same. However, since we know pH isn't always the same, it must be true that water is never very pure. Any other compounds that can interact with hydroxyl's or proton's will affect the pH by altering the [H+] (proton concentration). Since [OH-] (hydroxyl concentration) is related to pH, pH can also be shown to be related to [OH-] also. Anway...if this isn't clear...let me know with a specific question.

Best Regards...Collin
 

wrightme43

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Its clear, I am just slow. I will read it a few more times and see if I cant get more out of it. Thank you for the quick response. I am very glad you are here.
 

cwcross

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Aug 18, 2004
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thanks for your welcome wrightme43. Don't feel bad. I have been learning chemistry since 85. I still get confused all the time with new stuff....C
 
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