- #1
Vulgar
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Hi,
My question is about the definition of alkalinity, I found this one:
Dickson (DOE, 1994; compare also Dickson, 1981): "The total alkalinity
of a natural water is thus defined as the number of moles of hydrogen ion
equivalent to the excess of proton acceptors (bases formed from weak acids
with a dissociation constant K < 10 -4.5, at 25°C and zero ionic strength)
over proton donors (acids with K > 10 -4.5) in one kilogram of sample. ''
Total Alkalinity = [HCO3-]+ 2[CO3-2] + [B(OH)4-] + [OH-]
+[HPO4-2] + 2[PO4-3] + [H3SiO4]- +[NH3] + [HS-]- [H+] -[HS04-] - [HF] - [H3P04]...(1)
So when u start adding a strong acid and eating the alkalinity away u get to a point where TA (total alkalinity) is zero (equivalent point). Then u get the proton condition:
[H+] +[HS04-] + [HF] + [H3P04]
=[HCO3-]+ 2[CO3-2] + [B(OH)4-] + [OH-]
+[HPO4-2] + 2[PO4-3] + [H3SiO4]- +[NH3] + [HS-]..........(2)
You can compare the state of the equivalent point with a solution u made from pure water and where u add H2CO3, NaH2PO4, B(OH)3, NH4Cl, NaF, Na2SO4 and H4SiO4 in all the equivalent amounts. You can't find these subtances in the proton condition because they are by definition the zero proton levels.
My first question is, why is H2SO4 not in (1) and consequently (2)? Seeing that SO4- is considered zero proton level, H2SO4 should be in the left side of (2), seeing as it is considered a proton acceptor.
My second question is concerning the fact that NaH2PO4, NaF and NaSO4 are the zero proton levels and not H3PO4, HF and H2SO4. When H2PO4- is formed it can still absorb an equivalent amount of H+ by forming H3PO4, so actually there is more alkalinity then (1) says. The same for F-, it can take H+ to form HF. So my definition would be:
Total Alkalinity = [HCO3-]+ 2[CO3-2] + [B(OH)4-] + [OH-]
+[HPO4-2] + 2[PO4-3] + [H3SiO4]- +[NH3] + [HS-] + [F-] + [H2PO4-] + [HSO4-] +
2[SO4-] - [H+] .................(3)
So (3) doesn't look at pKa values of all the acid-base systems like (1) does, but just gives all neutral substances zero proton level, and the alkalinity of (3) will be higher than (1). Now your proton condition at the equivalence point after adding a strong acid is:
[H+] = [HCO3-]+ 2[CO3-2] + [B(OH)4-] + [OH-]
+[HPO4-2] + 2[PO4-3] + [H3SiO4]- +[NH3] + [HS-] + [F-] + [H2PO4-] + [HSO4-] +
2[SO4-] ...................(4)
and this will be the same as a solution starting from pure water and adding equivalent amounts of H2SO4, H2CO3, H3PO4, B(OH)3, NH4Cl, HF and H4SiO4
And my last question is, how can u determine the total alkalinity according to (1) experimentally of an unknown sample, when do u know when to stop, is there some pH jump?
My question is about the definition of alkalinity, I found this one:
Dickson (DOE, 1994; compare also Dickson, 1981): "The total alkalinity
of a natural water is thus defined as the number of moles of hydrogen ion
equivalent to the excess of proton acceptors (bases formed from weak acids
with a dissociation constant K < 10 -4.5, at 25°C and zero ionic strength)
over proton donors (acids with K > 10 -4.5) in one kilogram of sample. ''
Total Alkalinity = [HCO3-]+ 2[CO3-2] + [B(OH)4-] + [OH-]
+[HPO4-2] + 2[PO4-3] + [H3SiO4]- +[NH3] + [HS-]- [H+] -[HS04-] - [HF] - [H3P04]...(1)
So when u start adding a strong acid and eating the alkalinity away u get to a point where TA (total alkalinity) is zero (equivalent point). Then u get the proton condition:
[H+] +[HS04-] + [HF] + [H3P04]
=[HCO3-]+ 2[CO3-2] + [B(OH)4-] + [OH-]
+[HPO4-2] + 2[PO4-3] + [H3SiO4]- +[NH3] + [HS-]..........(2)
You can compare the state of the equivalent point with a solution u made from pure water and where u add H2CO3, NaH2PO4, B(OH)3, NH4Cl, NaF, Na2SO4 and H4SiO4 in all the equivalent amounts. You can't find these subtances in the proton condition because they are by definition the zero proton levels.
My first question is, why is H2SO4 not in (1) and consequently (2)? Seeing that SO4- is considered zero proton level, H2SO4 should be in the left side of (2), seeing as it is considered a proton acceptor.
My second question is concerning the fact that NaH2PO4, NaF and NaSO4 are the zero proton levels and not H3PO4, HF and H2SO4. When H2PO4- is formed it can still absorb an equivalent amount of H+ by forming H3PO4, so actually there is more alkalinity then (1) says. The same for F-, it can take H+ to form HF. So my definition would be:
Total Alkalinity = [HCO3-]+ 2[CO3-2] + [B(OH)4-] + [OH-]
+[HPO4-2] + 2[PO4-3] + [H3SiO4]- +[NH3] + [HS-] + [F-] + [H2PO4-] + [HSO4-] +
2[SO4-] - [H+] .................(3)
So (3) doesn't look at pKa values of all the acid-base systems like (1) does, but just gives all neutral substances zero proton level, and the alkalinity of (3) will be higher than (1). Now your proton condition at the equivalence point after adding a strong acid is:
[H+] = [HCO3-]+ 2[CO3-2] + [B(OH)4-] + [OH-]
+[HPO4-2] + 2[PO4-3] + [H3SiO4]- +[NH3] + [HS-] + [F-] + [H2PO4-] + [HSO4-] +
2[SO4-] ...................(4)
and this will be the same as a solution starting from pure water and adding equivalent amounts of H2SO4, H2CO3, H3PO4, B(OH)3, NH4Cl, HF and H4SiO4
And my last question is, how can u determine the total alkalinity according to (1) experimentally of an unknown sample, when do u know when to stop, is there some pH jump?
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