Titrasi Bebas Air
5
The major reason is that many acids and bases can’t be quantitatively in water at reasonable concentrations (~0.1M). The problem occurs when the pK A or pK B of our material is > 7. The endpoint is not a sharp break so is very difficult to detect. pK A 8 pK A 7 pK A 6 pK A 5 pK A 4 pK A 3 pH As the pK A increases, it becomes more difficult to detect the equivalence point. K B = K W K A = [ HA ] [ OH - ] [ A - ]
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Transcript of Titrasi Bebas Air
The major reason is that many acids and bases can’t be quantitatively in water at reasonable concentrations (~0.1M).
The problem occurs when the pKA or pKB of our material is > 7.
The endpoint is not a sharp break so is very difficult to detect.
pKA 8
pKA 7
pKA 6
pKA 5
pKA 4
pKA 3
pH
As the pKA increases, it becomes more difficult to detect the equivalence point.
KB = KW KA
= [ HA ] [ OH- ] [ A- ]
KW KA
= [HA] [OH-] [A-]
(10-4)2 0.1 = = 10-7
With amphiprotic solvents, the solvent plays a significant role in determining the observed acid-base chemistry and titration curves.
Non-ionizing solvents only act to transport on pairs.
Aprotic solvents only contribute solubility.
We’ll now look as several amphiprotic solvents.
This amphiprotic solvent is commonly considered the ‘universal solvent’
High dielectric constant - 78.5 KW = 1.00 x 10-14 pH scale 0 - 14
Strong acids are considered to dissociate completely producing H3O+.
So H3O+ is the only aqueous acid and OH- is the only aqueous base.
Our acids and bases can’t be any stronger than these species.
Weak acids and bases have as an additional limit incomplete ionization.
It makes sense that many of our strong acids are significantly different -- why would they be of the same strength?
Another amphiprotic solvent
2 CH3COOH CH3COOH2+ + CH3OO-
pKHAC = 14.45
While the pH range is similar to water, CH3COOH2
+ is a stronger acid and CH3OO- is a stronger base.
It also has a lower dielectric constant.
As a result, strong acids will still dissociate completely but, because of the low dielectric constant, tend to exist as as ion pairs.
Perchloric acid is the strongest acid in acetic acid.
KA = = 10-4.87 [H+HAc ] [ ClO4
- ] [ HClO4 ]
Since acetic acid is less basic than water, it does not level strong acids.
This increases the number of acids and bases we can titrate.
The expanded pH scale also helps somewhat.
Acetic still levels aliphatic amines and simple aromatic amines.
This is the most common acid to use in non-basic solvents (like acetic acid)
It also serves as an excellent example of what is possible when using a solvent other than water.
While perchloric acid is more hazardous than other acids, in acetic acid, it produces the sharpest endpoint.
HCl
HNO3
HClO4
pH
COO-
COOH
COOH
COOH
HClO4 HClO- + +
