Download - TITRATIONS IN NON-AQUEOUS SOLVENTS
TITRATIONS IN NON-AQUEOUS SOLVENTS
B.KIRUTHIGALECTURER
DEPT OF PHARMACEUTICAL CHEMISTRY
WATER, as SOLVENT
ADVANTAGES: ☻ cheap, clean (can easily be purified)
☻ high relative permittivity (ε): good solventDISADVANTAGES: 0 - 100 °C temperature range
apolar substances can not be disolved
Kw = 10−14, therefore Kd ≤ 10−7 unmeasurable
WATER actively participates in all type of reactions:
- acid and base: acid-base reactions take place through connectionto water first (amphoteric)
- complex formation: cations: aqua complexanions : H-bond
- precipitation: dissolves precipitate of ionic lattice (high hydration energy)
- oxidant and reductant; range of redox potential: 0 −1,23 V theoretical-0.8 − 2,1 V practical
CLASSIFICATION of SOLVENTS
ACIDIC (protogene): H2SO4, CH3COOH, HCOOH, acetone proton donor
AMPHOTERIC (amphiprotic): H2O, alcohols, acetonitrile
BASIC (protophyl): pyridine, liq. NH3, amins, dioxane proton acceptor
APROTIC: liquid SO2
INERT: CCl4, CHCl3, benzene, carbohydrates
REACTIONS in NON-AQUEOUS SOLUTIONS
AUTOPROTOLYSIS EQUILIBRIA determines the ionic product :
2 H2O H3O+ + OH− 10−14 0 - 14 7
☻ neutralization (protolytic solvents) ≈ 90 %
☻ complex formation, precipitation, redox ≈ 10 %
solvent K pH scale neutr. point
2 CH3COOH CH3COH2+ + CH3COO− 10−13 0 - 13 6,5
2 NH3 NH4+ + NH2
− 10−32 0 - 32 162 C2H5OH C2H5OH2
+ + C2H5O− 10−19 0 - 19 9,5
− pH scale depends on the value of KHL = [H2L+][L−]
NEUTRALIZATION ANALYSIS in NON-AQUEOUS SOLVENTS
REACTIONS in NON-AQUEOUS MEDIUM
E.g. HClO4 + pyridine (Py) in glacial acetic acid
acid: HClO4 + CH3COOH ClO4− + CH3COOH2
+
K = [CH3COOH2+][CH3COO−] = 10−13
base: Py + CH3COOH PyH+ + CH3COO−
ClO4− + CH3COOH2
+
PyH+ + CH3COO− PyH+ ClO4− + 2 CH3COOH
2 CH3COOH
− Brönsted equation can be used
− reactions take place through reaction of acids or bases with the solvents
ADVANTAGES of USING NON-AQUEOUS SOLVENTS
☻ 1. More than 3 acids/bases can be measured in mixture due to the wider pH range
compared to water
E.g.methyl-ethyl-ketone water0 - 25.7 pH range 0 - 14 pH range5 comp. measurable max. 3 acids (3 x ΔpH(4) = 12)HClO4 - HCl - Salicylic acid - Acetic acid - Phenol(can titrated with TBAH (C4H9)4N+OH−)
☻ 2. Differentiation - levelling effect (Kd ~ 10−12 can be measured)
a) Differentiation effect:in water: HClO4 ≈ HCl ≈ HNO3
in CH3COOH: HClO4 > HCl > HNO3
in HF: medium > weak > base
acidConclusions:Strong acids (in water) can separetely be measured in acidic solventsStrong bases - ″ - in basic solvents
in water: HCl > CH3COOH > benzoic acidb) Levelling effect:
in pyridine: HCl ≈ CH3COOH ≈ benzoic acid
EXPLANATION by the protonaffinity
ADVANTAGES of USING NON-AQUEOUS SOLVENTS
Conclusions:Weak acids (in water) can be measured in basic solventsWeak bases - ″ - in acidic solvents
☻ 3. Determination of organic acids and bases which have a limited solubility in water.
☻ 4. Application of new reagents and indicators is possible due to
ADVANTAGES of USING NON-AQUEOUS SOLVENTS
expensive
volatile
toxic
removal of water is necessary, can take water (humidity) from the air
DISADVANTAGES of USING NON-AQUEOUS SOLVENTS
− HClO4 in glacial acetic acid− HCl in propylene-glycol /chloroform mixture
STANDARD SOLUTIONS
ACIDIC :
BASIC : − TBAH (C4H9)4N+OH−) in pyridine− KOH in ethanol
application: - weak bases: Kb : 10−7 − 10−12
e.g. aromatic amines, amides, alcaloides, etc.- high-molecular-weight organic bases, that have limited solubility in water
E.g. Determination of „Lidocain” (Lidocainum Ph.Hg. VII.)
NH CO CH2 NC2H5
C2H5
CH3
CH3
application: - weak acids: Ka : 10−7 − 10−12
e.g. carboxylic acids, phenols, enols etc.- high-molecular-weight organic acids, that have
limited solubility in water
END POINT DETECTION
CHEMICAL:(INDICATORS)
− crystal violet (in glacial acetic acid)
INSTRUMENTAL: − potentiomety : glass electrode in glacial acetic acid− conductometry
− phtaleins (phenolphtalein) (e.g. in pyridine)− azo compounds (methyl red) (e.g. in alcohol)
R +|C-R|R
ibolya
R + R-H 2+
| |C-R + H+ C - R| |R R
ibolya zöldeskék
R + R-H 2+ R-H 3+
| | |C-R + H+ C - R + H+ C-R| | |R R R-H
violet green yellow
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