Aquametry
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- Created by: Rscottqub
- Created on: 28-11-19 15:06
Aquametry is
an analytical process to measure water present in a product
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water can be present in a product as ..
Solvent/diluent . Absorbed (unbound) . water of crystalisation
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what can happen if water content is not = to amount specified
growth of microbes. Degradation by hydrolysis. Increased weight- dosing disrupted. Changes in physical properties. granulation and tabletting affected . Decreased stability
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Too much water in tablets can
cause them to swell
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too little water in tablets
can make them brittle
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Pharmaceutical substances can exist in ....
hydrous or anhydrous state
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Water of crystalisation
stoichiometrically bound water within a crystal which will dissociate on heating
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Stochiometrical
a defined no of H2O molecules bound and therefore can be quantified
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Molecular mass is dependent on
hydration
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Factors affecting hydration
1. Efflorescence 2. HyGroscopy and 3. deliquescence 4. Exsiccation
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Efflorescence
the loss of water to the atmosphere when exposed to air
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If vapour presssure of hydrous product is higher than Vapor pressure of atm ....
substance will lose water to the atm - this will continue until equilibrium is reached
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How can we prevent Efflorescene ?
Lid tightly only container straight after use. Pack tightly to reduce air space
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Hygroscopy
the ability of a sub to attract water molecules from the surrounding atm by absorption or adsorption- sub will phiscally change - increase vol and stickiness
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Deliquescence
if a sub is v. hygroscopic - can attract enough water to dissolve in it
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Exsiccation
removal of water by heating - heat is applied until constant mass is achieved
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3 methods we use to reduce water content
1. Titration methods 2. Drying methods 3. Distillation
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Advantages of Titrations
fast , water specific
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Disadvantages of Titrations
sub could react with I2
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Disadvantages of drying methods
not water specific,time consuming
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disadvantages of distillation
large sample required, not suitable for trace amounts of water
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Equivalence point
the theoretical completion of reaction
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End point
what is actually measured
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Indicator error
difference between Eq point and e.point
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KFT
Karl fischer titration- method of choice for water analysis
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Based on
the bunsen reaction - KF modified it - uses XS SO2 and I2
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KF uses
Primary alchol - methanol. Base pyridine
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KF reagent
Solution of I2, SO2 in mixture of methanol and pyridine
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Pyridine
colourless, unpleasant fishy smell, carcinogen
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So many Newer KFR have
imidazoles/primary amines instead of pyridine . methanol can also be swapped too
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KFT reaction
2 stage process 1. alcohol reacts with SO2 and base --> suphite salt +base 2. Is reduces suphite salt --> sulphate salt. THIS REACTION CONSUMES WATER
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How do we quantify water from KFT
water and I2 are consumed in 1:1 ratio- if we know how much I2 used we know how much water
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2 types of KFT
1. Volumetric 2. Coloumetric
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Volumetric KFT
involves titrant
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2 types of V. KFT
1. one component 2. two component
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One component
titrant in burette contains all parts of KFR - I2, SO2, base and OH - time consuming as requires calibration
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two component
titrant in burette nly contains I2 and OH - other parts are in sample cell. Expensive but faster
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Volumetric KFT is preferred for ...
samples containing a lot of water
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KF reagents = v unstable , this means that ...
titer must be determined repeatedly to acheive accurate,reliable results
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Coloumetric KFT
no titrant used , I2 is generated in situ @ the annode
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2 types of C. KFT
Fritted cell. Frit-less cell
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Fritted cell
Diaphragm (fritt) separates the annode and cathode to prevent I- being reduced back at the cathode.
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Which section is larger
annode section
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Fritless cell
no frit - H2 bubbles act as a barrier between electrodes
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How is water quantified in C.KFT
based on total charge passed Q=Current x Time(s)
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Farraday constant
magnitude to e.charge per mole of electrons F= 96486 C/mole
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Q is proportional to
amount of I2 generate and also water content
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How do we measure end point in KFT - 3 methods
1. Visual detection 2. Colourmetric detection 3. amperometric detection
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Principle of end point -
when all water is used up - I2 will be in XS
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Visual detection
Active KFR is brown, when this enters the sample it turns yellow (spent KFR) as the I2 is being used in the reaction. When I2 is in XS - KFR entering will remain BROWN
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Colourmetric detection
active KFR will absorb at 525nm. Spent KFR will not . when an aborption is detected by colourimeter - I2 is in XS
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Amperometric detection
(Preferred method) aka dead stop method. 2 Pt electrodes . I2 depolrise annode. I- depolarises cathode. For current to flow both must be depolarised. When I2 is in IX current stops
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Other cards in this set
Card 2
Front
water can be present in a product as ..
Back
Solvent/diluent . Absorbed (unbound) . water of crystalisation
Card 3
Front
what can happen if water content is not = to amount specified
Back
Card 4
Front
Too much water in tablets can
Back
Card 5
Front
too little water in tablets
Back
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