WEBVTT
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Interactive numerical educational resources
for the analysis of complex media
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Titration of base salts by potentiometry
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Today, I am going to present you the pharmacopeia titration
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of an organic base hydrochloride
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which is a potentiometric titration
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by titrated sodium hydroxide (soda) in alcoholic media
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I will perform this titration on lidocaine hydrochloride
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A test sample of near 70 mg of lidocaine hydrochloride has been taken
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in order to get a theoretical endpoint volume
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around 5 ml of soda
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I also add a stirred bar in the beaker
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First, I am solubilising the sample
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with around 50 ml of ethanol
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The ethanol is a reactant
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therefore the quantity has been determined
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in order to ensure a good homogenization of the media
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and a good immersion of the bulb of the electrode
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Before starting the potentiometric titration
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I am recording the potential at the beginning,
at time 0
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which is 240 mV
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The reactant has to be added regularly
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and the potential has to be recorded also regularly
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0.25 ml
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214
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0.5 ml
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187
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2.75
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75
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3 ml
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65
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So, after 3 ml
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I will try to slow down the adding
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with smaller intervals
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in order to well straddle the potential fall at the equivalent volume
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3,2
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56
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3,4
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50
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4,4
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7
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4,5
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-1,6
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4,6
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-8
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6
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-325
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Now, I can take again bigger intervals
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since the potential fall is smaller
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6,25
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-333
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6,5
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-338
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7
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-345
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As you can see, I am getting to another plateau
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Let’s make 2 more addings
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7,5
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-348
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8
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-351
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I will continue up to9 ml
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in order to get a nice plateau
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-356
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Now, I need to draw the potential curve
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depending on the volume of soda
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in order to get the equivalence point
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I am going to enter my experimental data
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in a software which enables to draw the titration curve
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and to directly calculate the equivalence point
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The first column represents the volume in ml
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the second column is the measured potential in mV
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Now, if I am looking at the graph mode
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The titration curve
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which is the potential depending on the added soda volume
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appears
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If I click on the inflexion point
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the software presents the tangents
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and, between them
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I get the inflexion point of the curve
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So, in that case
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I have added 5.275 ml of soda
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to titrate 70.7 mg exactly weighed of lidocaine hydrochloride
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Titration of base salts by potentiometry
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Interactive numerical educational resources
for the analysis of complex media