Ion pair extraction titration

EXPERIMENTAL PROCEDURE

Raw material of lidocaine hydrochloride:

Step 1: In a 250 mL Erlenmeyer, introduce an exactly weighed test sample of lidocaine hydrochloride (note down the mass of this test sample) previously calculated for an expected volume of 5 to 6 mL of tittered DOSS solution out of the burette at the equivalence point (the exact titer of the DOSS given in the laboratory must be taken into account); solubilize in around 20 mL of distilled water by stirring well.
Add (approximately): 5 mL of pH=2.8 buffer, 5 mL of mixed indicator (green) and 70 mL of dichloromethane. Add a bar magnet.
Fill the burette with 0.01M DOSS (the exact titer is displayed)
Proceed to a vigorous magnetic agitation and add the DOSS drop by drop; stir vigorously after each addition to extract the ion pair; the dye remains green
After a given volume of added DOSS, the indicator turns from green to a dirty grey. You are close to the equivalence point
Slow down, or even stop the addition; stir vigorously to monitor the change in color.
At the equivalence point, an additional drop of DOSS is combined with the colored indicator; turning mauve; let it settle if necessary.
1 mL of 0.01M DOSS corresponds to 2.71 mg of C₁₄H₂₃ClN₂O (anhydrous substance).
Remember to use the exact displayed titer of DOSS for the calculation:
[DOSS] = 0.0184 M
  Calculate the % purity of your lidocaine hydrochloride raw material.
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