Conductivity detector is an electrochemical technique which applies alternating current (AC) signal — Analytical Chemistry Chemistry Question
Conductometric Titration
Conductivity detector is an electrochemical technique which applies alternating current (AC) signal to two identical electrodes. The applied AC signal induces ions to move in solution and switching of polarity of the AC voltage avoids electrolytic reaction at the electrode. As a result, the mobility of ions in solution relates to the conductivity of ions in solution. Conductivity probe was used in acid-base titration of 25.00 cm3 HCl vs 0.100 mol dm-3 NaOH. The conductivity signal was recorded during NaOH addition. The conductometric titration setup is shown in Fig 1. When standard NaOH solution was dropped under gravity force to the titration cell at 3 drops per second and the conductivity value from conductometer recorded, the plot of conductivity of solution vs titration time is shown in Fig 2.
Explain why the slope of titration curve before and after the turning point is different.
Model Answer
The conductivity value before turning point comes from the mobility of H+ and Cl- from HCl. After NaOH was added to the titration vessel, the H+ reacted with OH- and the solution conductivity due to H+ decreases. After the turning point, NaOH becomes excess in solution. The conductivity value increases with amount of added NaOH. The ion mobility of H+ is higher than that of OH-, hence the slope of titration curve before and after turning point is different.
Calculate the concentration of HCl, if the volume of NaOH of each drop is 0.029 cm3 and the turning point is at 108 second.
Model Answer
From titration curve, the turning point is the end point, and is at 108 seconds. Flow rate of NaOH = 3 drops / sec. Therefore, the volume of NaOH is 108 x 3 = 324 drops = 324 × 0.029 cm3 = 9.39 cm3. The concentration of HCl = (9.39 × 0.100) / 25 = 0.038 mol dm-3.