pH Titration Curves / Electrochemistry

December 6

you've used it previously during the lab work on acids and bases

• A brief review
  • the electrode seals a solution of H+ inside a glass electrode
  • a potential develops across the glass
  • this voltage is proportional to the log of the [H+] outside the glass
    • an internal electrode is used to monitor this voltage
  • a second electrode (reference electrode) completes the electrical circuit
    • in practice, the two electrodes are usually combined into a single device
  • the pH meter translates the voltage to pH readings
    • the pH meter generally needs calibration with a buffer

Titration Curves

• One can record the pH during a titration
  • you measure the pH of a solution (say a weak acid)
  • then you add a small volume of titrant (say NaOH)
    • mix and record the pH and volume
  • continue to do this
    • typically, you need 25-250 data points to get a smooth titration curve

Automated Titration Curves

• We will connect the pH meter to a computer data system
  • this includes an amplifier
• The computer will measure, record and plot the pH at regular intervals
• The titrant is added at a constant rate with a precision pump
  • this is not as precise as a buret
  • volume measurements are typically 1-3% with our pumps
  • volume measurement in a buret is typically 0.2%
  • The solution is continuously stirred

Samples to Titrate

• 1. Titration with a strong Base, 0.1M NaOH
  • a strong acid, HCl
  • b. a weak acid, Acetic acid or Benzoic Acid
  • c. a diprotic acid, H₃PO₄ (we won't get the third proton off in aqueous solution)
    • the first proton represents a strong acid, the second is a weak acid
  • d. a mixture of strong and weak acid (HCl and Acetic acid)
  • e. titrating our classic green salt
    • it behaves as a weak acid
• 2. Titration with a strong acid, 0.1M HCl
  • a. a weak base, ammonia
  • b. a weaker base, sodium bicarbonate
  • c. a a weak base, sodium carbonate
    • (two end points-- one for carbonate and one for the bicarbonate)
  • d. a mixture of carbonate and bicarbonate
• 3. Titration with a weak acid
  • a. titrating a weak base, ammonia

Experimental:

• The titration equipment will be set up (with a specific titrant in the pumps)
  • Measure a suitable sample and place it in a 100 ml beaker
    • Add a magnetic Stirring bar
    • Lower the electrode until it is immersed
  • Start the program; enter descriptive data
    • At the point you trigger data collection, turn on electrical power to the pump
  • Observe the titration curve
    • Print and post the titration graph
    • Determine the endpoint volume
    • Calculate the strength of the sample

Apparatus:

• The magnetic stirrers are an inexpensive solution
  • the motors are reclaimed from old 3.5" floppy disk drives
  • this are very thin motors so we can make a shallow stirrer
  • the read heads fail long before the motors
  • the motors turn at a fixed speed that is ideal for small titrations
  • the motors work on 12V DC (same voltage as our pumps)
  • old PC power supplies can power a number of these units

• the titrant pump is a device called a peristaltic pump
  • peristalsis it the technique your intestines use to move food along
  • muscles contract, squeezing the food forward
  • a motor rotates a shaft
    • on the shaft are several rollers
    • these squeeze a length of flexible tubing
    • this drives liquid forward
    • the pumping speed is fixed by the motor rotational speed and the tubing diameter

• The stand is designed to hold all the components
  • the stirrer is in the base
  • the pump is mounted on the base
  • an adjustable clamp holds the electrode and the tubing delivery tip
  • a built in amplifier converts the pH electrode signal (a few millivolts) to the voltage needed by the computer

• Each computer includes an Analog to Digital Converter that actually measures the voltage
• Software converts voltage to pH and plots the titration curve