Chemistry 564 Instrumental Analysis
Take-Home Examination
January 30, 1998
Due Friday, February 6, at 11:30 a.m.

Instructions: Answer the following 6 equal-valued questions on your own. Answers may be turned in either neatly hand written or typed (printed). Be sure to put your name on your response. Most everything required for successful performance is in the book or on the handouts received in class. You may not share your results with other students prior to the examination being graded and returned. You may ask me questions, either in person, or by e-mail (sbialkow@cc.usu.edu). If I respond with important information, this information may be shared with the class, by me only, either verbally or by posting on the Chemistry 564 internet home page (http:\\www.chem.usu.edu\faculty\sbialkow\Classes\564\index.html).

1) Thevenin equivalent circuits and measurement error

The electronic circuit for a common combination pH electrode may be accurately represented by a Thevenin equivalent circuit for a voltage (potential) source.

  1. The internal resistance of a pH electrode is very high, typically RS=20 MW (see page 611 in the text), because the glass membrane is not very conductive. The potential for this source is, of course, variable and dependant on the solution pH, ionic strength, temperature, and presence of interfering species. Draw the Thevenin equivalent circuit for the electrode.
  2. Assuming that a measurement device can be represented as a single "load" resistance, RL, draw the circuit that results when the measurement device is being used to measure the potential.
  3. Measurement devices have different load or input resistances. Calculate the relative % errors produced using the following measurement devices;
    1 An oscilloscope with an input resistance of 1 M    W
    2 A low-quality voltmeter with an input resistance of 100k     W
    3 A voltage follower operational amplifier circuit with input resistance of 10 M    W
  4. Which measurement device would you prefer to use? Why?

2) Complex impedance and passive filters

  1. Using the imaginary capacitor reactance, show that the gain formulas (gain=Eout/Ein) for the high- and low-pass frequency filters using resistors and capacitors in Figure 2-11 of the textbook are correct.
  2. Give values for the resistor and capacitor that will attenuate 50% of a signal at 60 Hz.
  3. What are the input impedences of these circuits at 100 Hz?

3) A zener regulated power supply.
Consider the power supply circuit shown below. (also see the text, Figures 2-22, 2-23, and 2-24)

  1. What is the output potential of this power supply?
  2. Explain the function of each component.
  3. What is the RC time constant? Is this sufficient to filter the bridge rectifier output?
  4. How much current passes through the resistor, assuming that the DC potential at the capacitor is 6.3 V, with no load resistor attached?
  5. When the power supply is attached to a load, the potential may drop below that regulated by the Zener diode. At what load resistance does this occur?

4) Operational amplifiers

  1. What is the output potential, as a function of the two input potentials, for the circuit shown above?
  2. Why is the voltage follower an important component of modern chemical instrumentation? When would you use such a circuit?
  3. Do problem 3-1 in your textbook.
  4. Using the fact that the complex reactance of a capacitor is -1/iwC, to deduce the frequency response of the circuits in 3-1 (d) and (e).

5) Analog versus

  1. What are the main differences between analog and digital signals? Which of the two are easier to store?
  2. What is the difference between serial and parallel data representation?
  3. What devices are used to connect analog to digital? Describe the operation of the circuit shown in Figure 4-8 of the text.

6) Signals and Noise

  1. List the three main types of noise, give the formula, and give a physical explanation for each noise type.
  2. How is interference different from the fundamental noise described above.
  3. Assuming that only thermal and shot noise source terms are important, use the Thevenin equivalent circuit for the electrode in Question 1, measured with a 1 MW instrument, to estimate the noise voltage. For a pH electrode that follows the Nernst equation, how many pH units does this noise represent?
  4. What type of filter would be used to reject noise from a DC signal?