Acid-Base Chemistry

AB1

Acid-Base Titrations

One important topic for any general chemistry course is acid-base chemistry.  Many principles taught in earlier curriculum can be explored through these reactions, and with moderate ease, you can acquire chemicals and design experiments to reinforce these principles.  The most common application of titrations is quantitative: How much of the analyte is present?   This is usually implemented with an equivalence-point/ indicator titration.  Through these titrations, students can investigate stoichiometry by comparing volumes of strong base required to neutralize mono- and diprotic acids.  They can study equilibrium properties and determine constants, Ka or Kb, for weak species.  There is a lot more information available if one is able to track and plot volume with respect to pH, temperature, conductance or other solution properties.  Observing temperature changes during an acid-base titration introduces heat of reactions and students can begin contemplating enthalpy.  Not to mention the availability of indicators to provide additional sensory information for nearly any acid-base combination.

ABDCDrop counters and inexpensive drop dispensers are a useful and cost-effective alternative to burettes.  They will not break and their resolution is about 50% better than a burette.  And they do not get tired or distracted.  Titrations can be made quick and easy with MicroLab’s Model 154 Drop Dispenser and Model 226 Drop Counter in combination with an FS-522 interface and MicroLab software.  MicroLab’s patented Model 226 Drop Counter uses a reflective infrared sensor and background correction circuit to accurately count drops of titrant added regardless of room lighting conditions.  The speed of collection makes it possible to perform titrations with a series of acid-base pairs and, in the same lab, discuss the results and draw conclusions from the data.  They can determine the effect of ion concentration, acid/base strength and identify desirable species for buffer selection in a given pH region.  The data shown for phosphoric acid demonstrates its ability to maintain near physiological pH (between 6.5 and 7.5), which is why phosphate buffer is so common place in biochemistry and biology.

AB2
With MicroLab software, students can make derivative plots and easily determine the equivalence point.  Removal and neutralization of the first and second hydrogens show clearly as inflections in the graph.  A first derivative plot identifies the equivalence points—one can match the maximum derivative value and pH with volume to calculate equivalence points to a titrant volume within 0.034 mL.  Let them compare this with varying indicator end points, and they can easily see why indicator selection is an important consideration.

AB3Students also have strict control of the experiment setup through MicroLab software.  They can modify when each pH measurement is made relative to when a drop of titrant is added.  This ensures consistent stir time for each drop.  Accurate results rely on adequate and consistent solution mixing, the response time of the pH electrode (close to one second), and the reaction rate of the compounds involved.  Strong acids react more quickly than weak acids.  The result is that, if drops are dispensed faster than about one every 1.5 seconds, the pH reading does not reflect the true chemistry of the solution.

Multi-Variable Titrations

PhAcid

The above phosphoric acid titration is an example of a multi-variable titration.  With the MicroLab FS-522 interface, software and sensors, you have the ability to track more than just volume and pH.  And the high resolution and low noise data produced by MicroLab equipment makes it possible to track phenomena that could not be observed with similarly priced sensors.  The rate of temperature change during the neutralization of the first hydrogen is greater than that for the second.  Demonstrating how the first hydrogen is more easily removed and has a greater heat of reaction than the second.  The total temperature change in this experiment was only about 0.9 °C, and it was run in a Styrofoam cup.

Similar multi-variable experiments can be designed combining pH with conductance, absorbance, scatter (with the integrated FASTspecTM or a light sensor), pressure, etc.  Let your imagination be your guide.

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