pH measurements

The goal of this lab is give you experience reading technical manuals for standard chemical analysis, interpret these manuals in the context of your particular analytical application, and communicate your analytical results in a complete and succinct report.

Background:  We will discuss the specifics of pH measurements in great detail in the final third of the semester.  For now, pH measurements will be made using an electrochemical cell with both the sensing (pH) and reference electrode built into a single combination electrode.   The potential of the combination cell in millivolts is directly related to the pH of the solution.

electrode potential (E) = E’ – slope (pH)  (1)

where E’ is a reference potential and the slope is the Nernst slope (theoretically 59.16 mv/pH at 25 0C).  Notice that equation 1 is a simple linear equation of the form Y=b-mX.   By measuring the electrode potential of two or more standards of known pH you can determine E’ and the slope.   We will use three standards and linear least squares to do this analysis.  Most pH meters perform this analysis automatically using the built in calibration procedure.   Our task is to evaluate the effectiveness of this calibration procedure in the context of real pond samples.

Please refer to The Theory of pH Measurement datasheet for more practical considerations of pH measurement.

Analytical tasks: (You may collect data in pairs)

  1. Calibrate the pH meter using the meter’s calibration function (follow the directions that come with the meter) using three pH standards. Record the electrode potential and temperature of each pH standard for subsequent analysis.
  2. Measure the pH of two Johnson Pond samples from different locations on the lake. Record both pH as displayed on the meter and electrode potential. Record the location and temperature of each sample. Why is it important to know the temperature of the sample? How do temperature effects influence your results?  How will you solve the temperature problem?
  3. Use a least squares analysis procedure to calculate the pH of samples in step two using the electrode potential of three standards measured in step one. Calculate the uncertainty of each pH measurement from the uncertainties in the electrode potential and uncertainties in the least square slope and intercept.  How well does the pH display of your meter represent the analytical uncertainty of the pH measurement?
  4. Report the results from tasks 1-3 using the Analytical Chemistry guide to authors. Every student should have his or her own write up.  Please limit your writeup to two pages.   Include title, introduction, methods, discussion, and citation sections in your writeup.   Use tables or figures as appropriate to describe your methods and results.

The following is the grading scheme used for this writeup.

40 %. General Style  – Does the report conform to the ACS – Analytical Chemistry guide to author’s style?

  1. Title  – informative, including key words related to the topic
  2. Authors – your name
  3. Abstract – not necessary for this short work.
  4. Introduction – a short introduction to the analytical problem, past analytical work relevant to the problem (with citations), and a overview of the work described in this paper.  Often the introduction is used to detail the theory behind the analytical method.
  5. Methods – a description of all methods use in the work described at a level so that an expert could reproduce the experiment using this work and other work referenced by this work.   Figures of a new apparatus or tables of reagents should be included in the methods.
  6. Results/conclusions/discussion – often combined for short papers.   A complete description of the results and significance of this work.   Figures showing instrument response (not linear data) and data tables listing experimental results are located in this section.
  7. Citations – you will always have one or more citations

 20% Quality of written work – the lab report should be written in complete sentences, without spelling errors, and written in a clear and concise style.   Avoid a conversational voice in your writing.  Focus on the completed procedures and results of the work instead of a narrative of your afternoon in lab.

 40 % Answers to the specific questions should be addressed in the lab write up.  Did you perform the analysis of the data correctly?