Alkalinity (Potentiometric Titration)

Background

In this analysis, you will determine the alkalinity of samples. To do this, you will perform a potentiometric titration. In such a method, a titrant (in this case the strong acid \(\ce{H2SO4}\)) is slowly added to the sample, which contains a weak base ("alkalinity" – actually multiple chemical species that we treat as one thing). As the acid is added, the base becomes neutralized, and the volume of acid it takes to completely neutralize the sample is related to the alkalinity of the sample. You will use a pH probe to monitor the reaction and determine when you have reached the endpoint. The equivalence point can then be determined through multiple computational methods, including the inflection point method and Gran plot method.

Safety

This lab requires sulfuric acid \(\ce{ (H2SO4) }\), which will be provided to you prediluted to approximately 0.16 N. Even diluted, this solution is very acidic and corrosive, and will quickly damage skin and eyes and may create holes in clothing, rags, and paper towels.

• Wear gloves and goggles at all times when handling sulfuric acid!

• Wear appropriate lab attire when handling sulfuric acid (closed-toe shoes, pants, and a shirt that fully covers your upper body.

• No food, drinks, or cosmetics should be present in lab, and you should wash your hands before leaving lab to handle these items.

Procedure

1. Use the USGS method (PDF) for alkalinity. Before you begin any analysis, thoroughly go through the method with your lab group to develop your procedure for your samples.

   ⚠ Note

   You will use the "Buret Titrator" option in the USGS method, not the "Digital Titrator" option.

   • Draw a cartoon of the procedures you will use to (1) standardize the sulfuric acid, (2) collect and preprocess the sample, and (3) titrate the sample. Use pictures and be as specific as possible. (For example, the method for low-alkalinity and high-alkalinity samples will be different, and your procedure should reflect this.

   • Use the data from Example IPT-2 in the USGS method (and corresponding data table) to reproduce the inflection point (IPT) graph shown in the USGS method.

   • Use the data from Example IPT-2 in the USGS method (and corresponding data table) to reproduce the Gran plot shown in the USGS method, including the linear fit.

   • Use the data from Example IPT-2 in the USGS method to determine the alkalinity of the example sample. Ensure your results are reported in mg/L "as \(\ce{ CaCO3 }\)". The "Calculations" section provides a detailed explanation of how to do the calculations for this step.

   • Make sure your plots are correct and that they are publication quality before turning in your work.

   ⚠ Note

   There is a template provided for you on Dropbox (Dropbox > data > alk-calc.xlsx) to help with the calculations and plots.

2. When you are confident in your procedure and ability to perform the calculations, move on to standardizing the titrant. Make the primary standard and perform the standardization according to the instructions in the USGS method. Use the IPT method to determine the equivalence point.

   ⚠ Note

   Make sure you calibrate your pH probe before beginning this step. You will need Vernier LoggerPro, LoggerLite, or Graphical Analysis to use the pH probe.

3. Collect and process your sample, and then determine the alkalinity in triplicate. Calculate the alkalinity using both the IPT and Gran plot methods for comparison.

What to Turn In

• When you are satisfied your data is both accurate and precise for your project site, turn in the completed Excel template for your samples. You will need to turn in 3 separate Excel files, 1 for each replicate. Do not alter the layout of the template – make sure all columns in your data are in the same position as the template (e.g. column A in your data should correspond to column A in the template) and that the data starts in row 5.

  • Your sample volume should be in cells L4 and N4 for the IPT and Gran methods, respectively.

  • Your equivalence points should be in cells L5 and N5 for the IPT and Gran methods, respectively.

  • Your final alkalinity values, including and dilutions, should be in cells L6 and N6 for the IPT and Gran methods, respectively.

• There is no post-lab quiz for this lab, but you will see this information on quizzes and/or exams in lecture.

Terminology You Should Know After Completing This Lab

• Alkalinity

• Acid Neutralizing Capacity

• Titrant

• Titrand

• Equivalence point

• End point

• Potentiometric Titration

• Buret (also spelled Burette)

• TD vs. TC

• Normality vs. molarity

• Standardization

• Standard solution

• Inflection point method (IPT)

• Gran plot method