The concentration of chloride in water is often indicative of human activities. Instructions are provided in the laboratory manual for determination of chloride using two different techniques:
These two techniques are very different. The use of an ion chromatograph requires a major initial outlay for the instrument. Once the instrument is set up, this technique is more convenient and accurate than the potentiometric method, and will provide the concentrations of a number of different anions from one analytical procedure.
You may wish to have your students perform replicate analyses on their samples. Alternatively, have a group of 4-5 students analyze the same sample and average their results.
Reading in Laboratory Manual
Background on chloride in natural waters: p. 90
Background on analytical method for chloride: 91-93
Lab procedures for chloride determination: pp. 96-99
15-20 minutes to assemble the chloride-sensitive electrode
15-20 minutes per sample for titration to endpoint
Remember to take the sample bottles out of the refrigerator about 30 minutes prior to analysis. Samples need to be at room temperature before any measurements are made. Have a map of the site prepared with sampling sites labeled. Post this map in the classroom. Collect all the field measurements on one map. Post these as well so students have something to compare their data with as they carry out their analyses.
Post data entry sheets or set up a computer spreadsheet for students to enter their results into. Photocopy masters for data entry sheets can be found in Appendix B in this instructor's manual.
Discuss the fact that chloride in natural waters is a good indicator of human activity, since it passes unchanged and barely used from the digestive tract. Chloride in natural waters can also result from the application of salt to roads to melt snow or from intrusion of ocean water into a fresh water supply.
Present the potentiometric method for chloride determination. If students have had redox chemistry, this is a good chance to develop the idea of potentiometry in more depth. If not, it is best to leave the technique more as a "black box," since there are a number of fairly difficult principles underlying the technique that require more time to cover than is typical for a laboratory period.
Remind the students to enter their data in the posted sheet (or a computer spread sheet) before leaving the lab.
If you are using ion chromatography to determine chloride ion concentrations, please refer to the ion chromatography section of this instructor's manual on p. 57.
Calculations for the Chloride Determination
The equivalence point should first be determined from the plot of mV vs. mL of AgNO3 added by noting the location with the steepest slope. The volume of silver nitrate solution is then read off of the plot and converted to moles of AgNO3 solution. For example, for a titration requiring 10.42 mL of 0.0040 M AgNO3 to reach the equivalence point:
Because the reaction stoichiometry of silver and chloride is 1:1, this number is also the number of moles of chloride in the solution. The final step is conversion to ppm of chloride:
Field-Ready Alternatives for Chloride Determination
The Hach company (see Appendix A for address) offers a field kit for the determination of chloride ion in water. The kit (Catalog # 26018-00, $19.50 per 100 tests) comes complete with all necessary glassware and full instructions. Also available are multi-parameter kits such as the nine-parameter kit (Catalog # 2430-02, $185 per 100 tests)
Instructor's Notes on Chloride Determination
The optimum concentration range for good endpoints is between 5 and 80 ppm of Cl- in the sample. More dilute samples show very little inflection in the titration curve and more concentrated samples require refilling the buret. If you are working with seawater or saline solutions, you will need to increase the concentration of the silver nitrate titrant accordingly or dilute the sample.
This stockroom preparation list applies to a room of 25-30 students working individually and titrating one sample each.
Post data entry sheets or set up a computer spreadsheet for students to enter their results. Photocopy masters for data entry sheets can be found in Appendix B in this instructor's manual.
Materials for Construction of Copper Reference Electrode
Materials for Acid Washing of Glassware
Since acid washing of glassware must be done regularly, it is useful to put out large carboys of the cleaning materials and be sure they are kept filled throughout the experiment.
Store acid carboys in the hood and be sure to label well with "Caution! Strong acid! Wear safety glasses! Wear gloves!" signs.
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