4 filtrate/ & re/idue/
edited by Frank Schulz Johnsburg Central School North Creek, NY 12853
The Titration Project Cary Kilner
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P.O. Box 595, Exeter, NH 03833
Anyone who is interested in offering a quality laboratory experience realizes that the beauty and elegance of titration is lost on the first-year chemistry student who does only a cursory lab or two. This is what I discovered in my early years of teaching; the students had barely begun to familiarize themselves with the myriad details and subleties of the titration process, let alone develop any mechanical skill, before we had moved on to a new topic, I noticed, however, that many students who had slogged along up to this point with minimal interest in chemistry suddenly came alive and enjoyed titration immensely. Furthermore, students with a low conceptual ability often had a dormant natural engineering sense which then came to the fore during the project. By expanding titration from a one-shot lab to a more indepth activity via a sequence of related titrations with a central purpose would give the students time to gain some manual expertise and, thus, provide a major new learning tool. In the course of the project, my students acquire the ability to concentrate and to follow directions, organization skill, self-discipline, attention to detail, patience, and an appreciation for error analysis. Since the project comes in my syllabus in May, I take great pleasure in observing their rapt attention no matter how warm and sunny it is outside! At Exeter High School we deliver courses in chemical fundamentals with emphasis on lab so that students will be able to do something with the subject they are studying. During the second semester, we follow the Modern Chemistry (Holt, Rinehart, and Winston) sequence from the kinetic molecular theory, through solutions and electrolytes, into acids and bases, using whole-number pH. The titration project, therefore, tends to be the culminating lesson and experience for the year and utilizes many previously learned skills, especially those involving the mole concept. You will need a collection of volumetric flasks and lots of burets and clamps. These must be set up and left in one place for at least two weeks. There are no lab partners; each student works independently except at the very beginning of the project when they help each other to familiarize themselves with the equipment and the process of titrating, and later when they prepare solutions in pairs. For weeks prior to the project, we make frequent but vague references to “the fun last year’s classes had”, and how it “uses everything you’ve learned so far”. Until the students actually begin, however, they have no idea what the project entails, and the mystery surrounding it tends to increase their anticipation of it greatly. In preparation they have page references to read and a vocabulary list to peruse, though we do not expect them to derive much from this material until they actually start
titrating. The project has built into its introduction some mechanical exercises and a “quick and sloppy” titration. The goal is to have the students learn what skillful titration is by doing preliminary titrations outside the main sequence. In this way, you can realistically demand and ultimately 80
Journal of Chemical Education
expect precision from your students of ±0.003 M in their later runs. The titration project consists of 12 pages and contains all the information necessary (any lecture of text work is strictly supplemental). The setup checklist has them prepare their workspace and become familiar with the burets by adjusting, filling, and reading levels. In the introductory set, the students are given approximately 0.1 M HC1 and 0.1 M NaOH, then are told that the NaOH is exactly 0.102 M and that they are to find the molarity of the HC1 to three significant figures. Before they start the intro set, but after they have completed the setup check list, I demonstrate one complete run so they can see how all the steps on the intro set fit together. The check list for the intro set takes them in detail through one complete run with calculations, which they repeat as many times as necessary to obtain agreement of ±0.003 M. Most students get the hang of it after three to five runs; those few who have “decided” that they “can’t possibly do it” soon change their minds as their classmates (and often their best friends) move quickly and effortlessly on to the main body of the project. They come to understand the necessity for all the detail in order to obtain agreement. In Set A, they prepare a potassium hydrogen phthalate standard solution with a “pan drop” of solute (rather than trying to obtain exactly 0.100 M) and calculate its molarity, then use it to standardize 0.1 M NaOH to ±0.003 M. In Set B, they find the molarity of acetic acid in vinegar, using their standardized NaOH, and then calculate the mass percent. In Set C, they standardize 0.1 M HC1 solution with their NaOH to ±0.003 M. In Set D, they find the molarity of ammonium hydroxide in nondetergent Windex with their standardized HC1, then calculate the mass percent ammonia. In both these sets, student results are tabulated on the board, and class averages are calculated and used for discussion. In optional Sets E and F, they find the molarity of a saturated solution of Ca(OH)2 with their HC1 and calculate its solubility (and Ksp in advanced classes), and standardize some unknown H2SO4 with their NaOH, thereby using normality in both sets of calculations. Prediction and testing of
pH can also be done. Finally, they tabulate their results and package all their data sheets and calculations into a report in a prescribed format. After such disciplined work, it is always great fun to see their imaginations run rampant as they design covers and booklets for their reports. You can see that students ultimately get practice in choosing appropriate indicators via transition intervals, (we use phenolphthalein and bromthymol blue for pH « 7, phenolphthalein for pH « 10, and methyl orange and orange IV for ®pH 4), using the Mettler balance, preparing solutions in volumetries and keeping track of dilution ratio, handling normality, converting molarity to mass percent, and a myri-
ad of subtle skills and processes that would be impossible to teach didactically. The bottom line is that this is real chemistry, which the students know and appreciate.
A copy of the Titration Project can be obtained by sending $1.00 for postage and a promise to let the author know how it works in your class.
Volume 65
Number
1
January 1988
81
