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Appendices: a few final lollipops

APPENDIX ii: SOME HINTS FOR QUANTITATIVE ANALYSIS
(TITRATION)
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A2.1 DO PRACTICALS TAKE TOO MUCH TIME FOR THE AMOUNT LEARNED?

A lot of the techniques listed in, for example, section 2.1.3.i within module 2 of the OCR specification will have bean learned and practiced in you practical classes. It is all too easy pay insufficient attention to what you do in practicals. In each practical stop from time to time to note the techniques you are using and to think about how you would describe them in words. Take mental photographs of equipment, the chemicals you use and those you make - are they solids, liquids (including solutions) or gases, what colour are they and so on? And take mental videos of any changes you observe. Then think about how you would describe those mental images and videos in words (and diagrams). And, of course, make sure you understand the chemistry involved!

If you're not learning enough, you are probably not thinking enough.

A2.2. SOME EXTRA TIPS FOR TITRATIONS

1) Make sure that equipment is cleaned properly, especially between experiments.

Remember that final rinsing of burettes, pipettes and measuring cylinders should be carried out with the solution to be used in them - distilled water would dilute the solution.

BUT, the titration flask should be rinsed finally with distilled water. Using solution would add to the quantity measured into the flask, whereas dilution of the measured quantity does not affect the absolute amount of dissolved substance.

2) Always take burette readings from the same part of the meniscus - the bottom if you can see through the solution.

3) The last quantity of solution from a pipette should be removed by touching the pipette on the inside of the titration flask. DO NOT blow out the last drop of solution unless the pipette is marked "Blowout".

4) Before starting the titration, the part of the burette below the tap should be filled with the solution by running some solution out of the burette before filling it to 50cm³. At the same time check that the tap works, and that it has a smooth action.

5) Never add too much indicator. (Remember indicators are usually acids themselves.) Starch should be added near the end point in iodide/iodine titrations.

One of the main reasons for carrying out a rough titration is to discover what changes occur at, or near, the end point. During the rough titration, take readings as soon as you think you have reached the end point, but continue the titration (dropwise), and continue taking readings, until you are sure you have seen the genuine end point.

6) Keep the contents of the flask swirling continuously during the titration. This is often extremely important.

The best arrangement is to have the tap on the right hand side of the burette and operate the tap with your left hand around the burette. This helps you to avoid pulling the tap out. Hold and swirl the flask with your right hand.

7) Do not let solutions run out of burettes too quickly, and do not help solutions out of pipettes by blowing. In both cases solution may be left behind on the walls.

8) Make sure you fully understand how to use balances. One of the most common inaccuracies in exam practicals is in the making up of solutions. This is apparently due to uncertainty in weighing.

9) In exam practicals there are many cases of burette readings given as whole numbers. Burettes are graduated 0.1ml divisions.

10) Make sure you understand moles, and particularly the difference between concentration measured in mol dm^-3 and concentration measured in gdm^-3 (section 1.6.).

11) In exam calculations do not quote an excessive number of significant figures, but equally do not quote too few. There are many instances of answers quoted to one significant figure, and even cases of numbers being grossly rounded to one significant figure and then used to calculate numbers with more significant figures. Unless it is spelled out in a question, three significant figures is a reasonable number to choose for a final answer, unless one of your measurements makes this too many.

12) When drawing graphs, choose scales which make best use of the graph paper. Also distinguish between graphs which require adjacent points to be joined by straight lines (any point between those plotted is meaningless) and those which require the best single line through the points.

REMEMBER, practical work is the foundation of chemistry. Observation leads to theory (in the wider sense), which is tested by experiment and further observation. It is all too easy to separate theoretical work from practical work. Hopefully, the language used in this book will help you to keep the two areas integrated. As you read theory, you should continuously think of experiments to test it.

REMEMBER FINALLY, it is events which determine theory, not theory which determines events.

Unless otherwise stated, all materials in this web version of appendix 2 are © 2007 Adrian Faiers MA (Oxon) MCIPR

What 's the connection between a dozen eggs and a garden mole?

Answer: Not a lot, really, but see Chapter 1