MadSci Network: Chemistry

Re: Testing the pH of a salt solution

Date: Thu Apr 22 18:13:28 2010
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Chemistry
ID: 1270595334.Ch

ACID BASE theories:–base_reaction
Also consult a good college chemistry text

Lesson Plans:
or just google or bing ACID BASE if you look around you will find more.

Lab Safety:

To write lab procedures for chemistry students one should be a chemist who has studied general college chemistry, had relevant laboratory experience, had training in and exposure to standard lab safety procedures and studied quantitative analysis, physical chemistry, and organic chemistry and has had training as a teaching assistant.

If you haven't had appropriate training DO NOT WRITE procedures.

I have seen three serious industrial accidents using insufficiently tested procedures involving concentrated acids, one severely disfiguring a young woman and causing her continuing anguish. In each case the procedure was not sound and was assigned to a technician without supervised testing and the technician was afraid to complain. Of course the technician was blamed for not following instructions properly. Tthe training and general knowledge of the potential users must be taken seriously.

From reading your question I feel that you are not yet qualified to write laboratory procedures. So find a introduction to Chemistry lab manual and read up on what should be in a lab procedure and lab notebook write up. Then find a procedure that pertains to your lesson objective [Your objectives also seem vague in your question].

You indicate 4 possible objectives: 1. a simple understanding of what acids and bases are. This is most simply explained by the Arrenhius theory in aqueous solution. Acids form H3O+ and bases form OH- in water. After giving some examples with balanced equations you should mention the more advanced theories; some students will be interested. Explain the differences between concentrated and diluted acids and bases and the care necessary in using these. The large heats of dilution can be explained here. Explain the differences between strong acids and weak acids giving examples of HCl, acetic, carbonic, citric acids and the common manifestions such as stomach acid and vinegar and lemon juice and soda water [sodas also have phosphoric and citric acids].

2. Pure water is in equilibrium with equal amounts of H3O+ and OH-. The concentration is very low about 1X10-7 of each. 2 H2O = H3O+ + OH-. Solutions of salts from the reaction of a strong acid and a strong base have about the same equal concentration and the solution is neutral. Reactions involving weak acids and weak bases are usually not neutral at the equivalence point.

3. Water solutions of acids react with water solutions of bases to give additional water and a solution of salts. If the base and acid are added in exact equal molar amounts they are said to be neutralized. This principle is used in titration to determine amounts of acids or bases. The equivalence point is best determined by electromeric titration using a pH electrode. Then an indicator is chosen that changes color near the pH of the equivalnce point. In the case of reaction of a strong acid such as HCl with a strong base such as NaOH the equivalence point is essentially the same as in pure water at pH 7.

I would recommend use of phenolphthalein [red in base, clear in acid] and methyl red [yellow in base, red in acid]. Using the mixed indicator the solution is yellow when neutral care being taken not to add excess of either acid or base. pH paper is less reliable. Caution is necessary because if there remains excess of either acid or base concentrating the solution to recover the salt increases the acid or base concentration possibly to dangerous levels.

4. Recovery of NaCl: I recommend slow evaporation in an oven set about 100oC or maybe lower. Rapid evaporation of water from salt solutions can result in bumping and spattering if done too quickly and the spattered salt is hot. If indicators are used the salt crystals will be tinted This can probably be removed by rinsing with an alcohol water solution or just ignored. The cubic crystals of the salt should be evident.

Your question about the heat of neutralisation. it is 14000 calories /mole If you neutralize 0.1 Molar solutions you can calculate the heat generated per mL and the temp rise hint it is about 1 degree so the heat generated in neurtralisation of DILUTE solutions is of little concern

Safety: General Lab Safety to include safety glasses, a lab apron, good lab gloves; when working with bases a lab glove that leaks can be more dangerous than no glove. 0.1 N solutions are relative innocous and can be handled without gloves provided the students are trained in the handling and transfer of liquids. See the links above.

General lab procedure outline:

1 Purpose of experiment
2. relevant chemistry BALANCED EQUATIONS
3. Table of reactants and predicted products
4. List of and possible arrangement of needed equipment
5. Step by step procedure including lab safety reminders
6. Results and data
7. cleanup procedure
8. Discussion and conclusion
9. Answer questions about a learning experiment. for the formation of NaCl you could ask how are hydrochloric acid and sodium hydroxide produced? Is the reaction a reasonable way to make NaCl? how is NaCl produced?

Any procedure should be tested by yourself and by a knowledgeable colleague. Then it should be tested under supervision by a person of the target skill level to find bugs. Classroom or lab use must be monitored to find hidden flaws and possible improvements.

Enjoy your studies.

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