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Acid-Base Reactions
The easiest way to make many of the simpler chemicals in the recipe is by doing a titration with a strong acid and strong base. For example, if you need to make calcium nitrate, you should mix calcium hydroxide and nitric acid. These will react to make calcium nitrate (actually, dissolved calcium ions and nitrate ions) and water. To figure out how much acid and base to mix, you must use stoichiometry. For example:
The reaction for calcium hydroxide and nitric acid is:
Ca(OH)2 + 2HNO3 --> 2H2O + Ca(NO3)2
So to find out how much of each to use, you would calculate:
Note that you need the molar masses of calcium nitrate and calcium hydroxide and the molarity of the nitric acid in order to find these amounts.
Materials needed:
appropriate acid and basebeaker or flask
deionized water
fume hood
stirring rod
chemical scoop
graduated cylinder
balance
filter paper or container to hold base while measuring
pH paper or pH meter
When mixing the nitric acid and calcium hydroxide, first put some deionized water in a beaker or flask. You must use deionized water because tap water usually contains ions that can mess up the reaction. Then carefully measure out the appropriate amount of acid in a graduated cylinder, and add the acid to the water- never add water to acid! Do this part under a fume hood because it will generate noxious fumes, and be very careful not to get any of the acid or base on your skin. Measure out the hydroxide on a balance with a piece of filter paper or a container to hold the chemical. Add the hydroxide slowly to the acid while stirring, and make sure it all dissolves. Be aware that a reaction between strong acids and strong bases such as these can generate a lot of heat.
Once you mix the acid and the base together, the reaction most likely will not be exact due to measuring error. If you don't correct for this, you may end up working with solutions of very high or very low pH. To correct for this, measure the pH of the mixture with pH paper or a pH meter, and then add extra acid or base according to the pH until the pH reaches about 7. You will then have a neutral solution of the chemical you need dissolved in water, and you can add this to the bottle of concentrated solution where you would normally add the chemical.
Focus Questions
- How do you figure out how much acid and how much base to mix?
- Why must you always use deionized water when you are mixing chemicals? What is wrong with tap water?
- What precautions must you take when mixing acids and bases?
- How do you adjust the pH of the final mixture to 7?
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© 2001 S.H.A.R.P.: The Super Hydroponic Awesome Radish Project. All rights reserved. Photographs from this page may not be used without permission.
