Have you seen
an acid or base solution?
If your answer is no, you must be an alien because we earth
people encounter acids and bases in everyday life. Orange juice,
which we drink everyday, is a citric acid
solution. Vinegar? Acetic acid.
Commercial antacid? Magnesium hydroxide (base).
Household cleaning products? Ammonia (base)...Hmmm...
What is the common property of orange juice and vinegar? Sour
taste, right? Acids have a sour
taste and acid solutions are capable of dissolving certain metals such
as iron and zinc. In contrast, bases
have a bitter taste and slippery feel.
What is acid-base
Acids were defined by the
Swedish chemist Arrhenius as substances that, when dissolved in water,
produce hydrogen ions (H+). For example, gaseous hydrogen
chloride reacts with water to give hydrochloric acid and increases the
amount of hydrogen ion in the water.
Bases are also defined as substances
that when dissolved in water, yield hydroxide ions (OH-).
For example, sodium hydroxide (NaOH) dissolves in water and increases of
the amount of hydroxide ions in the water.
cf. The symbol (g) is used to designate the gaseous
state, (s) for the solid, and (aq) for the aqueous state.
How can we know
it's an acid or base?
Tasting a lab chemical is very dangerous and you don't want
to taste a liquid to determine whether it is an acid or not. Then
what would you do?
One easy way is using indicators. One familiar example is a litmus
indicator. Litmus indicators change the color from blue to red when added
to acid solutions. In contrast,
base solutions change litmus indicators
in color from red to blue. Many natural dyes found in fruits, vegetables,
and flowers act as indicators, too. For example, red cabbage extract is
red in acidic solution and blue in
However, the indicators cannot show which acid solution is more acidic
than the other acid solutions or which base solution is more basic than
the other base solutions. Then what would you do to measure the acidity
Scientists use the "pH" scale to determine how acidic or basic
a solution is. pH is a measure of the hydrogen ion concentration
of a solution and is defined as the equation.
Usually the pH scale goes from 0 to 14 and acids are found between 0
to 7 and bases are from 7 to 14. The middle point of the pH scale
is 7 and distilled water is exactly 7. In other words, water is neither
acidic nor basic, but neutral. The lower pH value an acid solution has,
the more acidic it is. In contrast, the higher pH value a base solution
has, the more basic it is. According to the pH values, some common
acids and bases are arranged in the following figure. Now, can you
tell which one is more acidic? lemon juice or vinegar ?
What would happen
if an acid and a base were mixed up?
Let's think about it this way. What would happen if a
hydrogen ion (H+) from an acid reacted with a hydroxide ion (OH-) from
a base? That's right. Water would be produced.
If the spectator ions (other than H+ and OH- ions) from the acid and
the base are put back into the equation, it would read, for example,
showing that an acid reacts with a base to yield water plus an ionic
compound called a salt. This acid-base reaction is called a neutralization
What is titration?
Using the neutralization reaction, you can determine the concentration
of an acid or a base solution. As shown in the previous equation,
one hydrogen ion from an acid reacts with exactly one hydroxide ion from
a base to produce a water molecule. If adding 100 hydroxide ions
to an acid solution makes it a neutral solution (only water and salt),
there must be 100 hydrogen ions in the original acid solution and you can
calculate the concentration of the acid solution.
To titrate an unknown acid/base solution, take a certain amount of the
unknown solution and add a standard reagent of the known concentration
carefully until the neutralization reaction is completed. This point where
the number (or mole) of hydrogen ions and hydroxide ions are equal is defined
as the equivalence point. To determine the equivalence point, scientists
use an indicator or a pH meter. With the data of volume of the standard
reagent used, the concentration of the unknown solution can be calculated.
This whole process is called "titration."
|First, you need to choose volume of an unknown
acid or base solution and put it in an erlenmeyer flask.
Second, fill a buret with a standard reagent of known concentration
and read the initial volume of the solution. Of course, if you put
an acid in the erlenmeyer flask, you need to put a base in the buret and
vice versa. A buret is a good apparatus for the determination of
an equivalence point in acid-base titration because you can accurately
read the volume of solution used.
Third, add a couple of drops of an indicator in the flask for titration.
An indicator is a soluble dye that changes its color noticeably over a
fairly short range of pH. Different indicators show color changes at different
pH values and it is important to determine an indicator to be used according
to the expected equivalence point.
If a pH meter is available, put a pH electrode in the flask.
Fourth, slightly open the cork of the buret and add the standard reagent
into the unknown solution. Around the expected equivalence point
of the titration, you need to drop the solution very slowly and mix the
solutions very well because, around the equivalence point, just one drop
of solution from the buret can make a radical pH change in the mixed solution.
If the color of the solution in the erlenmeyer flask changes, record the
volume of the solution in the buret and add a few drops of the solution
to make sure the the equivalence point you found is correct.
Finally, using the data from your acid-base titration, you can calculate
the concentration of the unknown solution. The equation for this procedure
is as follows. (Here, M means molarity (concentration) of solution , and
V means volume of solution.)
For example, if you choose 10.0 mL hydrochloric acid solution as a concentration-unknown
solution and it takes 10.0 mL of 0.100 M sodium hydroxide solution to titrate,
the concentration of the hydrochloric acid is 0.100 M.
If you want to use a polyprotic acid or base (which
can donate two or more hydrogen or hydroxide ions per molecule) for titration,
you need to multiply the molarity of the solution by the number of hydrogen
or hydroxide ions it can donate per molecule.
titrate some acids and bases you'd like.
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