Acids and Bases in Water.
(please print out and bring to class)
Introduction
Many household, industrial and agricultural chemicals are acids or bases.
Their widespread use has increased the stress of such compounds on the
environment. Industrial processes such as the combustion of fossil fuels has
also increased the presence of acids in the environment through acid rain.
Therefore, it is of great importance to the environmental scientist to be able to
understand the chemistry of acids/bases. Since many acids and bases are very
soluble in water, this is one of the environmental compartments most strongly
affected by these compounds. Therefore, the accurate measurement of the
acidity of water is of great importance.
In the present experiment, we will determine the acidity of various
substances (household and industrial) and of natural water samples. For this
purpose, two methods will be employed, pH paper and the pH meter. But before
going into the details of the laboratory procedure, the terms acid, base and pH
need to be defined. For the purpose of this lab we will define acids as those
compounds able to produce hydronium ions (H3O+ also described as H+, or
protons), while bases are defined as those compounds able to generate
hydroxide ions (OH-). Since hydroxide ions readily react with protons to form
water, bases can also be defined as compounds able to accept protons and
acids as compounds able to donate protons. For many purposes, especially
when dealing with very small concentrations of acids, it is difficult to express the
acid concentration (protons) or the base concentration (hydroxide ions) based
on a weight per volume basis. Therefore, a very convenient method was
proposed in 1909 which expresses the acid concentration by the hydrogen ions
(proton) exponent, or pH relationship:

pH = -IO91o[H+] or [H+] = 10-pH

The quantity pH is thus the logarithm (to base 10) of the reciprocal of the
hydrogen ion concentration, or is equal to the logarithm of the hydrogen ion
concentration with negative sign. The pH scale ranges from 0 to 14. A solution
with very high acid concentration would have a pH of 0, while a solution
containing elevated amounts of base will have a pH of 14. A neutral solution has
a pH of 7; acidic solutions have a pH of <7 and solutions containing base
(alkaline solutions) have a pH >7. During the laboratory practice, pH values of
various different solutions will be determined in two ways; one by using pH
paper, the other by using a pH meter.

Experimental section:
EXPERIMENT 1.
Acids and bases are conveniently detected by indicators. Litmus paper
contains such indicators, but shows only whether the solution is acidic (paper
turns red) or if it is basic (paper turns blue). For more precise pH measurements
pH paper is used. Its color ranges from deep blue (pH 10 or higher) through
green (pH 8) all the way down to red (pH 2). The indicator paper should be
wetted with the liquid to be tested and compared with the pH color chart within
30 seconds.
Test the pH of the following substances first with pH paper, then with the
pH meter:
* Deionized water
* Coffee
* Lemon juice
* Vinegar
* Soap solution
* Rain water
* Ammonia
* Soft drink
* Sodium Bicarbonate
* Lake water
Select an acidic and a basic substance from the preceding set and verify
that they can neutralize each other. Describe your results. (EX. Try a sample of
vinegar; add soap solution; observe)

EXPERIMENT 2.
In this experiment you will neutralize a dilute solution of Hydrochloric acid
(HCL; ph = 1) with a dilute solution of sodium hydroxide (NaOH; ph = 13). With
the pipet, take 10 ml of the HCL acid solution and place it into a 100 ml beaker.
Take a buret (50 ml) and rinse it with approximately 3 to 4 ml of the NaOH
solution. Discard the hydroxide solution used for rinsing and fill the buret with
approximately 20 ml of the hydroxide solution. Add 3 drops of phenolthalein
indicator to the HCL acid solution in the beaker and gently stir. Now, introduce
the tip of the buret into the mouth of the beaker and add approximately 1 ml of
base (hydroxide solution) to the acid solution. Gently stir and measure and
record the pH. Repeat this procedure 5 times. Next add approximately .5 ml of
the NaOH solution to the HCL solution. Gently stir and measure and record the
pH. Repeat this procedure 10 times. Finally again add approximately 1 mi of
the NaOH solution to the HCL solution. Repeat this procedure 5 times, stirring,
measuring and recording the pH between each addition. Construct a graph of
the volume of base added vs. pH. You should obtain a S-shaped curve. You will
notice that the pH does not change drastically at the initial stages of the
experiment, nor at the final stages of the experiment. The strongest change in
pH is around a pH of 7 or a neutral pH. This is the pH where your indicator will
also have changed color. Make a graph of your data and describe your
observations.

NOTE: Before starting with your lab practice, the instructor will present a short
demo of the lab techniques to be used. It will benefit you to pay close attention
to this demo!