Changing the pH of salt solutions by an electric current
Juernene Tholel & Patrick Hack
Cygnus Gymnasium, Amsterdam, The Netherlands
Received June 2011
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Summary
pH is a measure of the acidity or basicity of an aqueous solution. A solution with a pH lower
than 7 is acidic and a solution with a pH greater than 7 is basic. The question answered in
this article is how the pH of two the same salt solutions, connected by a salt bridge, change
as a current is passed through them. Two solutions of magnesium sulfate-7H2O in water
showed that when a power supply supplied 9 volt to the circuit redox reactions took place.
These redox reactions ensured that the pH of both solutions changed. The solution on the
anode side of the circuit became acidic and the solution at the cathode side became basic.
Introduction
The presence of hydrogen ions(H+) and
hydroxide ions(OH-) in solutions determine
if a solution is acidic or basic. Acids have
a higher concentration of hydrogen
ions than hydroxide ions(pH below 7).
Bases have a higher concentration of
hydroxide ions than hydrogen ions(pH
higher than 7).
The pH in solutions can change because
of redox reactions (reduction and
oxidation reactions). Redox reactions are
processes where electrons move from one
molecule to another.
Water can be electrolyzed by passing an
electric current through it in an
electrochemical cell. Electrons from the
power supply are added to the water
molecules at the negative electrode.
A question relevant to this is: How does
the pH of two the same salt solutions,
connected by a salt bridge, change as a
current is passed through them?
The electrode connected to the negative
terminal of the power supply is called
the cathode. Adding electrons results in
the following reduction reaction:
2H2O + 2 e- → H2 (gas) + 2OHThe water will react with electrodes
supplied by the negative terminal of the
power supply. Forming hydrogen gas and
2 hydroxide ions.
The other electrode is attached to the
other, positive, terminal of the power
supply. At this electrode, the anode,
electrons are removed from the solution by
the electrode. So at the cathode electrons
are added and at the anode they are
removed. Because of this the circuit is
completed and the current can flow. At the
anode the water is oxidized by the
following reaction: : H2O → 1/2 O2 (gas) +
2H+ + 2eSo at the anode the water will react and
form oxygen gas, hydrogen ions, and
electrons.
The oxidation reaction and the reduction
reaction can’t happen if one of both
doesn’t happen. So both reactions always
happen at the same time.
It is essential that a current can flow from
the anode to the cathode, because else
the electrolysis isn’t possible. Because of
this dissolving magnesium sulfate in
purified water is a smart choice. Purified
water itself is a poor conductor, but
because the dissolved magnesium sulfate
become positive magnesium ions(Mg 2+)
and negative sulfate ions(SO42-, which
allow the current to from the anode to the
cathode. Magnesium sulfate-7H2O is a
good choice for this experiment because it
doesn’t react with the carbon electrodes,
so only water will react.
Essential is that the solutions stay
separated, but the current must be able to
pass. For this the a salt bridge is needed.
Our hypothesis is that there will be a
difference in pH at the end of the two
solutions because of the electrolysis of
water that will take place. The pH of the
solution on the cathode side will get higher
and the pH of the solution on the anode
side lower, because on the anode side the
oxidation of water will take place and on
the cathode side the reduction of water.
So in the solution on the anode side
hydrogen ions will be formed and solution
on the cathode side hydroxide ions will be
formed.
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Experimental procedure
Two cleaned measuring cups were filled
with 75mL of purified water. 20 grams of
magnesium sulfate-7 H2O were put into
one of the filled measuring cups and
another 20 grams into the other measuring
cup. After both mixtures had been stirred,
the magnesium sulfate-7H2O dissolved in
the water and it became a colorless
solution. Six drops of bromothymol blue
were added to both solutions. This gave
the solutions a green/blue color.
Bromothymol blue gives colorless
solutions a blue color when the pH of the
solution is higher than 7.6 and a yellow
color when the solution has a pH lower
than 6.0. Because both solutions were
neutral(pH = 7.0) the bromothymol blue
gave the solutions a green/blue color.
A power supply was connected to two
carbon electrodes with insulated wires and
an alligator clip. One carbon electrode was
connected to the supply’s positive side, so
this electrode became the anode. The
other carbon electrode was connected to
the supply’s negative terminal, making it
the cathode. At last a salt bridge
connected the solutions of magnesium
sulfate-7H2O.
A pH-meter was used to measure the pH
of both solutions every five minutes for a
total time of 35 minutes. To make sure
that the solutions didn’t mix while changing
the pH meter from one solution to the
other. The pH meter had been cleaned in
purified water every time it would be
switched.
Figure 1: Schematic draw of the experimental setup. Both solutions in the picture consist of:
Mg2+(aq) ions, SO42-(aq) ions, the bromothymol blue drops, which made the solutions turn
blue/green and of course H2O.
Results
The results obtained are shown in the
table and figures below:
Table 1: the change in pH of both magnesium sulfate-7 H2O solutions at the anode and
cathode side of the circuit.
Time (minutes)
pH
pH
Anode +
Cathode-
0
7.21
7.21
5
6.66
7.60
10
6.21
8
15
5.81
8.61
20
5.63
9.41
25
5.40
9.73
30
5.22
10.01
35
4.86
10.52
Figure 2: Graph of the change of pH of the magnesium sulfate-7 H2O solutions at the
cathode and anode side of the circuit.
Figure 3: Picture showing the change in color of both magnesiumsulfate-7H2O solutions
Another thing that became visible was that
bubbles got produced at the electrodes. At
the cathode side H2(gas) got produced(as
seen in the equations in the introduction).
And at the Anode side the oxidation of
water took place so O2(gas) got produced.
Discussion and Conclusion
It became visible and was measured that
the pH got lower at the anode side and pH
rose at the cathode side. This became
visible, because of the change of color of
the solutions. First both solutions were
green/blue but at the end the anode side
had a yellow color. The cathode side got a
blue color. The anode side turned acidic
and the cathode side basic.
So our hypothesis on the inquiry question
has been proved to be correct. The pH did
change on both sides. The anode side has
become acidic and the cathode side basic
because of the forming of hydroxide ions
and hydrogen ions. At the cathode the
reduction of water took place:
2H2O + 2 e- → H2 (gas) + 2OHBecause of the hydroxide ions(OH-) that
got produced at the cathode side the
solution became basic, so the pH rose. At
the same time bubbles became visible,
because of the H2(gas) that got produced.
At the anode side the oxidation of water
took place producing hydrogen ions(H+):
H2O → 1/2 O2 (gas) + 2H+ + 2eBecause of the Hydrogen ions that got
produced the pH lowered, so the solution
became acidic. At the same time also on
this
side bubbles became visible because of
the O2(gas) that got produced.
Evaluation
We believe, when evaluating our
experiment, that our results are valid. The
measuring cups we used were cleaned
thoroughly before we used it, making sure
that there weren’t any other substances
still in the cups. Also while measuring we
had to switch from one solution to the
other, because we only had one pH-meter.
Because we didn’t want the solutions to
mix we cleaned the pH-meter every time in
purified water when we switched the pHmeter from one solution to the other. By
this we made sure the solutions didn’t mix.
Something that could have been improved
was the duration of the experiment. We
did not have any time left at the time we
did the experiment and we had to stop
after 35 minutes. It would have been
interesting to see if something different
would have happened after a longer time.
Our school didn’t have a lot of magnesium
sulfate-7H2O and therefore we weren’t
able to repeat the experiment, which
would ensure even more that our results
were valid. Repeating the experiment is
something that we actually should have
done, although we couldn’t.
-------------------------------------------------------------------------------------------------Bibliography:
1. http://www.chemistryrules.me.uk
/tandp/optiontransitionelements.htm
2.http://en.wikipedia.org/wiki/Magnesium
_sulfate
3. http://en.wikipedia.org/wiki/PH
4. “BINAS”, NVON fifth edition, 2004,
Wolters-Noordhoff bv Groningen,
The Netherlands
5. Icy – road salt, January 2011,
Vrije Universiteit Amsterdam, The Netherlands
6. http://en.wikipedia.org/wiki/Electrolysis
_of_water