"Electrical Conductivity of Aqueous Solutions"

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Electrical Conductivity of Aqueous Solutions
Objectives
The objectives of this laboratory are:
a) To observe electrical conductivity of substances in various aqueous solutions
b) To determine of the solution is a strong or weak electrolyte
c) To interpret a chemical reaction by observing aqueous solution conductivity.
Background
Electrical conductivity is based on the flow of electrons.
Metals are good conductors of
electricity because they allow electrons to flow through the entire piece of material. Thus,
electrons flow like a “sea of electrons” through metals. In comparison, distilled water is a very
poor conductor of electricity since very little electricity flows through water. Highly ionized
substances are strong electrolytes. Strong acids and salts are strong electrolytes because they
completely ionize (dissociate or separate) in solution. The ions carry the electric charge through
the solution thus creating an electric current. The current, if sufficient enough, will light one or
both LEDs on a conductivity meter, shown at right.
Slightly ionized substances are weak electrolytes. Weak acids and bases would be categorized
as weak electrolytes because they do not completely dissociate in solution.
Substances that do not conduct an electric current are called non-electrolytes. Non-electrolytes
do not ionize; they do not contain moveable ions. The LEDs of a conductivity meter will not
light because there are no ions to carry the electric current. The table below lists examples of
strong, weak and non-electrolytes.
Strong Electrolytes
Weak Electrolytes
Strong Acids
Weak Acids
Hydrochloric acid
HCl (aq)
Hydrobromic acid
HBr (aq)
Acetic acid
HC
H
O
(aq)
2
3
2
Hydroiodic acid
HI (aq)
Carbonic acid
H
CO
(aq)
2
3
Nitric acid
HNO
(aq)
Citric acid
C
H
O
(aq)
3
6
8
7
Sulfuric acid
H
SO
(aq)
Phosphoric acid
H
PO
(aq)
2
4
3
4
Perchloric acid
HClO
(aq)
4
Chloric acid
HClO
(aq)
3
Strong Bases
Weak Bases
Sodium hydroxide
NaOH (aq)
Ammonia
NH
(aq)
3
Potassium hydroxide
KOH (aq)
Ammonium hydroxide
NH
OH (aq)
4
Calcium hydroxide
Ca(OH)
(aq)
Magnesium hydroxide
Mg(OH)
(aq)
2
2
Barium hydroxide
Ba(OH)
(aq)
Most other bases
2
Soluble Salts
Slightly Soluble Salts
Sodium chloride
NaCl (aq)
Silver chloride
AgCl (s)
Potassium carbonate
K
CO
(aq)
Calcium carbonate
CaCO
(s)
2
3
3
Copper(II) sulfate
CuSO
(aq)
Barium sulfate
BaSO
(s)
4
4
Non-Electrolytes
Distilled water
H
O (l)
2
Methanol
CH
OH (aq)
3
1
Electrical Conductivity of Aqueous Solutions
Objectives
The objectives of this laboratory are:
a) To observe electrical conductivity of substances in various aqueous solutions
b) To determine of the solution is a strong or weak electrolyte
c) To interpret a chemical reaction by observing aqueous solution conductivity.
Background
Electrical conductivity is based on the flow of electrons.
Metals are good conductors of
electricity because they allow electrons to flow through the entire piece of material. Thus,
electrons flow like a “sea of electrons” through metals. In comparison, distilled water is a very
poor conductor of electricity since very little electricity flows through water. Highly ionized
substances are strong electrolytes. Strong acids and salts are strong electrolytes because they
completely ionize (dissociate or separate) in solution. The ions carry the electric charge through
the solution thus creating an electric current. The current, if sufficient enough, will light one or
both LEDs on a conductivity meter, shown at right.
Slightly ionized substances are weak electrolytes. Weak acids and bases would be categorized
as weak electrolytes because they do not completely dissociate in solution.
Substances that do not conduct an electric current are called non-electrolytes. Non-electrolytes
do not ionize; they do not contain moveable ions. The LEDs of a conductivity meter will not
light because there are no ions to carry the electric current. The table below lists examples of
strong, weak and non-electrolytes.
Strong Electrolytes
Weak Electrolytes
Strong Acids
Weak Acids
Hydrochloric acid
HCl (aq)
Hydrobromic acid
HBr (aq)
Acetic acid
HC
H
O
(aq)
2
3
2
Hydroiodic acid
HI (aq)
Carbonic acid
H
CO
(aq)
2
3
Nitric acid
HNO
(aq)
Citric acid
C
H
O
(aq)
3
6
8
7
Sulfuric acid
H
SO
(aq)
Phosphoric acid
H
PO
(aq)
2
4
3
4
Perchloric acid
HClO
(aq)
4
Chloric acid
HClO
(aq)
3
Strong Bases
Weak Bases
Sodium hydroxide
NaOH (aq)
Ammonia
NH
(aq)
3
Potassium hydroxide
KOH (aq)
Ammonium hydroxide
NH
OH (aq)
4
Calcium hydroxide
Ca(OH)
(aq)
Magnesium hydroxide
Mg(OH)
(aq)
2
2
Barium hydroxide
Ba(OH)
(aq)
Most other bases
2
Soluble Salts
Slightly Soluble Salts
Sodium chloride
NaCl (aq)
Silver chloride
AgCl (s)
Potassium carbonate
K
CO
(aq)
Calcium carbonate
CaCO
(s)
2
3
3
Copper(II) sulfate
CuSO
(aq)
Barium sulfate
BaSO
(s)
4
4
Non-Electrolytes
Distilled water
H
O (l)
2
Methanol
CH
OH (aq)
3
1
Procedure
Safety
Be cautious with hydrochloric acid, nitric acid, sulfuric acid and concentrated acetic acid.
Although low in concentration, some individuals may have extreme skin sensitivities. If you
experience any tingling sensations or skin discolorations, rinse immediately with large amounts
of water for 15 minutes. Inform your instructor ASAP.
Personal Protective Equipment (PPE) required: lab coat, safety goggles, closed-toe shoes
Materials and Equipment
conductivity meter, wash bottle with distilled water, large beaker for rinsing/waste, small
beakers, Kimwipes, solid sodium chloride, solid calcium carbonate
Solutions: acetic acid, aluminum nitrate, ammonium hydroxide, calcium hydroxide, citric acid,
ethanol, hydrochloric acid, magnesium hydroxide, magnesium sulfate, nitric acid, potassium
iodide, sodium chloride, sodium hydroxide, sucrose
Conductivity Testing – Evidence for Ions in Aqueous Solution
1. The meter has a 9V battery, and two parallel copper
electrodes.
Use a wash bottle with distilled water and a large beaker
labeled “waste” to rinse the copper electrodes. Dry using a
Kimwipe tissue. When switched on, the lights should not be
lit any color. If they are, repeat the rinsing and drying.
NOTE: DO NOT EXPOSE THE CIRCUIT BOARD TO WATER.
Only the copper electrodes should be rinsed with water.
2. Place the meter so that the circuit board is facing up (the battery will be below). Always
place the meter in this way so that the circuit board will not get wet. On this side, there is a
guide to the possible conductivity measurements:
Scale
Red LED
Green LED
Conductivity
0
off
off
low or none
1
dim
off
low
2
medium
off
medium
3
bright
dim
high
4
very bright
medium
very high
Switch the meter on and dip the copper electrodes to test conductivity.
Thoroughly rinse with distilled water after each test, and dry with Kimwipes.
Switch the meter off between uses.
3. Place 5 mL of distilled water into a small, clean beaker. Test and record your results.
4. Place 5 mL of tap water into a small, clean beaker. Test and record your results.
2
5. Place about 0.2 g of solid sodium chloride (NaCl) into a small, clean beaker and test the
conductivity. Add 5 mL distilled water to the sodium chloride; test the conductivity of the
solution. Dispose of this solution in the sink and rinse the beaker.
6. Place about 0.2 g of solid calcium carbonate (CaCO
) into a small, clean beaker and test the
3
conductivity. Add 5 mL distilled water to the calcium carbonate; test the conductivity of the
solution. Dispose this solution in the sink and rinse the beaker.
7. Use 5 mL of each of the following in 100-mL beaker to test the conductivities.
Be sure to rinse and dry the electrodes between tests, using your wash bottle with waste
beaker, and Kimwipes.
Dispose the solution and rinse the beaker in the sink between tests. Dispose the waste beaker
solution in non-hazardous waste in the hood.
a. acetic acid, 0.1 M HC
H
O
2
3
2
b. aluminum nitrate, 0.1 M Al(NO
)
3
3
c. ammonium hydroxide, 0.1 M NH
OH (aq)
4
d. calcium hydroxide, saturated Ca(OH)
2
e. citric acid, 0.1 M C
H
O
6
8
7
f. ethanol, CH
CH
OH
3
2
g. hydrochloric acid, 0.1 M HCl
h. magnesium hydroxide, saturated Mg(OH)
2
i. magnesium sulfate, 0.1 M MgSO
4
j. nitric acid, 0.1 M HNO
3
k. potassium iodide, 0.1 M KI
l. sodium chloride, 0.1 M NaCl
m. sodium hydroxide, 0.1 M NaOH
n. sucrose, 0.1 M C
H
O
12
22
11
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