5.2 Determination of Chloride using an Ion Selective Electrode (ISE)
GLX Version
Introduction
Fresh water and salt water are the two major aquatic environments found on earth. Salt water is a
complex mixture of mainly inorganic anions and cations. The chloride ion is the predominant anion in
seawater present in concentrations in the range of 19,000 ml/L (ppm) while sodium is the predominant
cation present in concentrations in the range of 10,000 ml/L (ppm). Fresh water also contains inorganic
anions and cations but in concentrations several orders of magnitude lower than seawater. The biota of
these two aquatic environments is very different, with fresh water biota generally unable to survive in
salt water and vice versa. While these two aquatic environments are the two largest, the most critical
and important aquatic environments are those where freshwater and saltwater meet. These are
predominately estuarine and salt marsh environments where there is a saline gradient from fresh to
saltwater. These ‘brackish water’ environments are some of the most productive on earth, have the
largest diversity of species, are critical to the health of the entire aquatic environment, and are
important commercially. These environments are also very sensitive to changes in anion and cation
concentrations – an increase or decrease in salinity can cause large changes in the biota.
Global warming will have a profound impact on these ‘brackish water’ environments. Rising sea levels
will destroy present day salt marshes. If this occurs too rapidly it is doubtful that new salt marsh
environments can establish themselves fast enough to prevent the extinction of present day salt marsh
biota. Rising sea levels will also destroy current estuarine environments. In addition, changing rainfall
patterns, with large increases of rainfall in some areas and near drought conditions in others, will bring
added stress to estuarine environments.
Because of its high concentration in seawater, the chloride ion is a good monitor of change in these
estuarine and salt marsh environments from more to less saline or from less to more saline conditions.
The concentration of chloride in more ‘brackish water’ environments could be determined using a
gravimetric method but this would not work in the less ‘brackish water’ environments (for gravimetric
analyses the analyte must be present in concentrations > 10-50 ppt). In addition, gravimetric methods
are very time consuming and are not suitable for monitoring environmental systems. Ion selective
electrodes sensitive to chloride ions and ion chromatography are two alternatives for the determination
of chloride ions. They have a much larger useful concentration range, can perform an analysis within a
few minutes and are therefore more suitable for environmental monitoring where samples have large
concentration ranges of analyte and multiple samples need to be analyzed.
This lab was adapted for high school from a lab found on Dr. Ryan E. Sours webpage from Towson
University, Towson, MD
http://pages.towson.edu/rsours/
http://pages.towson.edu/rsours/docs/210/Lab/Experiment_6d.pdf
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Equipment needed
NaCl
Chloride ion selective electrode
1 250-mL bottle
LabQuest and power supply
6 60-mL bottles
Analytical Balance
1 10-mL Graduated Cylinder
Distilled water
1 100-mL Graduated Cylinder
Weighing paper
1 100-mL Volumetric Flask
1 250-mL beaker (for waste)
1 Wash bottle
1 Spatula
Procedure:
Cleaning
1. Carefully wash and rinse the 250-mL stock solution bottle and the 6 60-mL standard solutions
bottle. When you are finished rinse each of these with a small amount of distilled water to
remove any of the chloride ions that might be present in tap water.
Preparation of stock solution
2. Accurately prepare 100 mL stock solution of 20,000 mg/L NaCl. You must be very careful that
this is mixed up properly because all calculations will be based off of this solution. This will
represent the chloride stock solution.
How many grams of NaCl do you need to weigh out to prepare 100 mL of 20,000 mg/L Cl-1?
(Note: Remember to convert from grams of NaCl to grams of Cl-1)
_______________
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Preparation of Standard solutions
3. From the stock solution prepare 50 mL of 6 standard solutions with concentrations of NaCl
ranging from 20,000 mg/L to 50 mg/L using the following table:
Solution
Stock Solution
Standard 1
Standard 2
Standard 3
Standard 4
Standard 5
Standard 6
Volume of
Solution
needed
100 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
Concentration
Volume of Stock needed
to make 50-mL of each Standard
20,000 mg/L
10,000 mg/L
5,000 mg/L
1,000 mg/L
500 mg/L
100 mg/L
10 mg/L
Collecting Cl-1 ISE reading for Standard solutions
4. Rinse off the tip of the chloride electrode using distilled water from the wash bottle.
5. Place the electrode into the Standard 1 solution and record the relative chloride concentration
from the GLX in the data table.
6. Repeat this procedure for each additional Standard Solution. Rinse the electrode between
samples.
(NOTE: The Cl-1 ISE electrode has not been standardized, thus this is only a relative reading for the Cl1
concentration. You will have to generate a correlation graph between the known Cl-1
concentrations and the Cl-1 ISE readings, later on.)
Collect Cl-1 ISE reading for the unknown samples
7. Collect data for each of the unknown water samples:
(a) distilled water,
(b) tap water,
(c) creek water,
(d) sea water,
(e) swimming pool water
(f) bottled water (ex: Dasani, Evian, Aquafina…)
Making correlation graph of actual Cl-1 concentrations versus Cl-1 ISE readings
8. Graph the data with Cl-1 concentration versus Cl-1 ISE reading. Please see the Excel Instructions
below.
9. Graph the data with log [Cl-1] on the x-axis and the electrode potential on the y-axis.
10. Get a linear fit for this graph and determine the slope and y-intercept for the best fit data.
11. Write out the equation of the best fit line.
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(Note: This equation now relates the Cl-1 ISE readings to the actual concentration of Cl-1 in the
solutions. Thus, if you get a Cl-1 reading from an unknown solution you can plug that into the
equation to determine the actual concentration of Cl-1 in the unknown.)
12. Determine the Cl-1 concentrations in the unknowns using the equation of the best fit line you
generated from your graph.
Excel Instructions for Graphing a straight line:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Set up your data table by entering your data into two columns
Select Chart Wizard , -OR- Insert and then Chart.
Select:
, then
Select the Series tab, IF you see a graph proceed to step 5, otherwise press
.
Set the y data by clicking the
icon next to ‘y data range’. Use your mouse to select the
column of data you want displayed on your y axis. Then click the
icon again to set that data
as y axis on your plot.
Set the x data by clicking the
icon next to ‘x data range’. Use your mouse to select the
column of data you want displayed on your x axis. Then click the
icon again to set that data
as x axis on your plot.
Check to make sure values are displayed on the proper axes. If not, go back to step 4 & 5 to fix
it.
Click
On the Chart Options,
a. Enter in the Title for your graph.
b. Fill in Value (x) axis (title and units)
c. Fill in Value (y) axis (title and units)
On the Axes tab, check that X and Y are selected to be displayed
On the Gridlines tab, select major x and y
Legend tab , unselect “show legend” to get rid of this
Click
Select Object on new sheet, then
This puts the new chart beside your data so that you
can print off both on one sheet of paper.
On the tool bar above, select
, then
a. Type - Linear
b. Options
i. Equation (so you can get the slope)
ii. Display R2 value. (if desired; tells how well regression line fits the data. Best fit =
1.0, if this number is less than – 0.8 then this is not a straight line but some
other mathematical function.)
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Determination of Chloride using an Ion Selective Electrode
Name__________________________
Date________________
Data Table:
Solution
Concentration
Stock Solution
20,000 mg/L
Standard 1
10,000 mg/L
Standard 2
5,000 mg/L
Standard 3
1,000 mg/L
Standard 4
500 mg/L
Standard 5
100 mg/L
Standard 6
10 mg/L
Cl-1 ISE reading
(mV)
Slope of best fit line:
_____________________
y-intercept of best fit line:
_____________________
Regression factor ( |r| ):
_____________________
Write out the equation of the best fit line through your experimental data. (Write out the equation in
the form of y = mx + b) _______________________
Unknowns
Concentration
(calc using the eq. of a line for
the correlation graph)
Cl-1 ISE reading
(mV)
Distilled Water
Tap Water
Creek Water
Sea Water
Swimming Pool
Water
Bottled Water
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Questions
1. Which water sample had the highest concentration of Chlorine ions?
2. How do chlorine ions affect the environment?
3. How much Chlorine gas is considered toxic?
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