REFRACTIVE INDEX OF SALT SOLUTION
AS RELATED TO ITS SALINITY AND
CONCENTRATION
by:
Caculitan, Bonifacio Jr. D.
LEAP-CTP1
National Institute of Education, Singapore
October 2009
REFRACTIVE INDEX OF SALT SOLUTION
AS RELATED TO ITS SALINITY AND
CONCENTRATION
______________________________
A project work presented to
A/P Augustine Tan Tuck Tee
In partial fulfillment of the
Requirements in Physics
Pedagogical Knowledge
______________________________
by:
Caculitan, Bonifacio Jr. D.
LEAP-CTP1
National Institute of Education, Singapore
October 2009
REFRACTIVE INDEX OF SALT SOLUTION AS RELATED TO
ITS SALINITY AND CONCENTRATION
Caculitan, Bonifacio Jr. D.
LEAP-CTP1
National Institute of Education, Singapore
ABSTRACT
A study on refractive index of salt solution was conducted to determine its effect
to salinity and concentration. Specifically the study aimed to apply Snell’s Law in
determining the refractive indices of salt solutions at different salinity, and
concentration at constant temperature.
The following hypotheses were tested in this study: 1) There is no difference with
the computed value of refractive index at different salinity and concentration at constant
temperature; and 2) Concentration and salinity of salt solution do not affect refractive
index.
Results showed that to obtain the standard value of refractive index, salinity, and
concentration for water used in aquaria, at 40 degrees angle of incidence angle of
refraction must be 59.50 degrees. The results also showed that as refractive index of salt
solution increases, salinity and concentration also increases. Based on the result of the
study, the null hypotheses were rejected.
It is finally recommended that salinity can be obtained using Snell’s law.
Aquarists can easily maintain the salinity of water in their aquarium by simply
observing the angles of incidence and refraction without necessarily obtaining the value
of refractive index.
INTRODUCTION
Snell's law (also known as Descartes' law, the Snell–Descartes law, and
the law of refraction), is a formula used to describe the relationship between the
angles of incidence and refraction, when referring to light or other waves passing
through a boundary between two different isotropic media, such as glass, water, and salt
solution. The law says that the ratio of the sines of the angles of incidence and refraction
is a constant that depends on the media. Named after Dutch mathematician Willebrord
Snellius, Snell's law states that the ratio of the sines of the angles of incidence and
refraction is equivalent to the ratio of velocities in the two media, or equivalent to the
opposite ratio of the indices of refraction.
In optics, the law is used in ray tracing to compute the angles of incidence or
refraction, and in experimental optics to find the refractive index of a material.
Refractive index is one of the most important optical properties of a medium. It plays
vital role in many areas of physical science with special reference to fiber optics.
Similarly, measurement of refractive index is widely used in analytical chemistry to
determine the concentration of solutions. Some studies (Schwartz 1999, Olesberg 2000,
Shlichta 1986) provide more detailed discussion on the concentration mapping by the
measurement of refractive index of liquids. Several techniques are reported in literature
for the measurement of concentration dependence of refractive index of liquids
(McPherson 1999, Garcia 1999, Otalora 1999, Miyashita 1994). The present study uses a
relatively simple and effective technique, which can be used to measure the refractive
index of salt solution at different concentration.
On the other hand, according to Holmes-Farley (Reefkeeping Magazine, Volume
8, No. 3 April 2009 Issue), salinity is one of the most important parameters measured in
reef aquaria. It controls not only the salt balance between an organism and its
surrounding environment, but also the levels of a host of ions in seawater that aquarists
neither measure nor control independently. Consequently, aquarists must monitor
salinity to ensure that organisms are not stressed by moving between aquaria of
potentially different salinity, and that the salinity of the aquarium itself is controlled
within ranges that organisms thrive in.
Seawater is approximately 96.5% water and 3.5% salt, by weight. When the
salinity of seawater is referred to as being 35 ppt (parts per thousand), that is the same
as saying 3.5% salt. Seawater is composed of many different ions (salts), with the total
of all adding up to the 3.5%. Some ions are present at very high concentration and some
at very low concentration. However, even those present at very low concentration can
be important to organisms living in the water.
When salts are dissolved in water, a variety of the water's attributes change,
including its density, its refractive index and its conductivity. All of these changes can
be used by aquarists as a way to measure the total amount of salt in
solution. Consequently, aquarists need not dry out water to find the salinity by the
amounts of salts that remain behind, but can simply rely on tools that measure these
other attributes. Hence, salinity is considered in this study to determine its relation to
refractive index.
Objectives:
Specifically, the study aimed to:
1. Apply Snell’s Law in determining the refractive indices of salt solutions at
different salinity, and concentration at constant temperature;
2. Determine the effect of refractive index to concentration and salinity of salt
solutions.
Hypotheses:
The following hypotheses were tested in this study:
1. There is no difference with the computed value of refractive index at different
salinity and concentration at constant temperature.
2. Concentration and salinity do not affect refractive index of salt solution.
Significance of the Study
The study is significant to the aquarists who need to maintain the salinity of their
aquarium by just considering the angles of incidence and refraction to come up with the
refractive index. Likewise the study is significant to students and researchers who are
trying to determine the effect of refractive index and salinity of salt solution.
METHODOLOGY
Materials and Equipment
The following materials and equipment were used in this study:
Laser Refraction tank is used to measure the angle of incidence and angle of refraction
to come up with refractive index.
Beaker is used to measure the volume of purified water.
Digital Weighing Balance is used to measure the mass of purified water and iodized salt.
Teaspoon is used to transfer iodized from container to the solution container.
Laboratory thermometer is used to measure the temperature of the solution.
Purified water is used as solvent for the solution.
Iodized salt is used as solute for the solution.
PROCEDURE
The flow chart shows the stages of the procedure employed in this study:
Preparation and gathering of
materials
Measuring the volume and
mass of purified water that will
be accommodated in the laser
refraction tank
Measuring the angles of
incidence (40 O) and refraction
of purified water using the laser
refraction tank
Recording and analyzing the
results
Computing for the refractive
index using the equation,
𝑟
𝑛 = 𝑆𝑖𝑛𝑒 𝑖
Measuring the angles of
incidence (40 O) and refraction
of salt solution using the laser
refraction tank
Recording the concentration
and salinity of salt solution
Computing for the refractive
index using the equation,
𝑟
𝑛 = 𝑆𝑖𝑛𝑒 𝑖
Measuring the temperature of
purified water accommodated
in the laser refraction tank
Preparing the salt solution at
different concentration
Weighing the amount of
purified water and iodized salt
for the solution
Preparation and Gathering of materials. Before starting the experiment,
see to it that all the materials needed are already prepared. In the absence of purified
water, tapped water can be a good substitute. Table salt may be used in the absence of
iodized salt.
Measuring the volume and mass of water. It is also necessary to note the
amount of water just enough to fill one-half the laser refraction tank. Measure the mass
of this volume and record it. This amount is necessary in preparation of the salt solution.
Measuring the angles of incidence and refraction of water. Using the
laser refraction tank, set the angle of incidence at 40 degrees (40O ) then record the
resulting angle of refraction.
Computing for refractive index. Compute for refractive index, using the
equation,
n=Sin r /Sin i
where:
n – refractive index
i – Angle of incidence
r – Angle of refraction
Measuring and recording the temperature. It is necessary to measure and
record the temperature of the water or solution because temperature might affect the
result of the experiment.
Weighing the amount of water and salt. The volume of water
accommodated in the laser refraction tank is considered to determine the amount of
water and salt that will be mixed into a solution. For example, in 1000 ml of seawater,
96.5% of it is water and 3.5% is salt by mass. By obtaining 96.5%, there must be 965
grams of water and 3.5% or 35 grams of salt.
Preparation of salt solution at different concentration. In preparing the
salt solution, start with 0.2% of salt and 99.8% water, then 0.4% salt and 99.6% water,
then 0.6% salt and 99.4% water and so on.
Recording the concentration and salinity of the solution. Record the
concentration and salinity of the solution on a table. For the concentration, use % and
grams .For the salinity use parts per thousand (ppt).
Measuring the angles of incidence and refraction of the salt solution.
Using the laser refraction tank, set the angle of incidence at 40 degrees (40 O ) then
record the resulting angle of refraction in all different concentration of salt solution .
Computing for refractive indices of salt solution at different
concentration. Compute for refractive index, using the equation,
n=Sin r /Sin i
where:
n – refractive index
i – Angle of incidence
r – Angle of refraction
Recording and analyzing the results. Record and analyze the results
properly.
RESULTS AND DISCUSSION
Refractive Index of Salt Solution at Different Salinity and Concentration
Concentration Water
(ml)
of salt(g)
0.00
350.00
Salinity
(ppt)
0
Angle of
Incidence (o)
40
Angle of
Refractive
o
Refraction ( )
Index
59.00
1.333
3.50
346.50
10
40
59.25
1.336
5.25
344.75
15
40
59.25
1.336
7.00
343.00
20
40
59.25
1.336
8.75
341.25
25
40
59.25
1.336
10.50
339.50
30
40
59.25
1.336
12.25
337.75
35
40
59.50
1.339
14.00
336.00
40
40
59.50
1.339
15.75
334.25
45
40
59.50
1.339
17.50
332.50
50
40
59.75
1.342
19.25
330.75
55
40
59.75
1.342
21.00
329.00
60
40
59.75
1.342
22.75
327.25
65
40
59.75
1.342
24.50
325.50
70
40
60.00
1.347
26.25
323.75
75
40
60.00
1.347
28.00
322.00
80
40
60.00
1.347
Table 1 shows the refractive indices of water and salt solution at different
concentration and salinity. As shown in the table, column 1 is the concentration of salt
in grams, column 2 is the amount of water in milliliter (ml), column 3 is the salinity of
salt solution in parts per thousand (ppt), column 4 is the angle of incidence in degrees,
column 5 is the angle of refraction in degrees, and the last column is the refractive index.
It showed in column 3 that the angle of incidence was controlled at 4o degrees. The zero
value (row 2) on the concentration means that no amount of salt was added, meaning
the liquid used was pure water. At 40 degrees, the resulting angle of refraction in pure
water is 59 degrees. Applying Snell’s law, the computed value of the water is 1.333 which
is the same as the standard refractive index of water which is 1.333. When salt was
added, from row 3 to 11, the obtained value of refractive index differs. It showed that as
more salt was added to the solution to increase salinity and concentration, the greater is
the obtained value of refractive index.
Furthermore, the yellow rows show the standard, seawater in the open ocean has
a salinity of 1.339 as stated by Michael J. Kennish (2000) in his book “Practical
Handbook of Marine Science”. The obtained value of refractive index in this experiment
is 1.339 which is also the standard value. The results show that at 40 degrees as angle of
incidence, the angle of refraction must be 59.50 to come up with the standard value of
salinity. Aquarists therefore can maintain the salinity of water in their aquaria by simply
measuring the angle of incidence at 40 degrees and the angle of refraction must be close
to 59.5 degrees.
Hence the results show varied values of obtained refractive index, the null
hypothesis that there is no difference on the computed value of refractive index at
different concentration and salinity is rejected.
Refractive Index
Refractive Index vs Concentration Graph
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
2
4
6
8
10 12 14
Concentration (g)
16
18
20
Figure 1. Refractive Index vs Concentration Graph
Figure 1 shows the refractive index against concentration line graph. In
this graph 100 ml of water was used but the same concentration was maintained, this
was done for convenience of measuring the amount of salt. As shown by the slight
inclination of the line there is a slight increase of refractive index as the concentration
gradually increases. The obtained value of refractive index ranges between 1.333 to
1.447 which means that concentration of salt solution affects refractive index. The
gradual inclination of the line shows that concentration affects refractive index, thus,
the null hypothesis that concentration does not affect refractive index is rejected.
Refractive Index vs Salinity Graph
1.5
1.4
1.3
1.2
1.1
Refractive Index
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Salinity (ppt)
Figure 2. Refractive Index vs Salinity Graph
Figure 2 shows the refractive index against salinity graph. The straight line shows
the gradual increase of refractive index as salinity increases. Refractive index ranges
from 1.333 to 1.347 while the salinity ranges from 0 to 75 parts per thousand (ppt). The
slight inclination of the line shows that salinity affects refractive index. The null
hypothesis that salinity does not affect refractive index is rejected.
CONCLUSION AND RECOMMENDATIONS
Conclusion
Based on the results of the experiment the following conclusions are made:
1. Concentration and salinity affect refractive index of salt solution. Refractive index
increases as concentration and salinity of salt solution increases.
2. At 40 degrees angle of incidence, the required angle of refraction must be 59.50
degrees in order to come up with the standard salinity of an aquarium.
3. Snell’s law can help aquarists in maintaining the salinity of water in their
aquarium.
Recommendations
Based on the results of the experiment it is therefore recommended
that:
1. Salinity can be obtained using Snell’s law. Aquarists can maintain the
salinity of water in their aquarium by merely observing the angles of
incidence and refraction without necessarily obtaining the value of
refractive index.
2. The results of the experiment can be conveniently used by aquarists as
their basis in maintaining salinity of water in their aquarium.
3. A parallel study must be conducted further to determine whether the
results will be the same as the results of this study.
BIBLIOGRAPHY
REFERENCES
1.
Garcia-Ruiz, J. M., Novella, M. L., Otalora, F., Supersaturation patterns in
counter-diffusion crystallisation methods followed by Mach-Zehnder
interferometry, Crystal Growth, (1999) 196, 703-710.
2. Holmes-Farley, Randy. Reefkeeping Magazine, Reef Central, LLC Copyright
2008.
3. Kennish Michael J. Practical Handbook of Marine Science, Third
Edition (2000) 896 pp, Publisher: Lewis Publishers, Inc.
4. McPherson, A., Malkin, A. J., Kuznetsov, Y. G., Koszelak, S., Wells, M., Jenkins,
G., Howard, J., Lawson, G., Effects of microgravity on protein
crystallization: evidence for concentration gradients around growing
crystals , J. Crystal Growth, (1999) 196, 572.
5. Miller, A., Hussmann, E. K., McLaughlin, W. L., Interferometer for
measuring fast changes of refractive index and temperature in
transparent liquids Review of Scientific instruments, (1985) 46, 1635.
6. Shlichta, P. J., Feasibility of mapping solution properties during the
growth of protein crystals. J. Crystal Growth, (1986) 76, 656.
ACKNOWLEDGEMENT
The researcher would like to express his deepest gratitude to Associate Professor
Augustine Tan for his untiring support and guidance during the conduct of this study.
Likewise to National Institute of Education, Singapore and Ateneo de Manila University
Philippines, family and friends who supported the researcher in their own simple way.
Above all, to Almighty God for the wisdom and power.
The researcher