Marine Science Laboratory:
Salinity in Seawater
MS20 Laboratory
Seawater Salinity and Density
Introduction
As you perform these experiments, pay particular attention to the results –different methods produce
different levels of precision and accuracy. Care in your technique should produce repeatable
results, but do not expect that each technique will produce exactly the same numbers.
For each method you will be measuring the salinity of: one known sample, whose salinity is 35º/ºº
and two unknown samples, A and B. In this lab, you will:
determine salinity by
• conductivity
• evaporation
• hydrometer (and temperature and TDS diagram)
• refractometer
and determine density by:
• hydrometer (and temperature and TDS diagram)
•
refractometer
Procedures
A.
Determining salinity by conductivity (demonstration)
Figure 1. Light intensity increases as salinity increases. Therefore it is possible to make a
meter to measure electrical conductivity. However, in our experience, the meters
are expensive, difficult to maintain and give erroneous results.
B.
Determining salinity by evaporation
1.
Fill each beaker (labeled “35 º/ºº” “A” & “B”) to about 200 mL from the dispensers on the side
of the room, making sure that you have the correct sample in each beaker.
2.
Weigh each labeled Petri dishes (with the same labels as the sample beakers) to the nearest
0.01 gram. Record the masses of the dishes in Table B (page 4) on the answer sheet.
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Marine Science Laboratory:
Salinity in Seawater
3.
Using a pipette, transfer about 10 mL of each of the three salt solutions to the corresponding
labeled Petri dishes. Weigh each Petri dish with the water to the nearest 0.01 gram and record
the masses on Table B. Determine the mass of the water samples by subtracting the weight of
the dish only, and record the masses.
4.
Carefully take the Petri dishes to the back of the room, where your instructor will place them in
the drying oven. Leave them in the oven until dry – this will take the majority of the lab period.
You will be doing exercises C through F while waiting for the samples dry.
5.
Once the samples are dry allow them to cool for a few minutes, then weigh each Petri dish
and record the results on Table B (dish + salt). Subtract the masses of the dishes to determine
the mass of each of the salt samples and record the results on the answer sheet.
6.
Determine the salinity of each sample using the equation below. Record your answers on the
data sheet (Table B, page 4).
C.
Determining salinity by measuring density (using a hydrometer) and temperature
(using a thermometer).
Figure 2. As the density of the fluid increases, the hydrometer displaces less water. Since the
upper tube is very narrow this difference in volume corresponds to a significant change
in depth. The density of the fluid is read from the graduations on the stem.
NOTE: The entire hydrometer is sealed in thin glass, and most of the mass
is located in the metallic base. This device is carefully calibrated, very
fragile, and very expensive. Handle your hydrometer carefully, as the tiniest
crack in the glass makes it worthless for measuring density.
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Marine Science Laboratory:
Salinity in Seawater
1.
Pour approximately 220-240 mL of water from the known (35o/oo) salinity sample into a 250
mL graduated cylinder.
2.
Measure and record the water temperature and density in Table C.
3.
Determine the salinity using the TDS diagram and record in Table C.
4.
Repeat steps 1-3 above for the “A” and “B” unknown samples.
D.
Determining salinity using a refractometer
Figure 3:
The refractometer.
Before you begin, look through the eyepiece and focus by turning the eyepicece. The boundary of
the shaded region should cross the scale at the zero line. If it does not, inform your instructor so
that she/he can adjust it or assign you a different refractometer. .
1.
Place a drop of the water sample on the refractometer and lower the “flipper” into place. Look
through the eyepiece and aim toward a the light. The boundary between the upper blueshaded region of the scale and the lower white region crosses the calibration scale at a value
that represents the salinity of the sample.
2.
Rinse the prism and cover plate in deionized water and dry with a cloth towel.
3.
Repeat steps 1-3 for the “A” and “B” unknowns and record the salinities in Table D.
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Marine Science Laboratory:
Salinity in Seawater
Data Sheets MS20 Laboratory: Salinity in Seawater
Record all data in the appropriate metric units (centimeters, grams, etc.). Remember to use
significant figure rules and to indicate appropriate units (if the scale reads 13.4 g, your answer is
not 13.4, but 13.4 g (or 13.4 grams).
A.
Demonstration (No data to record)
B.
Determining salinity by evaporation
TABLE B:
SAMPLE
MASS OF
MASS OF
MASS OF
MASS OF
MASS OF
SALINITY OF
EMPTY DISH
DISH + WATER
WATER
DISH + SALT
SALT
SAMPLE
35 o/oo
A
B
C.
Determining salinity from a TDS diagram by measuring density (using a
hydrometer) and temperature (using a thermometer).
TABLE C:
SAMPLE
TEMPERATURE (°C)
DENSITY
(GRAMS/ML)
SALINITY (O/OO)
35 o/oo
A
B
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Marine Science Laboratory:
Figure 3:
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Salinity in Seawater
TDS Diagram.
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Marine Science Laboratory:
D.
Salinity in Seawater
Determining salinity using a refractometer
TABLE D:
SALINITY (O/OO)
SAMPLE
35 o/oo
A
B
Combined Salinity Data from Tables B, C and D
Record the given salinities for each exercise.
(B)
EVAPORATION
(C)
HYDROMETER
(D)
REFRACTION
35 o/oo
A
B
Note: Save your graduate of 35o/oo salinity water for the next procedure!
E.
Determining density as temperature changes
1. Using the 35o/oo salinity water in the graduate from the previous exercise, add a bit more
35o/oo until the graduate is as full as possible without overflowing when the hydrometer
is in it. Take the graduate and a glass thermometer to the warm water bath at the
side of the room.
2. Stir the water in the graduate constantly but gently (the thermometers are glass!) to insure
uniform heating of the water in the graduate. Warm the graduate in the water bath
provided until the temperature is 30° C. Take the graduate back to your table and
measure the density with a hydrometer and record both the density and
temperature in Table E2.
3. Fill a 2000 mL beaker (stainless steel) about ¾ full of cold tap water.
4. Place the 250 mL graduate containing 35o/oo into the beaker. Stir the water in the
graduate constantly with the thermometer except when you remove the thermometer
and put in the hydrometer to measure the density. Record densities and temperatures
in Table E2. Be sure to remove the hydrometer and thoroughly stir the water in the 250
mL graduate between density readings.
NOTE: The amount of time between readings is not as important as making sure that you
have a density recorded for every drop in temperature of about 2 degrees.
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Marine Science Laboratory:
Salinity in Seawater
o
5. When your 35 /oo water has reached a temp of 25º add a little ice to the water bath to cool
it down. Continue to measure temperatures and densities every 2º or so. Stop when your
salt water reaches a temperature of about 10º
Table E2. Temperature and density of a 35o/oo water sample
Temperature (°C)
Density (g/cm3)
30
28
26
Add ice here
24
22
20
18
16
14
12
10 stop
6. Plot Density versus Temperature from Table E2 in Figure 4 below.
o
Figure
4. Density
versus
Temperature for
/oo owater
sample
Figure
4. Density
versus
Temperature
fora 35
a 35
/oo water
sample
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Marine Science Laboratory:
Salinity in Seawater
F: Constructing a Swallow float
An aquarium has been set up to represent a model ocean with two water masses of different
densities. These water masses have been dyed with food coloring to help show the boundary
between the different density layers. Your job is to create a float that remains at the interface
of the two water masses.
1.
Samples from each of the water layers have been drawn and are in hydrometers
next to the aquarium. The blue layer is denser cold salty water (bottom) and the
yellow layer is lighter warm fresh water (top). Measure the density of each layer
using the hydrometers provided. Record the densities below.
surface (yellow) layer:_____________
2.
deep (blue) layer:_______________
Weigh a 0.5 dram glass vial (plus cap) to the nearest 0.01 g using a 200 g balance.
mass of the empty vial and cap:____________
3.
Fill the vial about 1/3 full with lead shot and measure its volume to the nearest 0.1 cm3
by immersing the vial in fresh water in a 25 mL graduate. Read the volume of water in
the graduate precisely before you add the vial. Tilt the graduate slightly and slide the
vial into the graduate slowly so that you don’t splash any water out or trap any air
bubbles! Then read the volume of water again. Subtract the first volume from the
second to arrive at the volume of the vial. Remember Archimedes principle!
volume of the vial: _____________mL
4.
Calculate the density needed to just sink through the upper surface and float on the
lower layer. In other words, calculate the density halfway between the upper and lower
layers. This is easily done by adding the density of the upper and lower layers and
dividing by 2.
(density of blue layer + density of yellow layer) / 2 = _____________g/cm3
5.
We now know the volume of the vial and the density it should be. We need to find the
mass. Recall that
density = mass / volume
so, by rearranging the equation
mass = density X volume
6.
Calculate the desired mass of your float by multiplying the desired density (step 4
above) by the volume (step 3 above)
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Marine Science Laboratory:
Salinity in Seawater
Calculated mass of float:__________ g
7.
Now adjust the mass of our vile to equal the calculated float mass. Place the empty vial
and its cap on the balance and slowly add lead shot until the mass equals the calculated
value. When you have done this, you are ready to test your float.
Tell your professor when you are
ready to launch the Swallow Float!
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Marine Science Laboratory:
Salinity in Seawater
Names__________________________ Lab day and time ________________
________________________________
________________________________
Questions
1.
Write a description of what you observed in the electrical conductivity demonstration.
_________________________________________________________________________
_________________________________________________________________________
2.
Consider the various methods of determining salinity. Which is the most accurate, which the
least, and why? (Define accuracy as being close to the "true" value). Think of “real-world”
conditions where you may be using this equipment.
________________________________________________________________________
________________________________________________________________________
_______________________________________________________________________
3.
Assume there was "dirt" (fine silt or clay particles) in the sample you evaporated. How would
this have affected the salinity as you measured it?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
4.
Could you have evaporated the salt out of the seawater at room temperature? If you could,
would this have affected the salinity measurement? Explain.
_________________________________________________________________________
_________________________________________________________________________
______________________________________________________________________
5.
Why might the Persian Gulf be significantly more saline than other ocean basins (hint: it is
the same reason that the Mediterranean Sea is more saline than the rest of the Atlantic,
and the same reason that the Dead Sea is the saltiest body of water on Earth)? (hint #2:
The answer is related to air circulation—remember “Horse Latitudes”?)
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
6.
A fully loaded ship moving from a fresh water port heads over to the Persian Gulf, an area
known for high salinities. Will the ship ride higher or lower in the Gulf compared with the
port it left? Explain.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
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Marine Science Laboratory:
7.
Salinity in Seawater
Does a ten degree change in temperature have a greater or lesser effect on density
than a 10o/oo change in salinity? (Use TDS diagram)
______________________________________________________________________
______________________________________________________________________
8.
Of the methods for determining salinity you have examined so far in this laboratory, which
takes the most time? Which method might the most difficult to do aboard ship?
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
9.
In Table E1, are your measured densities different from the theoretical values? How can
you explain these differences? (Hint: What are the possible the sources of error?)
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
10.
Is the change in density with temperature a "linear" change over your experimental
temperature range? (Figure 4).
_______________________________________________________________
11.
Assume you were heating up a liter of 20o/oo water with a sunlamp. The temperature
increases from 10ºC to 30 ºC. At the same time some of the water evaporated and the
salinity went from 20 to 40o/oo. What is the initial and what is the final density? Which is
the more important factor (temperature or salinity) affecting the density? (Hint see TDS
diagram)
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
12.
Would it be possible to make a Swallow float for a fresh water lake? Explain your
answer.
______________________________________________________________________
______________________________________________________________________
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