Marine Science Laboratory:
Salinity in Seawater
MS20 Laboratory
Salinity in Seawater
Objectives
•
To understand the definition of salinity
•
To learn the sources and sinks for salt in the ocean
•
To measure the salinity of a water sample by mass evaporation
•
To measure the salinity of a water sample using its electrical conductivity
•
To measure the salinity of a water sample using its density
•
To measure the salinity of a water sample using its refractive index
•
To measure the salinity of a water sample by chemical titration
Introduction
Salinity is a measure of the total dissolved salts in seawater. This sounds simple enough,
but measuring salinity becomes a problem when we realize that some of the salts do not
simply dissociate into ions but chemically react with water to form complex ions, and some
of the ions include dissolved gases such as CO2 (which converts to carbonic acid, H2CO3,
bicarbonate, HCO3-, and carbonate, CO32-).
Duxbury (1971) defines salinity as:
"The total amount in grams of solid material dissolved in 1 kilogram of
seawater when all the carbonate has been converted to oxide, all of the
iodine and bromine have been replaced by chlorine, and all the organic
matter has been completely oxidized."
For this laboratory we will use a simpler definition:
The total amount in grams of solid material dissolved in 1 kg of seawater.
Normally, salinity is expressed in parts per thousand (ppt), often written as o/oo. It is also
sometimes abbreviated as per mil, much the same as parts per hundred is abbreviated
as percent (%). For example, if you had 1000 g of seawater that contained 35 grams of
dissolved salt, you would have a 3.5% salt solution or a salinity of 35 parts per 1000 (35
º/ºº).
In this laboratory our salt water is not seawater, it is a solution of table salt (NaCI)
dissolved in de-ionized water. We only have two elements of the 11 commonly found in
seawater (and the dozens that are present in trace amounts); however, the methodologies
we use here will work for natural seawater as well as for our artificial seawater.
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Why is the sea salty? Why do we care?
The salt in the ocean comes primarily from the weathering of rocks on land. When rocks
are exposed at the surface of the Earth, minerals are weathered chemically to produce
various solid compounds as well as free ions such as calcium (Ca2+), potassium (K+), and
sodium (Na+). Volcanic eruptions produce CO2, hydrochloric acid (HCl), and sulfur dioxide
(SO2), all of which are soluble in water.
These solutes are brought to the ocean by precipitation and by river transport.
Evaporation of water concentrates these materials, bringing the salinity of the oceans well
above the salinity found in rivers and lakes. Some materials (such as calcium and
potassium) are gradually removed from the ocean by organisms that use them as
nutrients. Other materials are lost through sediment interactions: the sediments at the
ocean’s bottom behave much like a water filter, adsorbing various ions associated with
salinity and locking them away.
These processes of adding salt to the oceans, concentrating it through evaporation, and
removing it through various chemical and biochemical processes, have been active on
Earth for billions of years; the average salinity of the ocean is therefore a function of the
rates of these various processes, and has changed little through Earth’s history.
The water's salt content is one of the primary reasons that study of the ocean is different
from the study of fresh water lakes (known as limnology). Biologists, for example, are
concerned with the range of salinities at which an organism can live. Many species don't
usually live in brackish water as adults, but may spend part of their juvenile life cycles in
estuaries where food may be more available as they are maturing. On the other hand,
some fish have the ability to migrate from high to low salinity environments by regulating
their internal salt balance.
Water circulation is also affected by salinity. In estuaries, fresh water can "float" on saltier,
denser ocean water and even set up a current pattern which is based on the difference in
density between the two water types. In the deep ocean, high salinity water is often the
most dense and will sink and flow along the ocean floor under the influence of gravity.
Physical oceanographers, by knowing the salinity and temperature fields of the ocean,
can make predictions about the velocity and direction of these density-driven currents.
The salt content can even affect the way sediments are deposited on the sea bottom.
Very fine clay minerals tend to clump together (a process known as flocculation) in saline
environments. These particle aggregates have a greater ratio of mass to surface area,
leading to much more rapid settling rates.
Measuring Salinity
In this laboratory we will investigate five analytical methods for determining salinity. Some
methods are straightforward and some are complicated, some are simple but imprecise
and some are difficult but precise.
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Marine Science Laboratory:
Salinity in Seawater
As you perform these tests, pay particular attention to the results – the different methods
produce different levels of precision and accuracy, since the components of salinity vary in
their effects on the different measurement techniques. Care in your technique should
produce repeatable results, but do not expect that each technique will produce exactly the
same numbers.
Methods of salinity determination:
• evaporation of seawater
• conductivity of seawater
• measurement of water density by hydrometer
• refraction of light through seawater
• titration of the chloride ion (CI-)
For each method you will be measuring the salinity of three samples: one known sample,
whose salinity is 35º/ºº; and two samples, A and B, whose salinities are to be determined
by you.
A. Determination of salinity by evaporation
As we have defined salinity as the total mass of dissolved salts (measured in grams) in
one kilogram of seawater, the most straightforward way to measure salinity is to measure
exactly one kilogram of seawater, evaporate the water, and weight the salt that
precipitates out. Evaporating a full kilogram of water would take more time than we have
today however, so we will shorten the process by evaporating a small fraction of a
kilogram.
1.
Label three 250 mL beakers for the three samples: 35o/oo, and unknowns A and B.
Fill each beaker to about 200 mL from the dispensers on the side of the room,
making sure that you have the correct sample in each labeled beaker.
2.
Label three Petri dishes (with the same labels as the sample beakers) and weigh
each to the nearest 0.01 gram. Record the masses of the dishes in Table A on the
answer sheet.
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 the answer sheet. Determine the mass
of the water samples by subtracting the weight of the dish only, and record the
masses.
4.
Carefully bring 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 B through E 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 the answer sheet (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.
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Marine Science Laboratory:
6.
Salinity in Seawater
Determine the salinity of each sample using equation 1 (below). Record your
answers on the answer sheet.
(1)
B. Determination of salinity by electrical conductivity
Contrary to common belief, pure water is not a very good conductor of electricity. It
becomes a good conductor when soluble salts are added, due to the presence of
positive and negative ions that can donate and accept electrons. The transfer of these
electrons is what allows water to conduct, and the higher the concentration of ions in the
solution the better the conductivity (the tap water in your house has enough ions present
to be a good conductor).
Figure One:
Electrical conductivity as a function of salinity. Pure water (such as distilled or deionized
water) does not conduct electricity well enough to complete a circuit and light a small light
bulb. Water with dissolved salts contains both positive and negative ions that complete the
circuit. The higher the salinity, the greater the conductivity, and the brighter the bulb.
This can be seen in Figure One, in which a water solution is used to bridge an electrical
connection. The greater the conductivity of the solution, the more light produced by the
light bulb. One way to measure the conductivity of the solution would be to measure the
brightness of the bulb against some standard. Fortunately we can directly measure the
conductivity of the water with a conductivity meter (Figure two). Electrical conductivity in
water is a function not just of salinity but also of temperature, so we will need to consider
temperature in our measurements as well.
Conductivity meters can be made to measure a variety of salinity conditions.
Unfortunately devices that can handle the high salt concentrations found in the world’s
oceans (and in today’s lab) are quite costly. The device we will be using today is only
accurate at much lower salinities so we must dilute our samples by a known amount in
order to make our measurements.
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Figure Two:
1.
Salinity in Seawater
Using the conductivity meter. Pour the sample, diluted to 25% strength, into a 250 mL
beaker. Measure and record the temperature, then place the probe into the sample to take
the conductivity reading. Remember to multiply your reading by four, since you diluted the
sample by a factor of four.
Measure exactly 25 mL of the 35o/oo water into a 25 mL graduated cylinder. Carefully
pour this water into your 100 mL volumetric flask. Fill the volumetric flask exactly to
the measurement line with deionized water from your squirt bottle.
NOTE: It is important to dilute the sample using only deionized
water, as tap water contains ions such as sodium and calcium that
will alter the salinity of your sample and skew the results of the
experiment.
Remember that both the graduated cylinder and the volumetric
flask are calibrated so that the bottom of the fluid meniscus should
be at the measurement line for the most accurate measurement.
2.
If you have been careful the sample in the volumetric flask now has exactly ¼ the
salinity of the original sample. Transfer the sample to a 250 mL beaker. Measure the
water’s temperature (allow 30-60 seconds for the thermometer to equilibrate) and
record the temperature in Table B on the answer sheet.
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Marine Science Laboratory:
3.
Salinity in Seawater
Measure the electrical conductance of the water (the units of conductance are
mmhos/cm – the reciprocal of electrical resistance). Place the meter into the beaker
(be sure that the probe sensors are completely submerged) and allow at least 30
seconds for the readout to equilibrate. Record the conductance on your answer
sheet, remembering to multiply your measurement by a factor of four (since you
diluted the sample by a factor of four).
NOTE: The conductivity meter is a valuable instrument – handle it
carefully.
Have a small beaker of deionized water at your station and use it
to rinse the probe sensors between measurements.
4.
Repeat steps 1-3 for the unknown samples A and B, recording the temperatures and
conductivities in Table B on the answer sheet.
5.
Figure Three is a graph that relates salinity in parts per thousand (horizontal axis) to
conductivity in mmhos/cm (vertical axis). The sloping lines represent the relationship
between conductivity and salinity at several known temperatures. Use this graph to
determine the salinity values for each of your samples and record these values in
Table B on the answer sheet.
NOTE: Since we have only included lines for temperatures at 5
degree increments, you will be expected to interpolate your results.
For example: if your water temperature was 17.5°C, use your
measured conductivity and the graph to determine the salinity if the
water were 20°C and if the water were 15°C. As 17.5°C is halfway
between 15°C and 20°C, the salinity value is halfway between the
two values you determined for these temperatures.
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Marine Science Laboratory:
Salinity in Seawater
Figure Three: Conductivity as a function of salinity and of temperature. Lines show the relationship
between conductivity and salinity at 5 degree increments between 5°C and 25°C.
C. Determination of salinity by density using a hydrometer
Remember from the isostasy laboratory that density is simply the ratio of a sample’s
mass to its volume. The density of seawater is a function of both water temperature and
salinity, thus if we measure both the temperature and the density of a water sample we
should be able to determine its salinity.
To determine density we could take an exact volume of water and weigh it, then divide
the mass by the volume. Unfortunately it is difficult to determine both mass and volume
to the precision we desire with the laboratory equipment we have available to us.
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Marine Science Laboratory:
Salinity in Seawater
Instead of measuring mass and volume directly, we return to Archimedes' principle: "a
floating body will displace a volume of water equal to its own mass." If we use a fixed
mass that is less dense than the water sample, such as a hollow glass ball, it will sink
down into the water until it displaces its mass. The greater the density of the water the
higher the sphere will float.
Hydrometers are devices designed to measure fluid density. The hydrometer’s mass is
precisely fixed and it is concentrated at the bottom of the tube (like a buoy) so that the
hydrometer will always float upright. The narrow stem is precisely graduated so that as
the device sinks and displaces its own mass, the level to which it sinks is equal to the
seawater density (Figure Four). As density of the seawater increases, the volume of the
displaced seawater decreases (the hydrometer sinks less in the higher density fluid).
Figure Four:
Hydrometer function. 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 can be read from the graduations on
the stem.
Figure Five below shows a close-up view of the hydrometer stem: there are labeled
divisions representing 5/1000 gram/mL (1.010, 1.015, and so on); these are divided into
five parts (1/1000 g/mL) by secondary unlabeled divisions; these are divided into halves
(0.5 g/mL) by tertiary unlabeled divisions. If you are careful using this device you can
determine density to a precision of 5 parts in 10,000 (1/2 milligram/mL).
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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Figure Five:
Salinity in Seawater
Reading density from the hydrometer. The major lines are labeled to 5 parts in 1000, the
secondary unlabeled lines to 1 part in 1000, and the tertiary unlabeled lines to 5 parts in
10,000. This device can be read to the fourth decimal place.
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 in Table C on the answer sheet.
3.
Carefully lower (do not drop) the hydrometer in the graduated cylinder until it
floats. If there is not enough water to allow the hydrometer to float, more water can
be added until the hydrometer rises from the bottom.
4.
Read the density of the water from the graduated lines on the hydrometer and
record this value in Table C.
5.
Rinse and dry the hydrometer and the graduated cylinder with deionized water.
6.
Repeat steps 1-5 for the unknown samples A and B, recording the temperatures
and the density values in Table C.
7.
Figure Six (below) is a graph relating water density (vertical axis) to salinity
(horizontal axis). It can be read like Figure Three, as it has lines representing the
density-salinity relationship at several fixed temperatures. Determine the salinity of
your sample by finding the salinity that crosses the lines representing your
measured density and temperature. As before you may need to interpolate
between the temperature lines. Record your salinity values in the appropriate
space on Table C.
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Figure Six:
Salinity in Seawater
Density as a function of salinity and of temperature. Lines show the relationship between
density and salinity at 5 degree increments between 5°C and 30°C.
D. Determination of salinity by refractometer
The speed of light in a vacuum is 2.998x105 kilometers/second. Even in intergalactic
space however, photons of light periodically interact with stray atoms, molecules and
dust fragments that slow it down by some negligible amount. Even the thickness of
Earth’s atmosphere is not enough to decrease this velocity by more than a fraction of a
percent.
In water however, the speed of light decreases to 2.25x105 km/second and in glass it
decreases even more, to about 2.00x105 km/second. This decrease in light velocity is
due to the increase in density of the medium, as photons are constantly being absorbed
and re-emitted by atoms that they encounter.
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Salinity in Seawater
When light moves from one medium to another in which the velocity is different, the light
will bend, or refract, by an amount that is proportional to the difference in velocity
between the two media. Since the density of water increases with . We will take
advantage of this property by measuring the amount of that refraction in water samples.
This is possible using a device called a refractometer. A refractometer can precisely
measure the amount of refraction that is caused by the density. The instrument that you
will be using is temperature compensated. This means that the temperature effects on
refraction can be ignored for these measurements, and you will be able to read the
salinity directly from the refractometer.
The refractometer consists of a blue-tinted glass prism and a beveled, clear plastic
cover lens, each with a known refractive index. When a thin layer of water is placed
between these two lenses it will refract light through an angle that depends upon the
salinity of the water sample. When you look through the eyepiece you will see a
calibrated scale with an upper blue-shaded region and a lower white region. The
boundary between these two regions will cross the scale at a value representing the
sample’s salinity (Figure Seven).
Figure Seven: The refractometer. Place a drop of the water sample between the two lenses and lower
the upper lens into place. Look through the eyepiece and aim the refractometer toward a
light source. The boundary between the upper blue-shaded region of the scale and the
lower white region crosses the calibration scale at a value that represents the salinity of
the sample.
Before you begin, carefully rinse the face of the prism and the cover lens of the
refractometer with deionized water, and dry with a cloth towel. Place a drop of deionized
water on the lens and close the cover. Look through the eyepiece. 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. If you have
a problem reading the scale you can sharpen it using the focus ring.
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Marine Science Laboratory:
Salinity in Seawater
1.
Using an eyedropper, place one or two drops of your known (35o/oo) sample onto
the prism face as shown in Figure Seven. Close the prism cover being careful not
to trap any air bubbles in the water on the prism face.
2.
Holding the prism toward the light, look though the refractometer and note where
the intersection lies between the upper shaded portion and lower clear portions of
the scale. This boundary represents the salinity. Record to the nearest part per
thousand (º/ºº) and enter in Table D on the answer sheet.
3.
Rinse the prism and cover plate in deionized water and dry with a cloth towel.
4.
Repeat steps 1-3 for each of the unknowns and record the salinities in Table D.
E.
Determination of salinity by chemical titration
NOTE: This section of the laboratory will be done for you as a
demonstration by your instructor. You will be responsible for the
results. Also, in a few weeks you will be responsible for performing
a titration yourself, so pay close attention to how it is done.
The final method of salinity determination that we will demonstrate is very precise (that
is, it will replicate well) and accurate (it produces a result that is very close to the true
salinity). It is a bit complicated and normally would be more appropriate for a chemistry
laboratory; however, we introduce it here because for decades it was the standard
technique in oceanography for determining salinity.
In this section we will use a silver nitrate solution to precipitate out all of the chloride in a
known volume of artificial seawater. Although natural seawater contains a much wider
array of ions than a mixture of NaCl and fresh water, the analytical technique is the
same. When we add a solution containing silver nitrate (that is, Ag+ ions and NO3- ions)
to a solution containing sodium chloride (that is, Na+ ions and Cl- ions), the chloride ions
in the solution will react with the silver ions to produce a solid precipitate, silver chloride
(and leaving the sodium and nitrate ions in solution):
(2)
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Marine Science Laboratory:
Salinity in Seawater
Hypothetically we could then dry and weigh the precipitate AgCl, and from the weight
determine the amount of chloride in the sample. But first we have to calculate the mass
percentage of chloride in silver chloride. This is done in the following manner:
(3)
(4)
From this we can see that AgCl contains 24.7% chloride ions. If we precipitate 100
grams of AgCl from 1000 grams of seawater that would represent 24.7 grams of Cl-.
Chloride represents a majority of the negative ions present in seawater, but not all.
Fortunately the proportion of chloride is consistent. We know from chemical
oceanography that chlorides make up about 55% of the total salts (salinity) in seawater.
Therefore if we know the chlorinity (the mass of chloride present) we can determine the
overall salinity:
Salinity = 100/55 x mass Cl- = 1.8 x mass Cl-
(5)
In the above example we measured 1000 grams of seawater and found 24.7 grams of
chloride so the salinity can be determined:
Salinity = 1.8 x mass Cl- = 1.8 x 24.7g Cl- = 44.5 º/ºº
(6)
The problem with drying and weighing the AgCl precipitate is that it takes a lot of time,
and the mass of the precipitate is a function of the oven temperature you use for drying,
since the AgCl can hold onto a significant amount of water even when it has seemingly
dried out (similar problems occur when we determine salinity by evaporation).
So rather than drying and weighing the silver chloride, we can use the same reaction to
measure the salinity by knowing exactly how much AgNO3 was used to precipitate all of
the chloride. We can do this by adding another chemical reagent, potassium chromate
(K2CrO4), which acts as an indicator to tell us exactly when all of the chloride in the
seawater has reacted to form AgCl.
The potassium chromate indicator remains colorless as long as there is Cl- present, but
the instant the last of the Cl- is bound up as silver chloride, the solution turns orange.
The orange color occurs when the extra chromate ion (CrO42-) combines with the silver
ion to form a complex ion called silver chromate.
The following two reagents will be made available for the titration analysis: AgN03
solution (50 grams of AgNO3 dissolved into a 1000 mL solution) and the K2Cr04 solution
(3.5 grams of K2Cr04 dissolved into a 1000 mL solution).
The experimental setup is shown below in Figure Eight:
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Marine Science Laboratory:
Salinity in Seawater
Figure Eight: Set up for chemical titration. Note that the burette reads from top to bottom.
Your instructor will perform the following procedure as a demonstration. You are
responsible for observing, recording all of the numbers, going through all of the
calculations and answering questions about the procedure. The first part of the process
requires calibrating the AgN03 solution against the 35o/oo salinity sample:
1.
Pour approximately 20 mL of the 35o/oo sample into a 50 mL beaker.
2.
Using a graduated 5 mL pipette, transfer exactly 5 mL of this 35o/oo sample into a
125 mL Erlenmeyer flask.
3.
Add 5 mL of the K2CrO4 solution to the same flask. Place a magnetic stirrer in the
flask and set it under the burette on top of a stirring hot plate (Figure Eight).
4.
Fill the 50 ml burette with the AgNO3 solution, pouring it carefully down a funnel
into the top of the burette. This can be done once for this entire set of four
titrations. Record the starting point in Table E.
5.
Turn on the stirrer (not the heat) to a low setting. Add the AgN03 solution slowly
from the burette until you detect an abrupt color change from milky yellow to
orange.
6.
Record the new burette reading (the end point) in Table E.
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Salinity in Seawater
7.
Repeat the analysis for the known sample, again recording the starting and ending
points on the burette in Table E (the starting point for the second analysis should
be the same as the ending point for the first analysis).
8.
Repeat steps 1-6 to titrate the unknown samples, recording the data in Table E.
9.
Calculate the salinity of unknown samples A and B using the average amount of
AgNO3 required to titrate the 35o/oo using the equation below:
(7)
Rearranging the above equation:
(8)
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Marine Science Laboratory:
Name
Salinity in Seawater
Lab Section
Date
.
MS20 Laboratory: Salinity in Seawater
Answer sheet: 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.
Determination of salinity by evaporation
TABLE A:
SAMPLE
MASS OF
EMPTY DISH
MASS OF
DISH + WATER
MASS OF
WATER
MASS OF
DISH + SALT
MASS OF
SALT
SALINITY OF
SAMPLE
35 o/oo
A
B
Assume there was "dirt" (fine silt or clay particles) in the sample you just evaporated. How
would this have affected the salinity as you measured it?
Could you have evaporated the salt out of the seawater at room temperature? If you
could, would this have affected the salinity measurement? Explain.
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B.
Salinity in Seawater
Determination of salinity by electrical conductivity
TABLE B:
SAMPLE
TEMPERATURE (°C)
ELECTRICAL CONDUCTANCE
(MMHOS/CM)
SALINITY (O/OO)
35 o/oo
A
B
Is warmer or colder water a better conductor of electricity? Why might this be true?
Water at a temperature of 5ºC and 40 º/ºº will have the same conductivity as what
salinity water at 25ºC?
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C.
Salinity in Seawater
Determination of salinity by density using a hydrometer
TABLE C:
SAMPLE
TEMPERATURE (°C)
DENSITY
(GRAMS/ML)
SALINITY (O/OO)
35 o/oo
A
B
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.
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)?
Does a ten degree change in temperature have a greater or lesser effect on density than
a 10o/oo change in salinity?
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D.
Salinity in Seawater
Determination of salinity by refractometer
TABLE D:
SAMPLE
SALINITY (O/OO)
35 o/oo
A
B
Under what conditions might the refractometer method be the only procedure you could
use to determine salinity?
Of the four methods for determining salinity you have examined so far in this laboratory:
•
Evaporation
•
Conductivity
•
Hydrometer
•
Refractometer.
Which takes the most time? Which method might the most difficult to do aboard ship?
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E.
Salinity in Seawater
Determination of salinity by chemical titration
TABLE E:
SAMPLE
INITIAL READING
FINAL READING
ML OF AgNO3
SALINITY OF SAMPLE
35 o/oo
35 o/oo
average
A
B
Suppose that your glass burette were not exactly round, and that instead of
delivering 1 mL per graduated division, it really delivered 1.1 mL. Could you still use
it for a salinity titration? Explain!
You transferred exactly 5 mL of seawater with a pipette to an Erlenmeyer flask. Then
you went for a cup of coffee, and some of the seawater evaporated before you could
began your titration analysis. Would you have to start all over, or could you use the
evaporated sample? Explain!
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Salinity in Seawater
Name
Lab Section
Date
.
MS20 Laboratory: Salinity in Seawater
1.
Record the given salinities for each exercise.
(A)
EVAPORATION
(B)
CONDUCTIVITY
(C)
HYDROMETER
(D)
REFRACTION
(E)
TITRATION
35 o/oo
A
B
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).
3.
The silver nitrate titration actually directly measures the chlorinity of the samples, not
the salinity. A more exact relationship between salinity and chlorinity is given by:
Salinity = 0.03 + 1.8 x Chlorinity
Calculate the salinity of the following samples given the chlorinity:
4.
a.
Cl = 10 ppt
b.
Cl = 18.56 ppt
c.
Cl = 35 ppt
d.
Cl = 100 ppt
Sodium and chlorine are only two of the major ions in seawater. What are the
others that make up 99% of the total salts? (hint: see your class text)
Revised on 2/21/2005
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