Salinity and Density Lab
Apparatus
12
100 ml volumetric flasks
6
10 ml volumetric flasks
1
Drying oven set at 70-80°
12
Plastic beakers (20 ml)
5
Hydrometer and tables
1
Refractometer
1
Salinometer
Seawater ( 20 liters)
Distilled water ( 20 liters)
6
600 ml beakers
6
Hot plates
6
Magnetic stirrers and stirring bars
Food coloring
Silver Nitrate, 500 ml (37 g/liter)
Indicator, 100 ml (3.5 K2CrO4 g/liter)
6
Burettes
6
250 ml Erlenmeyer flasks
6
10 ml Pipettes
2
Triple beam balances
2
Thermometers
Procedures
Temperature
Oceanographers use two terms in describing temperature, one is the in situ temperature
while the other is the potential temperature, usually signified by the Greek letter theta ( ). The
in situ (Latin for in place) temperature is that measured in the water. The potential temperature
is what the temperature would be if raised to the surface adiabatically, that is without exchange
of heat or water with the surrounding water. Oceanographers are interested in the potential
temperature since deep water usually acquires its temperature characteristics while at the surface.
Thus potential temperature may give an indication of what the temperature was at the time the
water mass was formed.
1.
Why does temperature decrease when a sample is brought to the surface?
2.
A water sample has an in situ temperature of -1ºC at 4,000m. Use Table II-6 to
determine what the potential temperature ( ) is.
=
What is the percentage change in the temperature?
Density
The density of seawater is dependent upon its pressure, temperature and salinity.
Ignoring pressure effects, almost all of the ocean has a density in the range of 1.020 and 1.0279
g/cm3. The greatest effect on density is from the compressibility of water. A parcel of water
with a density of 1.028 at the surface would have a density of 1.051 at 5,000m.
Since the range of densities in the ocean is fairly narrow, oceanographers have adopted a
convention to make the numbers easier to use, that is to use only the decimal portion. The
following definitions apply
t
=(
S,T,p
- 1) x 103
= in situ density
=(
S,T,0
- 1) x 103
= density at the surface
=(
S, ,0
- 1) x 103
= density at the surface
Where is the density, S is salinity, T is Temperature, and p is pressure.
3.
Determine the
What is the
4.
t of
t
of a sample which was measured to be 1.0239 g cm-3.
a sample collected at 14.33ºC and 32.65‰ (use Table II-I)
Use a 100 ml volumetric flask and determine the weight of both fresh and seawater.
Then determine the density of both.
Wt. of Flask
Flask + Water
Wt. of Water
Density
Fresh
Seawater
How did you calculate density? What are the units? Do the units come out right?
5.
Weigh two plastic beakers. Using a 10 ml volumetric flask, very carefully measure 10 ml
of distilled water in a weighing dish, and do the same with seawater. Put the dishes in a
drying oven at 70-80°C. When dry, determine the weight of salt residue.
Beaker Wt.
Wt. with water
Wt. of Water
Wt. dried
Distilled
Seawater
What was the density? What are the units?
Distilled
Seawater
Compare the density to that found in #4. Are there differences? Why?
How does the presence of salt affect the density of water?
What is the purpose of drying a sample of distilled water.
Wt. of Solids
What are your conclusions from this?
What is the salt content of each sample in parts per hundred (%)? i.e. What is the weight
of salt in 100 g of water?
Fresh
Seawater
What is the salt content in parts per thousand (‰)?
Fresh
Seawater
6.
An alternative method of measuring density uses a hydrometer. Often these measure the
specific gravity of a fluid. This is defined as
Specific Gravity =
Where
Mx
Mw
Mx is the mass of some liquid or solid
Mw is the mass of an equal volume of water (usually at 4ºC).
Specific gravity is the density of a substance divided by the density of water. Since water
has a density of 1 gram/cm3, and since all of the units cancel, specific gravity is the same
number as density but without any units.
Use a hydrometer to determine the specific gravity of the seawater.
What else is necessary to determine density accurately with the hydrometer?
Follow the directions in 210.B Hydrometric Method below
Sample
Salt
Temp
Sp.Gr.
Density
Salinity
Fresh
* make sure you include units in the table
7.
In the following table collect the density (g/kg) data from the previous experiments.
Sample
Distilled
Fresh Wt.
Dry Wt.
Hydrometer
Salt Water
How do the three methods (the fresh wt., dry wt. and the hydrometer) compare in terms of
determining density? Why?
Salinity
Salinity may be defined as the content of dissolved salts in sea water usually expressed as
S‰. This is a convention which approximates the weight in grams, in vacuo, of the solids
obtained from 1 kg of sea water (weighed in vacuo). The definition states that this weight is
after the solids have been dried to a constant weight at 480°C, the organic matter completely
oxidized, the bromide and iodide replaced by an equivalent amount of chloride, and carbonates
converted to oxides. Ocean water contains slightly more salts (halides, carbonate, and
bicarbonate) than is expressed by its salinity value.
The median salinity of the ocean is 34.69 g/kg of seawater or 34.69‰, where ‰ stands
for parts per thousand. About 75% of the ocean falls into the range of 34.5-35.00‰. Therefore,
measurements of salinity have to be made with considerable precision because the range is so
narrow. For some processes, changes of only a few hundreths of a part per million are important.
For example, there is a slow decrease in salinity in the deep Pacific from about 34.70‰ at 30ºS
to 34.67‰ at 40ºN, which is evidence for a northward movement of this large volume of water.
The ions in seawater remain in the same relative ratios, no matter what the salinity. In
practice, because of this Principle of Constant Proportionality, the salinity may be defined in
terms of chlorinity by the Knudsen equation:
S‰ = 1.80655 x Cl‰
This equation is solely a definition and has no true meaning in any practical chemical sense.
Recent developments have made measurement of salinity by means of conductivity the
practical rule. Salinity has thus also been redefined in terms of conductivity, and these
measurements are often noted as psu, practical salinity units. However, measurements based on
silver nitrate titrations are still the accepted reference procedure. In addition, other methods are
in use depending on the ease of use and accuracy desired. Physical oceanographers may want to
know salinity to 4 decimals, while some biologists may be satisfied with ± 1‰. Some of these
other methods include the refractometer which is based in the change of the index of refraction
with changing salinity, and T-S-D diagram estimates based on temperature and density
measurements.
8.
One of the original methods, which is still used today, is the titration with silver nitrate.
This technique has a precision of about ± 0.02‰. Measure 15 ml of seawater into an
erlenmeyer flask, add 15 ml of indicator solution (K2CrO4). Titrate with the silver nitrate
(AgNO3) solution using the burette. (WARNING: silver nitrate stains your skin, so is an
indicator of how good your lab technique is.)
When you begin the titration, the solution starts off as a clear pale yellow liquid. Upon
titration a white precipitate starts forming and as the end point is approached a pink/red
precipitate forms. The pale yellow color of the solution turns to a full yellow then a
definite pale red as the endpoint is exceeded.
This titration determines the chlorosity (Cl/liter) in the sea water. Note that it is not in
terms of cubic meters. To get the salinity value from the chlorosity, use Table II in the
appendix of Strickland and Parsons. Chlorosity at 20 C is defined below.
Cl/liter(20) = V + Cb + Cs + Ct
V is the volume of titrant to three decimal places
Cb is a burette correction (usually within ±0.05 ml)
Cs is a standardization correction
Ct is a temperature correction
(For our purposes ignore the correction factors)
Chlorosity = V =
Salinity =
Find the density using the Table II, from Strickland and Parsons, provided by the
instructor or use the relationship S‰= 0.03 + 1.8050 x Cl/liter.
Salinity =
Does this make sense given where you took the samples?
9.
Evaporating seawater to determine salt content is tedious, impractical most of the time,
and error prone. Salinity titrations are also relatively slow. Two other techniques which
are faster rely on the refractive index (using a refractometer) and the now-preferred
conductivity method (using a salinometer).
The salinometer is based measuring electrical conductivity. Conductivity is dependent on
temperature and salinity. These can be measured with great precision in the lab. Good
instruments have an accuracy of ± 0.003‰.
The refractometer is a commonly used optical instrument. The refractive index of
solutions changes depending on the amount of dissolved solids in the solution. The
technique is generally accurate to about ± 1.0‰.
Measure and record the seawater samples again using these two instruments. Record the
results in the table below.
Refractometer
Salinometer
Titration
Hydrometer
Fresh
Temp
Salinity
Seawater
Temp
Salinity
10.
Are there differences in the salinity results from the four different methods (hydrometer,
titration, refractometer, salinometer)?
What might that mean?
Inter-relationships and Conversions
11.
There is a precise relationship among salinity, temperature and density. If any two of
these are known then the third can be determined using this relationship. Graphically,
this is done with a temperature-salinity-density (T-S-D) diagram.
Determine the density and t of the seawater solution from the information you collected
in #9 above and the T-S diagram.
Density
t
12.
Tables are also available to determine the density from measurements. Ours tables are
limited to salinities of 20-30‰ with temperatures of 12-30ºC or salinities of30-40‰ and
temperatures of -2.0 to 30ºC. Use your measured temperature and salinity (from #9) to
determine the t of the seawater sample using Table II-1 provided by your instructor.
Temperature
Salinity
t
How does this compare to the graphical method used in #11.
Which do you think is more accurate? Why?
Practical Implications
13.
Fill a large beaker about 1/2 full with seawater. Using a pipette bulb, carfefully suction
hot seawater into the pipette. Without disturbing the water in the beaker too much, place
the pipette tip near the bottom of the beaker. Hold the pipette steady (use a ringstand
with clamp if necessary). Then slowly release the hot seawater from the pipette.
Describe what happens.
Why does this occur?
14.
Using the same procedure as in #8 above, place dyed, room temperature freshwater
into the pipette.
Describe what happens.
Why does this occur?
15.
What happens to Saco River water as it flows into the ocean? Why?
16.
The water flowing out of the Mediterranean Sea has been subjected to lots of
evaporation. What does it do when it reaches the Atlantic Ocean? Why?
17.
Seawater near the poles is colder than that near the equator. What happens to polar
water? Why?
18.
Deep ocean water percolates through cracks in the earth's crust near the mid-oceanic
ridges and becomes heated by the magma. What happens to this water when it resurfaces
above the ocean floor? Why?