MadSci Network: Science History
Query:

Re: How would salinity in water be tested in the early 1960?

Date: Mon Mar 26 09:54:16 2001
Posted By: Lon Brouse, Faculty, Chemistry, Challenge Charter School
Area of science: Science History
ID: 984328602.Sh
Message:

Dear Tonya,

The chemistry of salt water is well understood and the techniques for 
measuring it have expanded over the years.  In the early 1960's salt 
content was most simply measured by titrating with silver nitrate (AgNO3) 
solution with a yellow potassium dichromate indicator.  As the silver and 
chloride reacted to form insoluble silver chloride (AgCl), the indicator 
would change from a bright canary yellow to a faint rose pink color at the 
end point.  By knowing the concentration of the silver nitrate, the volume 
of silver nitrate titrated into the sample, and the volume of the original 
salt water sample, the concentration of the salt could be determined.  The 
following formula relates the concentrations and volumes:

C1 x V1 = C2 x V2, where: 

C1 = Concentration of the silver nitrate delivered 
V1 = Volume of silver nitrate delivered 
C2 = Concentration of the salt in the original salt water sample, and 
V2 = Volume of original salt water sample.

The concentration of salt can also be detemined by mesuring the electrical 
conductivity of the water sample.  Pure water is a poor conductor of 
electricity, but if you dissolve salt in it, the water will conduct 
electrical current more easily.  By referring to a standard chart of 
conductivity vs. salt concentration, the salt content can be easily 
determined.

You mentioned a reftractometer.  These were also in use in the early 
1960's.  As the salt content increases in a water sample, the index of 
refraction increases.  That is, the more salt dissolved in water, the 
slower light travels through it.  This is measured by determining how much 
a beam of light is bent as it passes from air into the salt water or from 
glass into the water.

A hydrometer measures the density of the salt water solution.  The more 
salt dissolved in the water, the more dense it is, or the more dense it is.
A hydrometer floats in the water with a claibrated scale sticking out 
above the surface.  More dense liquids will cause the hydrometer to float 
higher in the fluid.  A less dense liquid will cause it to sink lower in 
the fluid.  The specific gravity can be directly measured from the scale.

A liquid's density can also be measured with a volumetric flask and an 
analytical balance.  This laboratory technique has been used for hundreds 
of years.  A known volume of water, say 100 cc., is accurately weighed on 
an analytical balance.  If the net weight is 103 gm., the density is 103 
gm/100 cc. or 1.03 gm/cc. for a specific gravity of 1.03.  Pure water has 
a specific gravity of about 1.00 depending on the temperature.  This 
difference is easily measured in the laboratory.

Saltiness of sea water vaires by depth and  by latitude on the earth.  
Different seas have slightly different salt contents from the main world 
ocean.  

The average surface salinity of all the world's oceans is about 3.5%.  
Saltier water is slightly denser than fresher water and will sink as some 
of the water is evaporated from the surface.  The Medeterranian Sea, for 
instance loses 100,000 tons of water per second to evaporation, increasing 
the salininty of that body of water to about 3.9%. The Red Sea is saltier 
still, at 4.0%.  The Dead Sea is about 10% salt by weight.

Beacuse of this evaporation process, the saltier water left after 
evaporation, sinks.  As alinity increases, density increases, and water 
with a high alinity will tend to sink to a level that achievess an 
equilibrium.  Thus, surface water in an ocean has a uniform salinity to a 
depth of a few hundred meters.  On the average, at a depth of about 500 to 
600 meters there is a sudden increase in salinity at aboundary called the 
halocline. 

This increased slainity continues for several hundred feet and then slowly 
increases very slowly to the bottom.

Contrary to popular belief, the salinity in the oceans did not slow 
increase with rain water flowing over the exposed land masses, dissolving 
salt as it went.  Much of the chlorides in the ocean are believed to have 
come from volcanoes and the outpouring of the gases that accompany 
eruptions.

The temperature also changes as the depth increases.  The surface water is 
the warmest, and therefore the least dense.  As one goes deeper into the 
sea, the temperature decreases.  There is a boundary layer at about 800 
meters where the temperature has reached a somewhat stable low temperature 
that continues to the bottom.  The coldest water in the ocean is on the 
very bottom.  The surface temperatures range from about 32 deg. F. in the 
high latitudes to about 82 deg. F. near the equator.

I hope this helps.

Life Nature Library, The Sea, Time Life Books, New York, 1963.
Introductory Oceanography, Joseph Weisberg, McGraw Hill Book Co., New     
York, 1974.
The Book of Popular Science, The Sea, Grolier Book Co., 1967.



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