Salinity (Hydrometric Method)
Salinity is determined by measuring specific gravity with a hydrometer, correcting for temperature, and converting specific gravity to salinity at 15° C by means of density salinity tables. Salinity is usually expressed in parts per thousand (ppt). When measuring how much salt is in water or more accurately, when measuring salinity, it is reported in parts per thousand (ppt or ‰). The salinity of ocean water is about 35 ppt. Seawater has about 35 parts of salt per 1000 parts of water. In other words, if you had 1,000 grams of water and dry up all the water, you would be left with 35 grams of salt. This means the ocean is about 3.5% salts. Salinity is not the same in all bodies of water. Most rivers, ponds, and streams have almost no salts with salinity ranging from 0-5 ppt. This range of measurement is considered fresh water. In an estuary (bay), the flow of fresh water from streams and rivers mixes with salty ocean water. That mixture is called brackish water, with a range of salinity from .05 to 30 ppt. In the Red Sea, the water is considered brine, with salinity up to 50 ppt. This is the saltiest lake in the world—even saltier than the ocean! Drinking water is less than 0.5 ppt.
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.
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. As density of the seawater increases, the volume of the displaced seawater decreases (the hydrometer sinks less in the higher density fluid).
Table 2‑1 List of bodies of water by salinity
|A Salty Lake (like the Red Sea)||36 – 50 ppt|
|The Ocean||30 – 35 ppt|
|Mouth of estuary by a river||1 – 15 ppt|
|Entrance of estuary by the ocean||15 – 30 ppt|
|Tidal fresh river||.05 – 14 ppt|
|Freshman Stream or River||< 1 ppt|
Types of water based on amount of dissolved salts in parts per thousand (ppt):
Fresh water <0.5 ppt ,
Brackish water 0.5 – 30 ppt,
Saline water 30 – 50 ppt,
Brine >50 ppt
2.1 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.
- Label three 250 mL beakers for the three samples. Fill each beaker to about 200 mL, while making certain that you have the correct sample in each labeled beaker.
- Label three evaporating dishes (with the same labels as the sample beakers) and weigh each to the nearest 0.01 gram. Record the masses of the dishes, M1.
- Using a pipette, transfer about 10 mL of each of the three salt solutions to the corresponding labeled evaporating dishes. Weigh each evaporating dish with the water to the nearest 0.01 gram and record the masses, M2. Determine the mass of the water samples by subtracting the weight of the dish only, and record the masses, Mw=M2-M1.
- Carefully bring the evaporating dishes to oven at the rear of the room and carefully place them in the drying oven. Leave them in the oven until dry – this will take the majority of the lab period.
- Once the samples are dry allow them to cool for a few minutes, then weigh each evaporating dish and record the results on the data sheet (dish + salt), M3. Subtract the masses of the dishes to determine the mass of each of the salt samples and record the results,Ms=M3-M1.
- Determine the salinity of each sample using equation 1 (below).
Salinity = x 1000‰(2.1)
Table 2‑2 Salinity by evaporation
|Weight of evaporating dish, M1 (g)|
|Weight of evaporating dish and water, M2 (g)|
|Mass of water, Mw=M2-M1(g)|
|Weight of evaporating dish and salt, M3 (g)|
|Mass of salt, Ms=M3-M1(g)|
2.2 Determination of salinity using the hydrometer
- Hydrometer jar – Use a 1000 mL graduated cylinder
- Thermometer – graduated in 0.2° C divisions
- Samples – Use set of 3 unknown samples of various grams of NaCl
The National Bureau of Standards for specific gravity of NaCl solutions should calibrate hydrometers at 15° / 4° C.
- Fill the hydrometer jar (1000 mL graduated cylinder) with sample.
- Hang the thermometer into the cylinder while the jar is sitting in the vertical position. Make certain the thermometer is totally immersed and you can read it through the side of the cylinder.
- Carefully remove the hydrometer from its enclosure and insert it into the cylinder, until it begins to float then give it a slight twist to remove bubbles.
**Caution: Make sure that the hydrometer does not hit the bottom hard (it may break). Also, take care that drops of water do not splash onto the hydrometer stem above the water level.
- Read and record the temperature of sample to the nearest 0.5°
- Read and record the specific gravity from the scale on the hydrometer stem to the nearest 0.001 (estimate three decimal place).
- Repeat procedure 3 times and obtain average of each.
Precautions in using hydrometer:
- Avoid dried salt from previous use
- Avoid grease from fingerprints
- Avoid water droplets on the portion of the stem not submerged. This will throw off your results.
Accuracy in reading hydrometer:
- Take the readings without touching the hydrometer
- Take readings with your eyes at the same level as the water surface in the hydrometer cylinder. Viewing the scale up or down at an angle can give an incorrect reading.
**Read at the point where the flat-water surface would cross the hydrometer stem. Notice that the water curves up slightly next to the wall of the stem (the curve is a meniscus). Make sure you measure your results from the bottom of the curve, not the top.
- Read the specific gravity at the 4th decimal place using the lines printed between the labeled graduations.
Figure 2‑1 Hydrometer Reading
- Make temperature corrections for specific gravity reading from factors listed in Table.
Table 2‑3 Salinity by Hydrometer method (Sample 1)
|Temperature (°C)||Specific Gravity of Sample||Salinity (‰)|
|Average Salinity (‰)|
- Run horizontally across the table until you find the column for the temperature at which you took the reading.
- Run down the column until you get to the row for the specific gravity you recorded.
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