Page 38
Copyright Wet Paper Publications 2010
P
ROJECT
2.5 M
AKING
A
SALINITY
HYDROMETER
Making the standard salt
solutions
500mL measuring cylinders work well for the graduation
process, they are deep and sufficiently wide at the top to
allowworkwith the tweezers, so it makes sense tomake up
500ml solutions
You need to make up standard salt solutions having
concentrations of 35g/L 30g/L 25g/L 20g/L 15g/L 10g/L
5g/L (see table below).
Step 1 For each concentration halve the quantity of salt.
Step 2 Weigh this out using an accurate balance.
Step 3 Place into a 500ml volumetric flask.
Step 4 Add 300 mL of distilled water to completely
dissolve the salt.
Step 5 When the salt is dissolved add distilled water.
These solutions can now be delivered to the
measuring cylinders.
Final concentration Mass of salt required to
g/L
make 500 mls
35
17.5
30
15
25
12.5
20
10
15
7.5
10
5
5
2.5
Background
A hydrometer is an instrument used to measure the specific
gravity (like density) of a liquid or solution.
Hydrometers work on Archemedes Principle and can be made
from anything that floats vertically.
• The denser the liquid, the more upthrust from the liquid
displaced by the hydrometer and the higher it will float.
Hydrometers can be used to obtain very accurate measurements
of salt concentration in water samples.
• Simple hydrometers can be used to determine salt
concentrations of solutions in g/L.
Materials
• Plastic transfer pipette (specifically non graduated narrow
stem polyethylene transfer pipette 15cm which can draw of
3.5 ml (ie suck up) Product code: A1503 Supplier:
Livingstone International School Supplies (see Page 120)
• Matches/lighter
• Fine dry sand
• Standard salt solutions 35g/L 30g/L 25g/L 20g/L 15g/L 10g/
L 5g/L in 500mL measuring cylinders
• Distilled water in large beaker (4 L)
• Curved pointed tweezers
• Fine permanent marker pen
• Salt
• 500mL volumetric flask
Procedure
Step 1 Two thirds fill the bulb of the pipettewith fine dry sand.
Use a scooping motion with the end of the pipette
through the sand or using a folded paper (see Figure
45.1 opposite).
Step 2 Holding the open end of the pipette between thumb and
forefinger, place the pipette into the beaker of distilled
water (0g/L salt).
Step 3 Add or remove sand until the pipette floats with about
25mm of the ‘stem’ above the water line.
Step 4 Remove the pipette and hold the open end over amatch
flame until it melts. Blow out the match and use it, or
the tweezers, to seal the open end of the pipette. (Don’t
use you fingers – molten plastic burns!!)
Step 5 Check for leaks then place back into the beaker of
distilled water.
Step 6 Using the pair of curved pointed tweezers grip the
pipette and place it into graduated cylinder with the 0g/
L solution.
Step 7 Nowgrip right on thewaterline (meniscus) andcarefully
lift it out taking care not to let the tweezers move.
Step 8 Using the fine tipped permanent marker, place a "0"
mark between the tweezers - the waterline.
Step 9 Repeat this procedure with all the salt concentrations,
marking carefully the point where the pipette floats in
each of 5, 10, 15, 20, 25, 30 and 35 g/L solutions.
Figure 38.1 Materials
Mick O'Connor
Notes:
1. Many organisms live in the sea which has a salt concentration
of 35 grams of salt per litre of sea water.
2. Fresh water streams have very little salt in their water, often
less than one gram per litre and pure water has no salt at all.
Fresh water organisms die if placed in salt water.
3. Estuarine species that live in brackish water are able to
tolerate a wide range of salt concentrations.
1,2,3,4,5,6 8,9,10