As part of KNAUER’s “Osmolalities of the World” project we thought we would look at the osmolalities of the Parramatta River and the sea outside. We took two samples from Parramatta River at different places and one sample from around the coast. We wanted to see how the osmolality of the river is affected by tidal flows. The samples were taken from next to Sydney Olympic Park Wharf, Mort Bay Park in Balmain and outside the harbour from the ocean water at Bondi Beach.
The Parramatta River is the main tributary of Sydney Harbour. It is defined as an intermediate tide-dominated, drowned valley estuary. The river is tidal up to Parramatta, approximately 30 kilometres (19 miles) from the opening of Sydney Harbour into the Tasman Sea.
We were interested to see how the osmolality of the river changed, with the fresh water downstream flow of the river interacting with the upstream flow of the tidal sea water.
Theory of Osmolality
It is worth stressing that these osmolality readings are not just measuring the amount of Na+ and Cl– ions in the water but all soluble ions and compounds. For instance, sea water is known to contain many other ions including sulphate (SO4)2-, magnesium (Mg2+), calcium (Ca2+) and potassium (K+) ions,
Osmolality is a general measure of the particle concentration in a solute. It is not dependant on the nature of molecules but just their number. Therefore, a two molar solution of a non-dissociating molecule has the same osmolality as a one molar solution of a fully dissociating salt composed of two ions. The osmolality of a solution is the same, even when molecules vary in shape as well as in size. Therefore, all solutions containing the same number of osmotically active particles — regardless of their chemical properties — exhibit the same osmolality.
Osmolality Readings (mOsmol/kg)
- Sydney Olympic Park: 450
- Balmain: 912
- Bondi Beach: 1058
Note: a reading of 0 (zero) mOsmol/kg indicates pure water with no particles dissolved in it.
The osmolality of the middle harbour at Balmain is twice as high as that further up the river at Olympic Park. Whereas the osmolality in the open sea at Bondi Beach is only about 14% higher than at Balmain. This non-linear drop in osmolality is probably due to the strong effect of the tide on the wider stretches of the Parramatta River from the harbour mouth up past the middle harbour.
In this experiment the samples were taken at mid-tide. It would be interesting to repeat the experiment just after low tide, when there would be a maximum flow of river water to the sea, and just after high tide when there would be maximum inflows up-river from the sea.
Osmolalities of the World
To see the osmolalities of other regions of the globe take a look at the Knauer “Osmolalities of the World” chart which can be seen at https://www.knauer.net/en/Systems-Solutions/Osmometry
If you would like you can send a sample of about 1ml of water from sea, lake, pond or river to Knauer for them to analyse. They will then add it to the World Map.
Send it to: KNAUER, Att: Dr Hanna Hiltunen
Hegauer Weg, 3814163 Berlin, Germany
Please include the GPS coordinates (found on Google Maps) and avoid using a container made of Polyethylene (PE) as this will contaminate the sample.