Service 2

Carbon system measurements 

The carbon system in seawater is quite complex but as this is a key part of controlling our experiments it is important that an accurate and large set of measurements are made on the carbon system. Without getting into complex chemistry, if two parameters from the carbon system are measured then all the other parameters can be calculated from these two if the salinity and temperature of the seawater are also measured.

Briefly (and simply) this is because of a series of chemical equations governing how carbon is passed between molecules in the seawater…

CO₂ reacts with water to form a weak acid but this quickly breaks up to form bicarbonate and hydrogen ions

CO₂ + H₂O —> H₂CO₃ —> HCO₃^– + H⁺

Bicarbonate can breakdown further to form carbonate ions, but more commonly the reverse reaction occurs: carbonate ions soak up hydrogen ions (acting as buffers) to form bicarbonate ions

CO₃^2- + H⁺ —> HCO₃^–

Dissolved Inorganic Carbon (DIC) is the total concentration of all the carbon forms in seawater

DIC = [CO_2(aq)] + [H₂CO₃] + [HCO₃^–] + [CO₃^2-]

Alkalinity is a more complicated measure of the carbon system but roughly it tells us how much buffering capacity there is in the seawater.

We are able to measure pH, alkalinity and DIC with a variety of machines. These three parameters, along with temperature and salinity, will be measured throughout the whole experiment to provide to all the scientists so that we understand what is going on in the carbon system. From alkalinity and DIC we will then be able to calculate important parameters such as the carbonate ion concentration and the saturation states of calcite and aragonite, which tell us whether the seawater is corrosive to calcium carbonate material, such as shells.

 

Make your own pH indicator dye at home or in the classroom (with adult supervision)

Red cabbage contains a coloured pigment which is one of a number of naturally occuring acid-base indicators. This means that the structure of the molecule changes when it is placed in an acid or a basic solution and this changes the colour. Red cabbage can be used to test solutions for their pH:

Ingredients:

Red cabbage leaves

Distilled (Fresh) water

A container (beaker or bottle)

Heating device (either a bunson burner or stove)

 

Extracting the dye (on a bunson burner – if you are doing this at home you can heat the cabbage leave in a pan on a stove instead):

1. Take a few leaves of red cabbage and tear or cut them into small pieces.
2. Fill a 250-mL beaker about three-fourths full of the cabbage leaf pieces, and add distilled water to the 200mL mark.
3. Place the beaker on the wire gauze of a ring stand and heat with a burner to a slow boil for about 20 minutes.  Be careful not to boil over the cabbage, or to let so much water evaporate that it burns.
4. Remove the beaker when you have a dark purple extract.  This will be used in the next portion of the experiment.
5. Pour the cabbage extract indicator solution into a labelled stock bottle. If you are going to keep the solution for a while, put it in the refrigerator.

 

Testing pH of solutions:

You can test a number of different household solutions to see whether they are acid or base. When the cabbage water is in its red form the solution is more acidic and when it is in blue form it is basic. For example, Lemon juice & vinegar have a pH 2, Orange juice has a pH 3, Banans have a pH 5, Milk has pH 6, pure water has pH 7, Sea water has pH 8, Baking soda has pH 9 and Soapy water has pH 12.

If you have access to a known concentration of acid (e.g. in school use 0.1 M  HCl – Wear goggles at all times during this experiment.  The acids and bases are corrosive, and could cause damage to your eyes) you can use serial dilutions to create a pH colour scale.

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