Let's take a look at the chemistry behind the story.
Firstly, Taree, a town located about 3 hours north of Sydney, is surrounded by farm land, land reclaimed from the wetlands. The sulfate ion, SO42-, is commonly found in fertilizers used in commercial farming. Recent rain, and flooding, has concentrated these acidic sulfates in the river.
A little later in the story we find that "Tests carried out by the university's water research laboratory show alarming amounts of acid, with a pH level of two - compared with a normal level of seven - meaning the Manning River water is roughly as acidic as lemon juice."
Chemistry students would realize that there are many factors that can effect the pH of river water, for example, if the river runs through limestone rocks the pH of the water will increase, but if the river runs through areas of peat the pH of the water will decrease.
The pH of river water typically lies within the range of about 6.5 to 8.5. Water with a low pH is said to be acidic, water with a high pH is said to be basic or alkaline. Most organisms, with the exception of some bacteria, can not live in water with a pH less than 6.5. Similarly, a pH greater than 8.5 also presents problems for the survival of most organisms in rivers.
The juice of a lemon often has a pH of about 2, and the vinegar you buy from the shop will also have a pH around 2. Both lemon juice and vinegar are acidic substances.
On the other hand, oven cleaner has a pH of about 13 and soapy water has a pH of about 12. Both oven cleaner and soapy water are basic solutions (or alkaline solutions).
Is river water with a pH of 2 "comparable to car batteries" as claimed in the story?
Lead-acid batteries, such as those found in cars, contain sulfuric acid, H2SO4. Sulfuric acid is a strong acid that undergoes dissociation in water so that an aqueous solution of sulfuric acid contains both hydrogen ions, H+, and sulfate ions, SO42-. The acidic river water will contain both hydrogen ions, H+, and sulfate ions, SO42-, if sulfate fertilizers have been used on the land where the river runs, so the acid in the car's lead-acid battery and the river water are comparable in that they contain the same ions.
The concentration of sulfuric acid in the lead-battery will usually be between 4 and 5 mol L-1 (let's just assume its 4.5 mol L-1 ).
If we assume the complete dissociation of sulfuric acid:
H2SO4 → 2H+ + SO42-
Then the concentration of hydrogen ions, H+, in solution is 2 times the concentration of the sulfuric acid:
[H+] = 2[H2SO4 ] = 2 x 4.5 = 9.0 mol L-1
We can calculate the pH of the battery acid, since pH = -log10[H+] = -log10[9.0] = -0.95
Battery acid is very, very acidic!
While you might be very happy to put vinegar on your chips (pH~2) and eat them, you should most definitely NEVER put battery acid on your chips and eat them!
Suggested Study Questions:
- Draw up a table with two headings; acid and base. Place each of the following substances in the correct column in the table : orange juice (pH =3), baking soda (pH = 9), milk (pH =6), tomato juice (pH =4), drain cleaner (pH =14), black coffee (pH=5).
- Calculate the concentration of hydrogen ions in each of the substances in the table, in mol/L
- Assume a drinking glass has a total value of 250 mL, and that a "full glass" of a drink is actually only 225 mL. Calculate the moles of hydrogen ions found in a "full glass" of
- orange juice
- black coffee
- Consider 225 mL of the river water with a pH =2. Calculate the moles of hydrogen ions present.
- Imagine you took 25 mL of orange juice (pH=3) and diluted it with water to a volume of 500 mL.
- Calculate the concentration of hydrogen ions in the diluted solution.
- Calculate the pH of the diluted solution.
- Sometimes cooks heat ingredients to "release their flavour". Acids, like vinegar, tend to have a sour taste. A cook has 200 mL of vinegar (pH=2.2) in a pan.
- Calculate the concentration of hydrogen ions present in the solution.
- On very gentle heating, the volume of the vinegar solution is reduced until it is only 50 mL. Calculate the pH of this concentrated solution.
- We could prepare a solution of sulfuric acid with a pH of 2 using the acid out of the car's lead-acid battery.
- Calculate the concentration of hydrogen ions present in 4.5 mol L-1 sulfuric acid.
- Calculate the concentration of hydrogen ions present in sulfuric acid with a pH of 2.
- If you had 10 mL of battery acid, what volume of water would you have to add in order to prepare a sulfuric acid solution with a pH of 2?
- Imagine the a dam with a volume of 250,000ML and a pH=2. How much water would have to be added to the dam in order for the dam to have a pH=7 ?