pH of the Manning River

"A POISONOUS plume of acid 'comparable to car batteries' is forming in the Manning River, near Taree in northern NSW, researchers from the University of NSW say." reports Ben Cubby in his article "Acid plume poisons river after floods"  in the Sydney Morning Herald, Thursday 28^th February 2013.
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, SO₄^2-, 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, SO₄^2-. The acidic river water will contain both hydrogen ions, H⁺, and sulfate ions, SO₄^2-, 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⁺ + SO₄^2-
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!

Reference:

 http://www.smh.com.au/environment/conservation/acid-plume-poisons-river-after-floods-20130227-2f6bh.html#ixzz2MA2XQ5AJ

Further Reading:
Calculating pH

Suggested Study Questions:

1. 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).
2. Calculate the concentration of hydrogen ions in each of the substances in the table, in mol/L
3. 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
   • milk
   • black coffee
4. Consider 225 mL of the river water with a pH =2. Calculate the moles of hydrogen ions present.
5. 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.
6.  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.
7. 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?
8. 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 ?

Drunken Fruit Flies

Wasps are a major killer of fruit flies. They inject their eggs inside fruit fly larvae, then, when the wasp egg hatches, the wasp larva starts eating the fruit fly lava from the inside!
Scientists at Emory University have found that fruit flies prefer to lay their eggs in an environment  with a "high" concentration of ethanol. The fruit flies have evolved a certain amount of tolerance to this toxic ethanol, but the wasps who inject their eggs inside fruit fly larvae find the ethanol level to be lethal. Furthermore, fruit fly lava that have been infected with wasp larva tend to prefer to eat food with a high ethanol content, this raises their blood alcohol level and helps kill the wasp larva.

The most common natural source of ethanol is rotting fruit. Yeasts on rotting fruit can ferment the fruit sugars, like fructose, to produce ethanol:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

This fermentation reaction takes place in anaerobic environments, that is, environments in which oxygen is not present.
The concentration of ethanol in rotting fruits has been  found to be between 0.04 and 0.72 v/v%. By comparison, the ethanol content in beer is usually between 3 and 6 v/v%, while the ethanol content of wine is between 8 and 11 v/v%.
Volume/volume (or volume) percent is a common way to refer to the concentration of alcoholic solutions. It refers to the volume of solute divided by the volume of solution which is then multiplied by 100, that is:

v/v% = V(solute)/V(solution) x 100

Beer that is 3 v/v% ethanol contains 3 mL of ethanol in every 100 mL of beer.

Wine that is 11 v/v% ethanol contains 11 mL of ethanol in every 100 mL of wine.

This preference for eating rotting fruit containing ethanol displayed by the fruit flies seems to be uncommon. Most animals, including humans, seem to prefer ripe, but not rotting, fruit.

References:

1. B. Z. Kacsoh, Z. R. Lynch, N. T. Mortimer, T. A. Schlenke. Fruit Flies Medicate Offspring After Seeing Parasites. Science, 2013; 339 (6122): 947 DOI: 10.1126/science.1229625
2. Neil F. Milan, Balint Z. Kacsoh, Todd A. Schlenke. Alcohol Consumption as Self-Medication against Blood-Borne Parasites in the Fruit Fly. Current Biology, 2012; 22 (6): 488 DOI: 10.1016/j.cub.2012.01.045

Further Reading:
Fermentation
Carbohydrates
Naming Alcohols
Density

Suggested Study Questions:

1. Calculate the volume of ethanol in a stubby (375 mL) of full strength beer (ethanol concentration 4.8 v/v%).
2. An average standard wine glass has a volume of 150 mL. What volume of ethanol is present in a standard wine glass of white wine with an ethanol concentration of 11.5 v/v%?
3. Port is an example of a fortified wine, that is, a wine that has had an additional distilled beverage like brandy added to it to increase its alcohol content to about 17.5 v/v%. A standard port glass has  a volume of 60 mL. Calculate the volume of ethanol in a standard glass of port.
4. The specific gravity (density) of ethanol is 0.789 g/mL. Calculate the mass of ethanol present in a stubby (375 mL) of
   • full strength beer (5 v/v% ethanol)
   • light beer (2.7 v/v% ethanol)
5. Spirits such as rum and vodka, have an ethanol concentration of approximately 40 v/v%. A standard "nip" is 30 mL. Calculate:
   • the volume of ethanol in a nip of vodka
   • the mass of ethanol in a nip of vodka
6. The alcohol content of Marsala wine is increased by allowing water to evaporate off it. The concentration of ethanol in Marsala wine will reach about 18 v/v%. Calculate:
   • volume of ethanol in a 750 mL bottle of Marsala
   • mass of ethanol in this bottle of Marsala wine
7. A particular type of wine barrel holds 225 L of wine. Calculate the mass of ethanol present if the wine in the barrel is
   • red wine (13 v/v% ethanol)
   • white wine (11.5 v/v% ethanol)
   • champagne (12 v/v% ethanol)

Neutral pH?

We hear this term a lot, often in advertising. But what does it mean?

From a Chemist's point of view, there are two different concepts involved in this seemingly harmless "neutral pH" expression. These two different concepts are:

• neutral
• pH

Let's take a look at the Chemist's definition of neutral first.
A solution is neutral if the concentration of hydrogen ions, [H⁺], is equal to the concentration of hydroxide ions, [OH^-].
Chemists often use square brackets to denote concentration, the concentration of  hydrogen ions can be written as [H⁺] and the concentration of hydroxide ions can be written as [OH^-].
So, for a neutral solution:

[H⁺] = [OH^-] = neutral solution 

Pure water is an excellent example of a neutral substance.

Some of the water molecules, actually very few of them, dissociate to form hydrogen ions and hydroxide ions:

H2O

H+ + OH-

Every time a water molecule dissociates, it produces one hydrogen ion, H⁺, and one hydroxide ion, OH^-, so that the concentration of hydrogen ions is always the same as the concentration of hydroxide ions.
Therefore, pure water is always neutral!

The pH of a solution is a measure of the hydrogen ion concentration in the solution. pH can be defined as:

pH = -log₁₀[H⁺]

This equation can be used to calculate the pH of our neutral water, but only if we know the concentration of  hydrogen ions in the water.

The concentration of hydrogen ions in water is not constant!

The concentration of hydrogen ions in water depends on the temperature of the water!
The dissociation of water molecules requires energy:

H2O + energy

H+ + OH-

If you put more energy into the system by heating it, then more water molecules dissociate, the concentration of hydrogen ions increases and the concentration of hydroxide ions also increases.
If you take energy away from the system by cooling it, then fewer water molecules dissociate, the concentration of hydrogen ions decreases and the concentration of hydroxide ions also decreases.
If we were to measure the concentration of hydrogen ions in pure water at various temperatures, we would find the following values:

Water temperature	[H+] x 10−7 M	pH
0°C	0.32	7.50
10°C	0.55	7.26
18°C	0.84	7.08
25°C	1.10	6.96
30°C	1.34	6.87
50°C	2.82	6.55
60°C	3.55	6.46
70°C	4.60	6.34
80°C	5.92	6.23
90°C	7.28	6.14
100°C	8.54	6.07

So what is the pH of water?
The pH of water is dependent on the temperature of the water.
Water is neutral for every value of pH because the concentration of hydrogen ions is always equal to the concentration of the hydroxide ions.
We can ONLY talk about the pH of water IF we state the temperature of the water.
For example, we can talk about water having a pH of approximately 7 at 25^oC, or we could say that the pH of water is approximately 6 at 100^oC.
Pure water is always neutral.
Pure water is neutral at 25^oC.
Pure water is neutral at 100^oC.

As Chemistry students, what we can't say is that water has a pH of 7, or that a neutral aqueous solution has a particular pH, unless we state the temperature of the system.

Further Reading:
Definitions of Acids and Bases
pH
Dissociation Constant for Water

Suggested Study Questions:

1. Plot a graph of temperature versus concentration of hydrogen ions in water. Describe the shape of the line, and write a generalization that links hydrogen ion concentration and temperature.
2. Plot a graph of temperature versus pH of water. Describe the shape of the line and write a generalization linking the  temperature of water and its pH.
3. Use your graph to find the pH of water at:
   • 12^oC
   • 22^oC
   • 32^oC
4. Construct a table giving the concentration of hydroxide ions in water at each of the temperatures shown above.
5. Plot a graph of temperature versus concentration of hydroxide ions in water. Describe the shape of the line, and write a generalization linking hydroxide ion concentration in water and temperature.
6. Use your graph to find the concentration of hydroxide ions in water at:
   • 12^oC
   • 22^oC
   • 32^oC
7. Explain why water is neutral at all temperatures.
8. Explain why the pH of water varies with temperature.