AI and chemistry

 How good are AI generated responses to typical chemistry questions?

No better than the source material used for the answer ... which can include common misconceptions.

One such question is "What is the pH of water?". To a chemist this is a nonsense question (comparable to asking what colour an apple is or what shape a building is) yet it is often asked in school science classrooms around the world. 

I asked ChatGPT "What is the pH of water?" This was the answer ...

The pH of pure water at 25 degrees Celsius (77 degrees Fahrenheit) is considered neutral and is defined as 7. A pH value below 7 indicates acidity, while a pH value above 7 indicates alkalinity. It's important to note that the pH of water can be influenced by dissolved substances, and natural water sources may have varying pH levels due to the presence of minerals and other compounds.

I asked perplexity.ai "What is the pH of water?" This was the answer ...

The pH of water is 7, which is considered neutral. A pH of 7 is neither acidic nor basic. It is important to note that the pH of pure water can change due to the presence of impurities or the absorption of carbon dioxide from the air, which can make it slightly acidic. However, under normal conditions, the pH of pure water is 7, indicating neutrality

2

Surprise, surprise, both answers are inaccurate and riddled with common misconceptions (although, to its credit, perplexity.ai does provide sources, but to its discredit it does not refer to a specific temperature which is critical!). ChatGPT emphasizes "pure water", while perplexity.ai is content with just "water". To a chemist, both terms are the same, "water" is a pure substance. If water has substances dissolved in it it is no longer "water" it is an "aqueous solution". Perplexity.ai's referral to "normal conditions" is baffling, perhaps it refers to standard conditions for aqueous solutions? After these issues, both AIs seem to perpetuate the misconception that acidity, basicity (alkalinity) and neutrality are defined by a pH value. This is not true. They are two entirely different concepts (although both are based on an Arrhenius description of aqueous solutions). pH is a measure of aqueous hydrogen ion concentration, independent of whether the aqueous solution has been designated acidic, basic or neutral. Acidic, basic, neutral are terms to describe a the relative amounts of hydrogen ions and hydroxide ions in an aqueous solution.

So, let's start by talking about the neutrality of "pure water". Pure water is always neutral (regardless of its pH). This is because the concentration of acidic protons in the water, H⁺(aq) or H₃O⁺(aq), is always equal to the concentration of basic hydroxide ions in the water, OH^-(aq), that is [H⁺(aq)] = [OH^-(aq)]

An acidic aqueous solution is one in which the concentration of acidic protons is greater than the concentration of basic hydroxide ions, that is, [H⁺(aq)] > [OH^-(aq)]

A basic aqueous solution is one in which the concentration of acidic protons is less than the concentration of basic hydroxide ions, that is, [H⁺(aq)] < [OH^-(aq)] 

So "pure water" is always neutral, but what is its pH? pH is a measure of the concentration of aqueous hydrogen ions in solution, the greater the concentration of hydrogen ions in aqueous solution, the lower the pH. The concentration of these hydrogen ions is very much dependent on the water temperature. The pH of pure water at 10^oC is approximately 7.27 but the water is still described as neutral because [H⁺(aq)] = [OH^-(aq)]. The pH of pure water at 50^oC is about 6.63 but the water is still neutral because [H⁺(aq)] = [OH^-(aq)].

 The pH of water at 25^oC is approximately 7 and is based, NOT on it being defined this way, but on a calculation of the hydrogen ion concentration of water at this temperature.

So, what happens to pH if other substances are dissolved in water to form aqueous solutions at 25^oC? Using an Arrhenius definition of acids and bases (and a pH of 7 which has 1 significant figure but even that digit is uncertain), if an aqueous solution has pH < 7  at 25^oC it could be described as acidic, but if the temperature is greater than 25^oC the solution may be neutral, it may even be basic, we simply do not have enough information to decide. Similarly, If an aqueous solution has pH > 7  at 25^oC it could be described as basic, but if the temperature is less than 25^oC it is not a justifiable conclusion.

It is important to note that if there are dissolved "substances", including "impurities", "minerals" or "carbon dioxide", then the "water" is not "pure water", it is an aqueous solution, and the AI discussions are not relevant. Furthermore, some solutes dissolve in water at 25^oC and increase its pH  (aqueous solution of sodium hydroxide), some decrease its pH  (aqueous solution of hydrogen chloride), and some may have no effect at all on its pH (aqueous solution of sodium chloride).

There is no simple, correct answer for the question "What is the pH of water?". The question does not contain enough information in order for it to be answered. IF the AIs simply stated that the pH of water at 25^oC is approximately 7, that would be a pretty good response, however, by providing additional information in the answer they are repeating, and encouraging, common misconceptions about the nature of "pure water", "pH", "acidity", "basicity" (or "alkalinity") and "neutrality". 

For a discussion on why pure water is always neutral and why its pH varies, see https://www.ausetute.com.au/kw.html

For a discussion on what determines whether an aqueous solution is acidic, basic or neutral, see https://www.ausetute.com.au/abneutral.html

pH After Mixing Weak Acids

 Acetic acid is a weak acid (K_a = 1.8×10^-5).  The pH of 100 mL of 0.10 mol L^-1 acetic acid is 2.9.

Hydrocyanic acid is a weak acid (K_a=6.3×10^-10). The pH of 100 mL of 0.10 mol L^-1 hydrocyanic acid is 5.1.

But what is the pH of the resultant solution if you mixed these two weak acids together?

AUS-e-TUTE has just added new resources to help you understand how to calculate the pH of solutions after mixing weak acids together. AUS-e-TUTE Members should log-in to access the new tutorial, game and test with worked solutions.

If you are not yet an AUS-e-TUTE member, you can access the "free-to-view" tutorial at https://www.ausetute.com.au/phmixwa.html

pH of Aqueous Salt Solutions

If you titrate acetic acid with sodium hydroxide, what is the pH at the equivalence point?

If you dissolve ammonium chloride in water, what is the pH of the solution?

To answer these questions you will have to calculate the pH of an aqueous salt solution!

AUS-e-TUTE has just added new resources to help you do just that!

Members should log-in to use the new tutorial, game, test, drill, worksheet wizard, and problem solving template.

If you are not an AUS-e-TUTE Member, you can access the "free-to-view" tutorial at https://www.ausetute.com.au/phsaltcalcs.html

pH of a Solution When Weak Acid and Strong Base are Mixed

What happens to the pH of a weak acid as you add strong base to it?

For example, aqueous sodium hydroxide solution (NaOH_(aq)) is a strong base and acetic acid (CH₃COOH_(aq)) is a weak acid.

If you add 10 mL of 0.20 mol/L NaOH_(aq) to 25 mL of 0.10 mol/L CH₃COOH_(aq), what is the pH of the resultant solution?

Find out how to work this out, and work through other examples with worked solutions at AUS-e-TUTE.

A "free-to-view" tutorial is available at https://www.ausetute.com.au/phmixwasb.html

AUS-e-TUTE Members should log-in to play the new game, do the test and get repetitive endless practice with the new drill.

Monoprotic acid pH with a concentration of 0.022M

Question: What is the pH of a monoprotic acid with a concentration of 0.022 M?

Answer:
1. Assume this is a strong monoprotic acid so that it fully dissociates: HA → H⁺ + A^-

2. [HA] = [H⁺] = 0.022 M

3. pH = -log₁₀[H⁺] = -log₁₀[0.022] = 1.7

Find out more, and see questions with worked solutions, at https://www.ausetute.com.au/phstronga.html

Neutral pH

What is a neutral pH?

An aqueous solution is neutral when the concentration of hydrogen ions in solution is equal to the concentration of hydroxide ions in solution:

Neutral solution: [H⁺(aq)] = [OH^-(aq)]

The pH of a solution is a measure of the concentration of hydrogen ions in solution:

pH = -log₁₀[H⁺(aq)]

For example, an aqueous solution will be neutral if  [H⁺(aq)] = [OH^-(aq)] = 10^-3 mol L^-1

For this solution the pH will be: pH = -log₁₀[H⁺(aq)] = -log₁₀[10^-3] = 3

For example, an aqueous solution will be neutral if  [H⁺(aq)] = [OH^-(aq)] = 10^-6 mol L^-1

For this solution the pH will be: pH = -log₁₀[H⁺(aq)] = -log₁₀[10^-6] =6

"Neutral" is NOT a pH.

Why do so many (non- Chemists!) believe a solution is neutral if the pH=7 ?
An aqueous solution can have a pH of about 7 IF the temperature of the aqueous solution is about 25°C.

At 25°C, a neutral aqueous solution has pH≈7

IF the temperature is less than 25°C, the pH will be greater than 7

Temperature <25°C, a neutral aqueous solution has pH>7 

IF the temperature is higher than 25°C, the pH will be less than 7

Temperature >25°C, a neutral aqueous solution has pH<7

The pH of a neutral aqueous solution depends on the temperature of the solution!!!

Read more at https://www.ausetute.com.au/kw.html

Suggested Study Questions:

1.  In an aqueous solution the concentration of hydrogen ions is the same as the concentration of hydroxide ions. Is this solution acidic, basic, or, neutral? 
2. In an aqueous solution, the concentration of hydrogen ions is 2.7 x 10^-6 mol L^-1 and the concentration of hydroxide ions is 2.7 x 10^-6 mol L^-1. Is this solution acidic, basic, or, neutral. Explain your answer.
3. The concentration of hydroxide ions in a neutral aqueous solution is 5.8 x 10^-6 mol L^-1. What is the concentration of hydrogen ions in this solution? 
4. The concentration of hydrogen ions in a neutral aqueous solution is  7.9 x 10^-7 mol L^-1 What is the concentration of hydroxide ions in this solution?
5. Calculate the pH of the solution in question 4.
6. Calculate the pH of the solution in question 3.
7. Calculate the pH o the solution in question 2.
8. Explain how you would calculate the pH of the solution in question 1, justifying any assumptions you need to make.
9. Critically evaluate this statement, "At pH 7 a solution is neutral".
10. Chris the Chemist tested the pH of the water in the local pool. Monday was a hot day, about 40°C, and the pH of water was 6.82. That night the temperature dropped dramatically and the next day was a chilly 10°C. When Chris tested the pH of the pool water it was found to be 7.59. Is the pool water acidic, basic, or neutral? Justify your answer.

Indicators for Strong Acid - Strong Base Titrations

When a strong acid is added to a strong base the products are water and a salt.
Water is neutral, that is [H⁺(aq)] = [OH^-(aq)]
 (or [H₃O⁺(aq)] = [OH^-(aq)] if you prefer)

The salt of a strong acid and base is made up of a cation that will not react with water to any appreciable extent, and an anion that will not react with water to any appreciable extent, so this salt does not affect the  [H⁺(aq)] and [OH^-(aq)] in the water, that is, the aqueous solution remains neutral.
At 25°C, K_w = [H⁺(aq)] × [OH^-(aq)] = 10^-14
Since [H⁺(aq)] = [OH^-(aq)]
K_w = [H⁺(aq)]² = 10^-14
√[H⁺(aq)]² = √10^-14
[H⁺(aq)] = 10^-7 mol L^-1
 So, at 25°C the pH of this salt solution will be pH = -log₁₀[H⁺(aq)] = -log₁₀[10^-7 ] = 7.0
A suitable indicator is one that changes colour at around pH = 7.00
Suitable indicators, for example, are bromothymol blue (colour change between 6.7 and 7.6) or phenol red (colour change between 6.8 and 8.4)

Phenolphthalein changes colour between pH 8.3 and 10. Phenolphthalein is NOT an appropriate indicator for a strong acid - strong base titration.

If we add a drop of phenolphthalein indicator to an aqueous solution of strong acid, the pH will be less than 7 and the solution will remain colourless. As we add strong base, hydrogen ions react with excess hydroxide ions to produce salt and water, so the pH increases. At pH = 7.0 all the strong acid will have been neutralised by the addition of strong base, BUT the phenolphthalein indicator will not have changed colour!
Phenolphthalein will not change colour until an excess of strong base (hydroxide ions) has been added and we have overshot the equivalence point for the reaction. The volume of strong base we record in this experiment will be too large!.

In strong base such as an aqueous solution of sodium hydroxide, the pH will be high and a drop of phenolphthalein indicator will turn the solution pink.
As we add a strong acid such hydrochloric acid, we will be consuming some of the hydroxide ions, and decreasing the pH. Somewhere between pH 8.3 and 10 we will decide that all our base has been neutralised by the acid because the indicator is now colourless instead of pink. But the reality will be that there is still excess hydroxide ions in solution waiting to be neutralised by the addition of more acid, so the volume of acid we have added, as indicated by the colour change of the indicator will be too low!

Learn all about how to choose an appropriate indicator for different types of acid-base titrations ar
https://www.ausetute.com.au/indicata.html

pK

You've probably heard of pH, maybe even pOH, but what is pK?
Does this sound like brand of chewing gum to you?
Or does that "p" mean something?
Time to find out, with AUS-e-TUTE's new pK resources!

https://www.ausetute.com.au/pk.html

What is the pH of a Strong Base After You Dilute It?

The problem with strong bases like sodium hydroxide and potassium hydroxide are that they absorb moisture from the air making it difficult to accurately weigh them. So we rely heavily on obtaining an accurately known concentration for an aqueous solution of a strong base (using titration techniques for example) and then diluting this stock solution to produce new, dilute solutions of known concentration and pH.
When you dilute an aqueous solution of a strong base :

• hydroxide ion concentration decreases (towards 10^-7 M)
• pOH increases (towards 7)
• hydrogen ion concentration increases (towards 10^-7 M)
• pH decreases (towards 7)

Visit AUS-e-TUTE's new tutorial to understand why, then log-in to the Members ONLY area to play the game, answer test and exam questions (which give you instant feedback), take the quiz, or, just for teachers, you could make a printable worksheet or quiz.

pH after mixing acid and base

If you add enough strong base to a strong acid you can neutralise the solution and its pH will be 7.
But what if you don't add enough base to the acid.
Will the solution be neutral?
Will the pH of the solution be 7?
These are all very good questions which is why AUS-e-TUTE has just added a new tutorial, game, test and exam on this topic.
AUS-e-TUTE Members should log-in to use these resources.

If you are not an AUS-e-TUTE Member yet, you can view the tutorial for free at https://www.ausetute.com.au/phmixsab.html

Effect of Dilution on pH of Strong Acid Solution

Need to know what the pH of your hydrochloric acid is after you dilute it?
Need to know how much water to add to your hydrochloric acid to make a solution with a particular pH?
AUS-e-TUTE has just added a new tutorial, game, test and exam to help you with this.
AUS-e-TUTE members should log-in to use the new resources.
If you're not an AUS-e-TUTE Member, there is a "free-to-view" tutorial currently available at https://www.ausetute.com.au/phdilsa.html

Sulfuric Acid Train Derailed

A locomotive and all 26 carriages containing sulfuric acid derailed in north Queensland on Sunday 27^th December 2015.
The 800,000 L of sulfuric acid was bought by Incitec Pivot Limited (IPL) to be used in the production of ammonium phosphate fertiliser at its plant at Phosphate Hill (south of Mount Isa).

Glencore Plc has an agreement with Incitec Pivot Ltd to provide sulfur dioxide from the copper smelter to the Mount Isa acid plant. Sulfuric acid is produced by collecting and cleaning sulfur dioxide before converting it, first to sulfur trioxide, and then to sulfuric acid. The acid plant has the capacity to take up to 80% of the sulfur dioxide emissions from the copper smelter, significantly reducing the emissions of sulfur dioxide into the atmosphere. The sulfuric acid is then taken by train to Phosphate Hill, more than 100 km away.

Phosphate Hill combines a large open-cut phosphate mine with a plant for the production of fertiliser. The proved reserve is 29 Mt at 24.6% P₂O₅. The phosphate ore is first crushed and washed before being dissolved in sulfuric acid to produce phosphoric acid. Gypsum, CaSO₄.2H₂O, a by-product of the process, is stockpiled. The phosphoric acid is converted into ammonium phosphate fertiliser using ammonia gas:
 NH₃ + H₃PO₄ → NH₄H₂PO₄ (known as MAP)
2NH₃ + H₃PO₄ → (NH₄)₂HPO₄ (known as DAP)
The ammonium phosphate is then sent by train to the port at Townsville about 900 km away for shipment.

Wet weather hampered access to the disaster site for several days, but testing of the waters 8 km downstream in Horse Creek revealed changes in acidity. Locally sourced limestone is being used to treat the water.

References:
http://statements.qld.gov.au/Statement/2015/12/31/environmental-testing-continues-around-train-derailment-near-julia-creek
 http://www.abc.net.au/news/2015-12-30/fear-sulphuric-acid-leaked-train-derailed-waterway-julia-creek/7060070

Further Reading
 Copper Smelting
 Electrorefining
 Sulfuric Acid Production
Density Calculations
Percentage Composition 
 Intermolecular Forces
pH Scale 
pH Calculations for Acids  
Neutralisation

Suggested Study Questions:

1. Convert 800 000 L of sulfuric to a volume in:
   • megalitires (ML)
   • kilolitres (KL)
   • millilitres (mL)
2.  Assume the density of this sulfuric acid is 1.84 g mL^-1. What is the mass of 800,000 L of sulfuric acid?
3. Assuming all the sulfuric acid was distributed equally between the 26 carriages, what volume of sulfuric acid was contained in each carriage?
4. Write the chemical formula for each of the following:
• sulfur dioxide
• sulfur trioxide
• sulfuric acid
• sulfurous acid
5.  Convert 29 Mt of phosphate ore to a mass in
   • tonnes (t)
   • kilotonnes (kt)
   • kilograms (kg)
   • grams (g)
6.   A phosphate ore contains 24.6% P₂O₅
   • What mass of P₂O₅ is present in 1 tonne of the ore?
   • What mass of the element phosphorus is present?
   • What mass of the element oxygen is present?
7.  Give the systematic IUPAC name for each of the following:
   • NH₄H₂PO₄
   • (NH₄)₂HPO₄
8. Calculate the percentage by mass of nitrogen in
   • NH₄H₂PO₄
   • (NH₄)₂HPO₄
9. In situations in which too much nitrogen in the fertiliser can adversely affect germination, which phosphate, MAP or DAP, should be used? Explain your answer. 
10. Hygroscopy is the ability of a substance to  attract and hold water molcules from the surrounding environment. Which phosphate, MAP or DAP, would you expect to be more hygroscopic? Explain your answer.
11. Sulfuric acid from the derailed carriages is believed to be leaking into the water of Horse Creek. Do you expect the pH of the creek to be rising or falling? Explain your answer.
12. If the pH of the creek water was 3.0, calculate the concentration of hydrogen ions in the creek water.
13. Explain what the term neutralisation means.
14. Assuming limestone is composed of calcium carbonate only, write a balanced chemical equation for the reaction between sulfuric acid and limestone.
15. Would you describe the chemical reaction described by the equation in question 14 as a neutralisation reaction? Explain your answer.
16. In the laboratory you could use sodium hydroxide to neutralise sulfuric acid. Why isn't sodium hydroxide used to neutralise the sulfuric acid at the train derailment site?

 

Cheesy Chemistry

Now here's the title of an article that sounds like it would make a great teaching and learning tool ..
"Food hacks: The science behind making perfect cheese melts and crispy cookies"
(Sydney Morning Herald, Monday 23rd November 2015)

"Science is great isn't it? ", writes the article's author.
Yes indeed, I couldn't agree more ... looks promising .....

"Even for those of us who find the periodic table of elements a foreign language, we can still reap the benefits of science's life-changing revelations."
Well, that's going a bit far (especially if you happen to teach/learn chemistry), but even so,  it still looks OK ......

"According to science, there's only one type of cheese for your toastie."
...mmm... possibly ...... "science" is rarely capable of making that kind of judgement ..... but we'll continue reading ....

until ........

" That cheese is the one with the right PH to balance the calcium, and release the casein (dairy protein) to create one big soft melty​ mess."
PH? Is that some kind of special food science thing? Could it be phosphorus monohydride?
No, it appears to simply be a mistake, which was, unfortunately repeated on the following line.
The author was referring to pH.

Nevertheless, did you know that different cheeses have different pH values?
I didn't!
So off I went to find the pH of some of my favourite cheeses:

cheese	pH
camembert	7.44
cheddar	5.90
cottage	4.75-5.02
cream	4.10-4.79
edem	5.40
gruyere	5.68-6.62
parmesan	5.20-5.30
stilton	5.70

Apparently, pH and temperature are both critical factors in the production of cheese:

• Addition of starter culture: temperature less than 20°C, pH = 5.1-5.3 (using rennet which contains enzymes for breaking down proteins)
•  Coagulation: temperature = 30°C, pH = 5.35 - 5.45
• Pressing: temperature 16-18°C (mild cheeses) or 25°C (hard cheeses), pH = 5.0-5.3
• Brining in salt solution: temperature 15°C, pH = 5.2
• Ripening: pH increases to optimum value as given in the table above.

A crumbly cheese, like a Cheshire cheese, has a low pH and low calcium content. At low pH the colloidal calcium phosphate between casein micelles becomes soluble and the size of these protein aggregates decreases, which, makes the cheese crumbly.

A low-acid cheese (high pH cheese) like Swiss cheese, has intact casein micelles which provide an extensive string of protein aggregates giving the cheese more elastic properties.

Further Reading:
http://www.ausetute.com.au/phscale.html
http://www.ausetute.com.au/phcalcs.html
http://www.ausetute.com.au/phhcalcs.html 
http://www.ausetute.com.au/enzymes.html 
http://www.ausetute.com.au/proteins.html
http://www.ausetute.com.au/aminoacid.html
http://www.ausetute.com.au/scientificm.html
http://www.ausetute.com.au/labreport.html


Suggested Study Questions:

1. What is meant by the term pH ?
2. Calculate the hydrogen ion concentration for each of the cheeses listed in the table above.
3. Arrange the cheeses in the table from lowest to highest pH.
4. Arrange the cheeses in the table from lowest hydrogen ion concentration to highest hydrogen ion concentration.
5. What is an enzyme?
6. What is a protein made up of?
7. Why do you think the temperature of the mixture during the addition of rennet and the coagulation stages is higher than at other stages during the production of cheese?
8. "According to science, there's only one type of cheese for your toastie."
    Do you think science can really tell you the best cheese to use for your toastie? Why or why not?
9. Who do you think the intended audience of this article is? Explain your answer.
10. Imagine you have just tested the pH the of various cheeses and that it is your results shown in the table above. Rewrite this article as if it were your lab report.
    

Calculating the pH of Sulfuric Acid

How do you calculate the pH of a strong diprotic acid, that is, how do you calculate the pH of sulfuric acid?
I'm glad you asked!
AUS-e-TUTE has just added a new set of resources designed to help you answer this question.

AUS-e-TUTE Members should log-in and go to the links for the Polyprotic Strong Acids tutorial, game and test, on the Test Centre homepage, or, alternatively, go to your AUS-e-TUTE syllabus study guide and follow the links from that.

Not a member?
There is a "free-to-view" tutorial currently available for evaluation purposes at
http://www.ausetute.com.au/polyproticsacid.html

Calculations for Strong Bases

AUS-e-TUTE has just added new resources!
As part of the update of our "Acids and Bases" topic, tutorials, games, tests, and exams have been added for the following:

• pOH Concepts
• Strength of Bases
• Calculating the pOH of Strong Bases (aqueous solutions)
• Calculating the Hydroxide Ion Concentration of Strong Bases (aqueous solutions)
• Calculating the pH of Strong Bases (aqueous solutions)
• Calculating the Hydrogen Ion Concentration of Strong Bases (aqueous solutions)

AUS-e-NEWS March 2013

Since the 1960's, superacids have become an essential tool in industry.

Without the powerful ability of superacids to react with, and break down, raw petroleum we would not have a supply of cheap, high-strength plastics.

But what is a superacid?

Read this super issue of AUS-e-NEWS to find out !

Links to the following new resources can be found in the Test Centre (http://www.ausetute.com.au/members/testcent.html)

                - Properties of Acids and Bases (tutorial, game, test)

                - Definitions of Acids and Bases (tutorial, game, tests, exams)

                - pH Scale (Tutorial, game, test, exam)

                - pH Calculations, introductory (tutorial, game, test, drill)

                - pOH Calculations, introductory (tutorial, game, test, drill)

                - Hydrogen Ion Concentration calcuations, introductory (tutorial, game, test, drill)

                - Hydroxide Ion Concentration Calculations, introductory (tutorial, game, test, drill)

                - Strength of Acids (Tutorial, game, test,exams)

                - Strong Acid pH Calculations (Tutorial, game, test, exam)

                - Strong Acid Hydrogen Ion Concentration Calculations (Tutorial, game, test, exam)

                - Strong Acid Hydroxide Ion Calculations Calculations (Tutorial, game, test)

                - Strong Acid pOH Calculations (Tutorial, game, test)

                - Oxidation of Alkenes (tutorial, game, test, exam)

If you have haven't received your issue of AUS-e-NEWS, please contact us.

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 ?

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.