What is an indicator?

What is an acid-base indicator?

What does it do?

Where do we use acid-base indicators?

These and other questions about acid-base indicators can be answered at 

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

AUS-e-TUTE Members also have access to the game, test and exam with worked solutions on this topic.

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Weak Acid - Strong Base Titration Curves

A titration of a weak acid with a strong base results in a titration curve with some notable features, including:

• buffer zone (weak acid in equilibrium with its conjugate base)
• point where pH = PK_a which can be used to determine pK_a, and hence K_a, for the weak acid
• equivalence point where pH > 7 due to the hydrolysis of the anion

Find out more at https://www.ausetute.com.au/titrcurvwasb.html

AUS-e-TUTE Members should log-in to use the new tutorial, game and test.

Question Impossible?

This was the first question on the 2019 Western Australia ATAR Chemistry Course Examination 2019

See if you can answer it ...

Boric acid, which is a weak acid, was titrated with standardised sodium hydroxide
solution.

Which one of the indicators listed below would be the most suitable to use in this
titration?

	Indicator	Range of colour change  (pH)
(a)	thymol blue	1 – 3
(b)	bromocresol green	3.8 – 5.4
(c)	cresolphthalein	8 – 10
(d)	alizarin yellow	10 – 12

(Note: no further information was given in the question nor in the accompanying Data Book).

You've probably narrowed the answer to either (c) or (d) using a couple of common assumptions:

• these are aqueous solutions of acid and base

• the temperature of the solutions is 25C

The salt of a weak acid and a strong base in aqueous solution at 25C will have a pH greater than 7.

So, what now? We need more information, like the value of the acid dissociation constant for boric acid and some information about concentrations and volumes.

Boric acid is a weak acid, Ka = 5.8exp-10 (this information was not provided!)

Because Ka is so low, boric acid is essentially a monoprotic acid, so the salt produced is essentially NaH2BO3

Sodium ions will not hydrolyse but H2BO3- will hydrolyse.

H2BO3- + H2O --> H3BO3 + OH-

Kb for the hydrolysis of H2BO3-: 
Kb = Kw/Ka 
= 1exp-14/5.8exp-10 
= 1.7exp-5
We need to know the concentration of the salt,
 this wasn't given in the question,
 so we are going to assume a 1.0 mol/L solution 
(because the numbers are nice)
[H3BO3] = x
[OH-] = x
[H2BO3-] = 1.0 - x 
and assume x is negligible compared to 1.0
 therefore [H2BO3-] ~ 1.0

Kb = [H3BO3][OH-]/[H2BO3-]

1.7exp-5 = x2/[1.0]
take the square root of both sides:
x = 4.1exp-3 = [OH-]
pOH = -log10 [OH-] 
= -log[4.1exp-3] 
= 2.4

For aqueous solutions at 25C:
pH = 14 - pOH 
= 14 - 2.4 
= 11.6

So..... drum roll please .... the answer is (d)


Ofcourse, if the concentration of salt was 1exp-5 mol/L, 
the pH of the solution would be 9.1 .... 
and then the answer would be (c)

If you were unfortunate enough to sit this exam...
 then the answer according to the examiners report was (c) ...
but no details of how they arrived at this answer are provided.

Precipitation Titrations

Have you ever tried to determine the chloride concentration in a water sample using gravimetric analysis? It's pretty tricky! There must be a better way .....
There is .... precipitation titrations (also known as argentimetric titrations or argentometric titrations).
AUS-e-TUTE has just added a new tutorial, game, test, exam and practical activity on this topic for members.
If you are not a member, you can access a "free-to-view" tutorial at
 https://www.ausetute.com.au/pptitration.html

Precipitation Conductometric Titrations

If you have an aqueous salt solution it conducts electricity because of all the ions in the solution. If you add a different aqueous salt solution to this it is possible that a precipitate will form, effectively removing some of the ions in solution so it should be possible to monitor changes in the electrical conductivity of a solution during a precipitation reaction to determine the equivalence point of the reaction. This is known as a precipitation conductometric titration (or as a conductometric precipitation titration) and AUS-e-TUTE has just added a new tutorial, game, test and exam to help our members understand this!
If you are not an AUS-e-TUTE Member, you can currently access a "free-to-view" tutorial on this topic at https://www.ausetute.com.au/conductpptn.html

Conductometric Titrations

Trying to titrate a weak acid with a weak base using an acid-base indicator is .... annoying!
The end point of the titration is extremely difficult to guage.
If only there was a better way ....
There is!
Let us introduce you to acid-base conductometric titrations in AUS-e-TUTE's new tutorial, game, test and exam resources. AUS-e-TUTE Members should log-in to use these new resources.
Non-members can currently access the "free-to-view" tutorial at https://www.ausetute.com.au/conductab.html

HCl as a Primary Standard?

Question: Why is HCl not suitable as a primary standard?
Answer:  There are three main reasons why HCl is not suitable as a primary standard:

• HCl is not a solid at room temperature and pressure. 
• HCl cannot be obtained at a very high purity.
• HCl does not have a high molecular mass.

You can find out more about what makes, and does not make, a primary standard at:
http://www.ausetute.com.au/titrstand.html

What is the best indicator for titration?

Question: What is the best indicator for titration?
Answer: That all depends on what you are titrating!

If you doing an acid-base titration, then you will need to know the relative strengths of the acid and base you going to be using.
Once you have determined an approximate pH for the equivalence point of the neutralisation reaction, you use this as the pH of the endpoint of the titration in order to determine the best acid-base indicator to use.

Concentration of Acetic Acid in Vinegar

Here's a strange thing. Most adults wouldn't dream of drinking wine that has been oxidized and "gone bad", just the smell is likely to put most people off! The same people, however, probably have no objection to consuming wine that has been deliberately oxidized, bottled, and marketed as vinegar.

Ofcourse, the chemistry of the oxidation of a mixture like wine is quite complex, but the most important constituent of the mixture is acetic acid (also known as ethanoic acid).

Determining the acidity of your vinegar, or how much acetic acid (ethanoic acid) is present in your vinegar, is quite easy. It's just a simple acid-base titration, and AUS-e-TUTE has just added new resources to help you understand how you can use acid-base titrations to find the concentration of acetic acid in vinegar.

AUS-e-TUTE Members have access to the new tutorial, games, test and exam.

If you are not an AUS-e-TUTE, you can take a sneak peek at the tutorial here

Methyl Orange

In March 2014, the Australian Competition and Consumer Commission (ACCC) prompted the recall of some clothing for having unacceptable concentrations of particular azo dyes.

Azo dyes are organic (carbon) compounds with the general formula R-N=N-R' and are named after their unusual N=N, azo, functional group.

They are synthetic dyes used to colour foods, cosmetics, carpets, clothes, leather and textiles.

The azo dye that most Chemistry students will be familiar with is methyl orange, an acid-base.

Read more in the September 2014 issue of AUS-e-NEWS for the chemistry of Methyl Orange

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Indicators and End Points

What colour is an indicator at the end point ?
At what pH does an indicator change colour?
Why is the colour change for an indicator always given as a range of pH values?

These questions are all addressed in AUS-e-TUTE's new Indicator End Point resources.
Read the tutorial, play the game, do the test and view worked solutions for the questions.

Not an AUS-e-TUTE Member?
There is currently a free-to-view tutorial available at http://ausetute.com.au/endpoint.html

Want to find out more about how an AUS-e-TUTE Membership will help you improve your understanding of chemistry, and, improve your exam results?
Then go to http://ausetute.com.au/membership.html

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Go to http://ausetute.com.au/register.html

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Contact us at http://ausetute.com.au/contact.html

Sulfuric Acid - Sodium Hydroxide Titrations

Why does a titration of sulfuric acid using sodium hydroxide have only one equivalence point?
Why isn't the pH 7 at the equivalence point?

These are both excellent questions.

So, we've written a set of resources to help you understand!

AUS-e-TUTE Members should log-in and go to the new tutorial at:
http://www.ausetute.com.au/members/titrh2so4.html
and you can follow the links to the game and test from this page.

Not an AUS-e-TUTE member?
Part of this tutorial is currently available free to non-members for evaluation purposes at
http://ausetute.com.au/titrh2so4.html

Titration Techniques

What piece of glassware should you use to make a standard solution?
What should you rinse a burette with before filling it with solution?
Should you force all the solution out of a pipette when you use it?

These and many other questions about Titration Techniques have been addressed in AUS-e-TUTE's new tutorials, games, tests, and exams.

Members should log in to use all these new resources which are listed under Volumetric Analaysis, or, use the links from your Syllabus Study Guide in the Members Only Test Centre.

A "free-to-view" tutorial on titration techniques is currently available for evaluation purposes for non-members: http://ausetute.com.au/titrtech.html

Standard Solutions

What is the difference between a primary standard and a secondary standard in volumetric analysis?
What substances can be used as a primary standard?
How do you make a standard solution?

All these questions are answered in AUS-e-TUTE's new tutorial on standard solutions.
Members should log-in and go to the Volumetric Analysis section on the index page, or,
follow the links from your Chemistry syllabus study guide.
Members will also find a game, test and exam as well as tutorial on this topic.
Teachers will find  a worksheet wizard which will let you make, and print off, a worksheet on this topic.

Not an AUS-e-TUTE Member?
There is currently a "free-to-view" tutorial at http://www.ausetute.com.au/titrstand.html

Ethanol Content of Beer and Wine

AUS-e-TUTE has added new resources for determining the content of ethanol in alcoholic beverages such as beer and wine.
Members should go to :
http://www.ausetute.com.au 
and log-in to the Members Only Test Centre to find the new Ethanol Redox Titration tutorial, game and test.
Teachers will find a new worksheet wizard for preparing Ethanol Redox Titration worksheets in the Teachers Only section of the website.

Why Use a Volumetric Flask?

In order to prepare an aqueous standard solution, you transfer the solute to a volumetric flask and then you add water "up to the mark", that is, until the bottom of the meniscus lies on the "mark" when viewed at eye-level. You can quickly convince yourself of the need to view the "mark" at eye-level just be preparing your standard solution as above and then changing the angle you view it from.
But what about this need to add water to the solute in this oddly-shaped vessel called a volumetric flask?
Why couldn't you just weigh out the solute into a beaker then add the required volume of water from a pipette?
Or, if the solute is a liquid or a solution, why not just pipette the required volume into a beaker then pipette the required volume of solvent into the same beaker?

So, why not try it?

Experiment (a) For solid solutes such as sodium chloride (table salt) or sucrose ("sugar") just pour the solid into a small DRY measuring cylinder until you have 5 mL for example. Pour this solid into a 100 mL measuring cylinder. Add 50 mL of water from a pipette (you will need to keep swirling the flask while you add the water). Record the volume of the final solution. Repeat the experiment using different volumes of solute and solvent.
Repeat this experiment using sand as the "solute" and water as the solvent.

Experiment (b)  For liquid solutes such as ethanol (ethyl alcohol) or acetone (propanone) use a pipette to place 50 mL of the solute into a 100 mL measuing cylinder then add 50 mL of water from a pipette and record the volume of the final solution. Repeat the experiment using different volumes of solute and solvent.
Repeat this experiment using vegetable oil as the "solute" and water as the solvent.

If a solute dissolves in a solvent, the volume of the final solution is not equal to the volume of the solute plus the volume of the solvent. Sometimes adding a solute to a solvent results in a solution with a volume less than that of the solvent + solute, and sometimes adding a solute to a solvent results in a solution with a volume greater than that of the solvent + solute. Right now, there are no good, general theories to explain this behaviour! But it is because of this that we use that oddly shaped piece of glassware called a volumetric flask when we make up a standard solution.

Introduction to Buffers

AUS-e-TUTE has just uploaded two new sets of resources:

• Buffer Concepts (tutorial, game, test, exam)
• pH of Salt Solutions (tutorial, game, test, exam)

There has also been a significant update to the tutorial on Titration Curves (Graphs) with improved graphs and explanations.

Members should log-in and go to the Acid and Base section to find these new resources.

Concentration of Calcium Ions in Hard Water

AUS-e-TUTE has just added new complexometric titration resources for the determination of calcium ions in hard water.
Members should log-in to see the new tutorial, game, test and drill.

New Titration Resources

AUS-e-TUTE has just added new Complexometric Titration resources for the determination of calcium and magnesium ions in hard water.
Members can log-in to use the new tutorial, game, test and drill.

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