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Acid-Base Titrations

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Kim Le

on 26 May 2014

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Transcript of Acid-Base Titrations

Dr. Persad
What is a titration?
Medical Uses:
Food Industry Uses:
-monitoring blood glucose levels
-pregnancy tests
-manufacture of antibiotics
Titrations and buffers are used in everyday life, making them an essential part of society.
Much in the same fashion, an acid-base titration is the process of finding a specific amount of a known basic or acidic solution through usage of acid-base reactions; we must figure out the amount of base needed in order to neutralize the acid.
Types of Titrations
1) A 40 ml solution of 0.7 M NaOH is titrated until neutralized into a 20 ml sample of HCl. What was the concentration of the HCl?
1) MacidVacid = MbaseVbase

Macid(20 ml)= (0.7 M)(40 ml)
Macid = 28 MmL/20 ml
Macid = 1.4 M
Acid-Base Titrations

Chapters 8.4-8.5
by Kim and Norita
Real Life Applications
Strong Acid with a Strong Base
Type of titration
Strong Acid with a Weak Base
Weak Acid with a Strong Base
Problem Set
Acid-Base Indicators
-testing amounts of different compounds in foods
-production of cheese, yogurts and sour cream
-testing a product's readiness for consumption (wine & cheese)
Biodiesel Production:
-determining the acidity of waste vegetable oil
-determining the desired pH level of the batch
May 22, 2014
A titration is the process of determining the concentration of an unknown solution by using the known concentration of another solution. This is done by adding small amounts of the known solution (
the titrant
) from a buret into the unknown solution (
the sample
) until the reaction is complete (
the endpoint
Determining the amount of titrant added allows us to determine the concentration and molar mass of the sample.
is the first step of a titration, because it involves determination of the titrant's concentration. From the titrant's concentration, we can use this information to help us figure out the unknown sample.
We use a
primary standard
to help us determine the concentration of the titrant; a primary standard is a pure and stable form of a chemical, and because of it's purity, we can be confident that the concentration will be accurate. Examples of common primary standards include Na2CO3 and KHC7H4O4.
that HCl and NaOH are not used as primary standards. This is because hydrogen chloride gas vaporizes and solid NaOH is
, that is, it gains mass by absorbing water from air.
The addition of the titrant is added to the sample until it reaches the
equivalence point
, where the reactant in the sample is consumed by the reactant in the titrant, or where the titrant neutralizes the sample.
Titration Curves
Characteristics of primary standards
- Solids at SATP (standard temperature)
- Not hygroscopic
- Do not vaporize
- Available in very pure form
- Produces colourless solutions
A titration curve (or pH curve) is a graph used to sketch the data obtained from the titration lab. The titrant volume is plotted on the x-axis and the pH level is plotted on the y-axis. The equivalence point is found on the midpoint of the graph and represents where the titrant has neutralized the sample.
One of the more common titrations occur between a strong acid and a strong base. The strong acid is usually HCl and the strong base, usually NaOH.
Consider the reaction between a weak acid and a strong base: the weak acid in this case will be HC2H3O2 and the strong base will be NaOH.
The reaction between a strong acid and a weak base can be analyzed with NH3 being the weak base and hydrochloric acid being the strong acid.
Titration curves hold a lot of information, including:
- equivalence points
- initial pH of solution
- number of quantitative reactions
- pH endpoints
- transition points
pH at equivalence point
Entity determining pH at equivalence point
strong acid and weak base
strong base and weak acid
strong base and weak acid
< 7
> 7
conjugate acid of weak base
autoionization of water
conjugate base of weak acid
Polyprotic Acid Titrations
Because polyprotic acids and polyfunctional bases are able to give away more than one H+ ion, they display two equivalence points.
2) In a titration, 20.00 mL of 0.300 mol/L HCl is titrated with standardized 0.300 mol/L NaOH. What is the amount of unreacted HCl after 10.0 mL has been added?
2) VNaOH = 10.00 mL CNaOH = 0.300 mol/L NaOH nNaOH = VNaOH x CNaOH = 10.00 mL x 0.300 mol/L nNaOH = 3.00 mmol
Indicator is added at the beginning of the titration so that we are easily able to see when the reaction has reached it’s endpoint; it changes colour rapidly to indicate that the reaction has complete.
Note that it is a conjugate weak acid-base pair
Le Châtelier's Principle
States that “If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established” (HighBeam Research, 2005).
pH curves with a weak acid or weak base have at least one buffering region, where the pH changes very little, even with appreciable amounts of acid or base being added. The solution near these points are known as buffer solution, or buffer.

Buffers are usually synthesized or exist naturally (i.e. blood) with a purpose, often times to counteract small amounts of acid or base that are accidentally added to a mixture.


Buffers must be able to maintain a relatively constant pH, because of it's importance in many biological processes where certain chemical reactions occur at a specific pH level.
Buffering actions can be explained using Bronsted-Lowry equations.
HC2H3O + OH > C2H3O2 + H2O
The small change and consumption of some of the OH- ions explains why the pH change is so small.
Capacity of a Buffer
There is a limit to the amount of acid or base that a buffer can neutralize before it's pH begins to increase rapidly.
A buffer's capacity can be determined by the concentration of its conjugate acid base pair.
Buffer Uses
The ability of buffers to maintain a constant pH is important for many processes.
Example: each enzyme carries out it's function over a small pH range.
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