Etiket Arşivleri: Acid – Base Titration

Principles of Titration and Errors ( Dr. A. Amsavel )

Principles of Titration and errors

By Dr. A. Amsavel

Introduction

Volumetric analysis
 Simple and easy
 Fast and can be done on site
 Less expensive
 Estimation of content or Assay
 Precise and accurate
 Depends on method and specificity

Requirements of a Titration Reaction

Reaction must complete by 99.9 % so that < 0.1 % (or 1 ppt) remains unreacted
Rxn must be rapid
Titration needs to be performed in a reasonable time period
The stoichiometry must be well defined, and known
It can be predicted from equilibrium constants
A method must be available to determine the equivalence point

Types of Titration

1) Precipitation
– A(aq) + B(aq) = AB(s)
2) Acid-Base rxn
– H+ + OH ̄ = H2O (strong acids or bases)
– HA + OH ̄ = H2O + A ̄ (weak acids)
– A ̄ + H+ = H2O + HA (weak bases)
3) Complexation rxn
– Zn2+ + 4NH3 = Zn(NH3)42+
4) Redox rxn (oxidation-reduction)
– Fe2+ + Ce4+ = Fe3+ + Ce3+

Standards

• Measurements are made with reference to standards
– The accuracy of a result is only as good as the quality and accuracy of the standards used
– A standard is a reference material whose purity and composition are well known and well defined
• Primary Standards
– Used as titrants or used to standardize titrants
– Requirements
• Usually solid to make it easier to weigh
• Easy to obtain, purify and store, and easy to dry
• Inert in the atmosphere
• High formula weight so that it can be weighed with high precision

Endpoint Detection

It is critical, to know the completion of reaction / determination
1) Visual indicators:
• Observe a colour change or precipitation at the endpoint.
– Rxn progress checked by addition of external or self indicator
2) Photometry:
• Use an instrument to follow the colour change or precipitation
3) Electrochemistry:
• Potentiometry – measure voltage change ( pH electrode)
• Amperometry – measure change in current between electrodes in solution
• Conductance – measure conductivity changes of solution
Later two used for coloured, turbid, end point accurate

Acid-base titration

 Neutralization titration
 Neutralization Indicators
 Indicators & mixed indicators
 Neutralization curve
 Non-aqueous titration


Acid – Base Titration Lab. Reports

PURPOSE:

The purpose of the titration is to determine the amount of acid it contains by measuring the number of mL of the standard NaOH need to neutralize it. The technique of titration will use to determine the concentration of solutions of acids and bases.

THEORY:

Titration is a laboratory technique designed to use the reaction of two solutions to determine the concentration of one or the other.  The titration with the HCl will be used to determine our NaOH solution concentration.

The technique of titration can be applied to different materials and different types of reactions. In this experiment, an acid-base neutralization between two solutions is used. The standard solution is 0.1 M NaOH, and the unknown solution is the mixture resulting from dissolving the antacid in excess “stomach acid”. This solution is still acidic; The reaction is:

NaOH + HCl à H2O + NaCl

An indicator is used to show when the solution has been neutralized. In this case, the indicator is methyl red, an organic compound which is yellow if it is dissolved in a basic solution, and red in an acidic one. As the excess stomach acid is neutralized by the NaOH, the solution changes from red to yellow; the exact neutralization point is orange.

At the neutralization point (or endpoint), the number of moles of OH-1 added exactly equals the number of moles of H+1 that remained after the antacid tablet “worked”. This number of moles of OH-1 can be found by multiplying the number of liters delivered by the burette times the molarity of the NaOH:

liters NaOH sol’n x moles NaOH = moles NaOH 1 liter sol’n.

The number of moles of HCl remaining after treatment with the antacid tablet is equal to the number of moles of NaOH calculated above. The total number of moles of HCl added to the flask can be found by multiplying the total volume of HCl used (in liters) by the Molarity of the HCl solution. The number of moles of HCl neutralized by the stomach acid is just the difference between these two values.

moles HCl neutralized by tablet = total moles HCl in flask – moles HCl neutralized by NaOH

The effectiveness of the antacid may be expressed in terms of mL of stomach acid neutralized, or moles of HCl neutralized, and may be calculated per tablet or per gram of antacid. See calculation page.

Burette Use and Titration Technique:

Typically, a special piece of glassware is used to measure out one or both of the solutions used in a titration. It is called a burette, and quickly and accurately measures the volume of the solutions delivered. A diagram of a burette, and instructions on how to use it are given below.

Before use, the burette must be rinsed with the solution it is to contain. Close the stopcock and use a small beaker to pour about 10 mL of solution into the burette. Tip the burette sideways and rotate it until all of the inside surfaces are coated with solution. Then open the stopcock and allow the remaining solution to run out. Again close the stopcock, and pour enough solution into the burette to fill it above the “0” mark. With the burette clamped in a vertical position, open the stopcock and allow the liquid level to drop to “0” or below. Check the burette tip. It should not contain air bubbles! If it does, see your instructor. Adjust the burette so that the liquid surface is at eye level, and take the initial burette reading as shown below:

M Na 2 CO 3 , 0.1 M HCl , 0.1 M NaOH and methyl orange indicator. Two flasks, burette, burette clamp, pipet, Bunsen burner

PROCEDURE:

1.  Determination of Concentration of HCl Solution:

Firstly, 10 ml of 0.1 M of Na 2 CO 3 was place in each of two flasks using a pipet. Then two drops of methyl orange indicator was added to each of the flasks and the solution was mixed with a gentle shaking. The burette was filled until full with HCl. One of the Erlenmeyer flasks was placed under the tip of the burette. The HCl was run from the burette into the Na 2 CO 3 solution until the end point is reached. Then the mixture was boiled for a few minutes. After the solution was cooled and added a few drops of HCl until the end point is observed again. When the end point is obtained, the volume of HCl added was recorded.

The solution of Na 2 CO 3 in the second flask was titrated following exactly the same procedure.

The molarity of HCl was calculated.

2. Quantitative Determination of NaOH:

Firstly, 10 ml of 0.1 M NaOH was placed in each of two flasks using a pipet. Then two drops of methyl orange indicator was added to each of the flasks and it was titrated as we did with Na 2 CO 3 . But the solution did not boil. The solution of NaOH in the second flask was titrated following exactly the same procedure.

The molarity of NaOH solution was calculated.

3. Unknown

Given an unknown solution of NaOH was titrated similarly above.

Laboratory‎ > ‎Acid – Base Titration

An acids-base titration is used to determine the unknown concentration of an acid or base by monitoring its neutralization reaction with a standard of known concentration in the presence of an indicator. For example, a base of unknown concentration may be titrated with an acid standard of known concentration or the reverse. The endpoint is the point of the acid-base titration in which just enough of the standard solution has been added to completely neutralize the solution of unknown concentration. Indicators are used to signal this point of the titration, and are generally selected according to the type of titration (strength of acid and based used in the titration). When the endpoint of the titration has been reached, one drop of standard solution past the endpoint will result in a color change of the indicator.

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