VOLUMETRIC ANALYSIS

CONTENT
⦁ Introduction to Volumetric Analysis
⦁ Apparatus Used in Volumetric Analysis
⦁ Basic Definitions in Volumetric Analysis
⦁ Different Indicators and their Colours in Different Media
⦁ Universal Indicator
⦁ Effect of the Wrong Use of Indicator
⦁ The Importance of Volumetric Analysis
⦁ General Procedure and Precautions During Acid-Base Titration
⦁ Steps in Recording Observations Made During Acid-Base Titration Experiment
⦁ Determination of Relative Molar Mass
⦁ Determination of Mole Ratio

Introduction to Volumetric Analysis
Volumetric analysis is an aspect of quantitative analysis which involves the measurement of the volume of reacting solutions in order to find the masses of substances dissolved in them. In other words, two solutions are given; one is a solution of an acid and the other a solution of a base. One of them is a standard solution and you are required, by titration to standardize the other.
The other aspect of quantitative analysis is gravimetric analysis, which involves weighing and determining the masses of reactants and products of a chemical reaction.
The most popular aspect of volumetric analysis is the acid-base (neutralization) reactions. The technique of determination is by titration. Other aspects are redox and precipitation reaction.
Apparatus Used in Volumetric Analysis
The apparatus used in volumetric analysis are:
The burette, pipette, beaker, flasks, funnel, wash bottle, chemical balance, dropping pipette and retort stand.
Basic Definitions in Volumetric Analysis
(i) Titration: It is the technique for carrying out volumetric analysis.
(ii) Concentration: The concentration of a solution is the amount of solute in a given volume of the solution.
(iii) Standard Solution: It is a solution containing a known amount of solute in a known volume of solution. An example of a standard solution is a molar solution.
(iv) Molar Solution: It is a solution which contains one mole of solute in 1dm3solution.
(v) Buffer Solution: It is a solution which resists a change in pH or H+ upon further addition of acid or alkali. Buffers   are usually made of solutions of a weak acid and its conjugate base or a weak base and its conjugate acid. The weak acid usually has a pH value lower than 7 and is therefore used to buffer systems in the basic region. Examples of buffers solution and their pH ranges over which they are effective are given below:

BUFFER  SYSTEM PH RANGE
Ethanoic acid/Sodium ethanoate(CH3COOH/CH3COONA) 4.25 — 5.25
Trioxocarbonate(iv)(carbonic) acid/Sodium hydrogentrioxocarbonate 5.87 — 6.87
Ammonia/Ammonium chloride(NH3/NH4Cl) 8.75 — 9.75
Sodiumtrioxocarbonate/sodiumhydrogentrioxocabonate 9.75 — 10.75

(vi) Molar Mass: This is the mass of one mole of a substance expressed in grams. Its unit is gram per mole.
(vii) End Point: This is the point at which the chemical reaction is complete during titration. The end point is detected with the help of an indicator.
(viii) Indicators: These are weak organic acids or bases whose colors change with pH of the solution. Indicators are widely used in monitoring titration involving colorless solutions of acids and bases.
Different Indicators and their Colours in Different Media

INDICATOR COLOUR IN
ACIDIC MEDIUM COLOUR IN
ALKALINE MEDIUM COLOUR AT
END POINT
Methyl Orange pinkish red yellow orange
Phenolphthalein colorless red colorless
Litmus red blue Pink/purple
Methyl red pink yellow orange
Screened methyl orange purple(violet) green grey
Bromothymol blue yellow blue green

Universal Indicator
This is a mixture of indicators used either as solution on a test paper to test the pH value of a solution. The pH scale is shown below:

Universal Indicator (pH Scale)
Effect of the Wrong Use of Indicator
The success of a titration exercises depend on the use of the correct indicator. Wrong use of indicator will definitely give wrong result. For instance, let’s consider a case of the titration of a solution of a wrong acid say HCl with that of a weak base say Na2CO3, methyl orange is the suitable indicator, but if phenolphthalein indicator is used instead, the end point will appear when only half of the weak base has been used up. This can then be represented with the following equation.

phenolphthalein indicator
____________________________
HCl + Na2CO2 NaHCO2 + NaCl

This happened because the phenolphthalein is sensitive to a weak acid such as Na2CO3.

S/N TITRATION EXAMPLE PH RANGE SUITABLE INDICATOR
1 Strong acid vs.
strong base (3 – 11) H2SO4(aq) and KOH(aq) 3.5 – 9.5 Any indicator is
suitable.
2 Weak acid vs.
strong base (7 – 11) H2C2O4 and NaOH 7.0 – 9.5 phenolphthalein
3 Strong acid vs.
weak base (3 – 7) HCl(aq) and NH3(aq) or
K2CO3(aq) or Na2CO3(aq)
or Ca(OH)2(aq) 3.5 – 7.0 Methyl orange or
screened methyl
orange
4 Weak acid vs.
weak base CH3COOH(aq) and
NH3(aq) No sharp
change No suitable indicator.
Or phenol red.

The Importance of Volumetric Analysis
⦁ Standardize unknown solution
⦁ Calculate molar mass, water of crystallization and solubility.
⦁ Determine the purity of substances.
⦁ Determine the masses of substances dissolved.
⦁ Faster and more convenient
General Procedure and Precautions During Acid-Base Titration

A. Steps Involved in Filling the Burette with Dilute Acids
Step 1: Clamp the burette  in a vertical position to avoid error due to parallax while taking reading. Rinse it with the given acid solution, allowing part of the acid to pass through the tap and the jet. Then close the tap.
Step 2: With the funnel at the top of the burette, fill the burette with acid to a desired level ensure that the jet is filled with acid to avoid air bubbles inside it.tightly close the burette tap to avoid leakages. Remove the funnel to avoid droplets of acid from it. Hold a piece of white paper behind the burette so that the acid level is clearly shown and note the reading of the burette.
B. Steps Involved in Measuring 25.00Cm3 (20.00Cm3) of Standard Base Solution
Step 1: Collect four conical flasks and wash with distiled water only. Make sure that none of the flasks contain any acid or alkali.
Step 2: Rinse the pipette with the given  base solution.
Step 3: Suck the base solution into the pipette until the level is about 1cm3 or2cm3above the forefinger. Gently release the pressure until the bottom of the concave meniscus is at the graduation mark,with the eye level aligned.
Step 4: While filing the pipette ensure that the jet lies well inside the base solution to avoid air bubbles in the pipette.

Step 5: Run the base solution into one of the conical flasks. The sides of the flask should not be splashed with the solution as the splashings may not be neutralised by acid later in the experiment. To remove the little alkali retained at the tip of the pipette after delivery, do not blow out the pipette or shake out the last drop of the solution but allow the pipette to drain for about 15 seconds and then allow the tip of the pipette to touch the bottom of the flask.
Step 6: Fill the remaining conical flasks as stated above.
C. Steps Involved in the Titration of the Alkali Solution with Diluted Acid Solution
Step 1: Add 2 drops of the given indicator to each conical flask to obtain a shape end-point.

Step 2: Place one of the conical flasks on a piece of white paper or white tile beneath the burette to aid easy recognition of colour change at the end point.Run in the acid from the burette adding about 1.00cm3 at a time.
Shake the conical flask gently during titration to ensure homogeneity of the two solutions but avoid spillage of the reacting solutions. Note the level of the acid when the colour of the indicator changes.this first titration is the trial or rough titration.

Step 3: Repeat the above procedures for the base in other three conical flasks. Take burette readings at the lowest concave meniscus for each titration.
These should be taken at the eyes levels to avoid error of parallax. The volumes of the acids used in the second,third and fourth accurate titrations should agree within ±0.20cm3 and should always be recorded to two places of decimals. These make a concordant or concurrent results to be obtained. Record all your readings as they are taken and do not try to remember them or write on scripts of papers.

Titration Precautions
⦁ The burette must be clamped vertically or not tilted.
⦁ Wash the burette and pipette with water and rinse with distilled water.
⦁ Rinse the burette with acid,the pipette with base (alkali)before putting these solutions into them.
⦁ Ensure no air bubbles in the burette or pipette.
⦁ Remove the funnel after putting the acid into the burette (if a funnel is used).
⦁ The content of the pipette should be allowed to run into the conical flask without blowing air into it.
⦁ Use a drop or two (small amount)of indicators.
⦁ Read the lower meniscus.
⦁ Ensure the tap of the burette is not leaking.
NOTE: Most examination bodies have different ways of assessing students practical skills and report writing procedures. For instance, the following are some of the penalties.
(i) Use of pencil: -2 marks
(ii) Alteration (cancellation) on table of titre values: -2 marks for each, maximum of  -4 marks.
(iii) No unit or wrong unit on table:  -1 mark.
(iv) Non concordant values (i.e. difference greater than plus or minus 0.2) for average titre value;  -1 mark.
Steps in Recording Observations Made During Acid-Base Titration Experiment
1. Construct the specimen table
Example:

Burette Readings in cm3 Trial or Rough 1st 2nd
Final Burette reading 24.80 24.00 24.00
Initial Burette reading 0.00 0.00 0.00
Volume of Acid used 24.80 24.00 24.00

2. Determine the average volume of acid used (i.e. average titre value).
Thus, Average Titre: 24.00+24.002=24.00cm3
Depending on the differences in readings of the titre values. All your readings must have their units, i.e.cm3.
NOTE: This value and that of the table should be recorded to two places of decimal.
3. Significant Figures
Student should learn to round  off their calculation numbers to three significant figures,since they are using four figure table for their calculations.
Significant figures start from non-zero digit and all other digits that follow including zero are considered Significant. As usual the digits, 5,6,7,8,9 are rounded up while 1,2,3,4, are rounded off.
Numbers Rounded off
299.30 299 to 3 s.f
0.0128 0.0128 to 3 s.f
6.0246 6.02 to 3 s.f
NOTE : for the sake of the examination, prepare your table on a sheet of paper and then neatly transfer it in INK to the answer sheet.