PRINCIPLES OF TITRIMETRIC (VOLUMETRIC) ANALYSIS

Titrimetric methods

as an analytical method in which the volume of a solution of known concentration consumed during an analysis is taken as a measure of the amount of active constituent in a sample analyzed

Analyte – or the active constituent in the sample

Titrant – solution of known concentration

Indicator – chemical which changes color at a point equivalent quantities of analyte and titrant have reacted

Stoichiometric point or Equivalence point – the theoretical point at which equivalent amounts of each have reacted

Endpoint – a sudden change apparent by use of indicators

Standardization – determination of the exact concentration of the solution

Standardized using 2 types of standards:

Primary standard – subs of known and high degree of purity; used in direct standardization

Secondary standard – std. soln used in indirect standardization

- std solution of known conc usually standardized by primary std.

Requirements for Primary Standard:

1. High purity = 99.9%

2. Definite and known composition

3. Not affected by drying

4. Soluble in water

Ex.

Stdzn. Of NaOH

4 primary stds:

1. Potassium acid phthalate (KHP)/ C6H4COOK . COOH

2. Sulfamic acid (HSO3)

3. Benzoic acid

4. Potassium acid iodate

Example: standardization of HCL

Standardize titrimetrically using sodium carbonate of known purity as a primary standard or

using standard NaOH as a secondary standard

Standardization using Primary Standard

Std. solutions Primary std Indicator

NaOH KHP/sulfamic acid php

HCl Na2CO3 php

AgNO3 NaCl K2CrO4

EDTA CaCO3 hydroxynaphthol

KMnO4 Na2C2O4

Iodine As2O3 starch

Sodium Methoxide Benzoic acid Thymol blue

Ceric sulfate As2O3 orthophenanthr

oline

Karl Fischer reagent Sodium tartrate

Na2S2O3 K2Cr2O7 starch

A solution of known concentration used to standardize another solution

Secondary

Standard

Indicator

HCl NaOH php

NH4SCN AgNO3 Ferric alum

Na2S2O3 Iodine Starch

ANALYTE TYPE TITRANT INDICA

TOR

Acetic acid Alkalimetric-direct NaOH php

Milk of Magnesia Acidimetric- residual NaOH mr

Aspirin Alkalimetric-residual HCl php

NaCl Volhard-precipitation. NH4SCN FAS

Povidone-Iodine NH4SCN FAS

Calcium lactate Complexation EDTA hydroxynap

hthol

H2O2 Redox-direct KMnO4

NaNO2 Redox- residual KMnO4

Ammonium

Potassium tartrateIodimetry Iodine starch

CuSO4 Iodometry Na2S2O3 starch

TYPE

Menadione (vit. K) REDOX

Selenium sulfide IODOMETRY

Phenol IODOMETRY- residual

Malic acid INDIRECT

PERMANGANATE

OXIDATION

Sulfonamide DIAZOTIZATION

NaHCO3 DIRECT ACIDIMETRY

ZnO ACIDIMETRY-

RESIDUAL/

EDTA-DIRECT

CaCO3 EDTA-DIRECT

Types of Titration:

DIRECT TITRATION – one titrant used, one volumetric solution

RESIDUAL TITRATION – two titrants, two volumetric solutions

1st VS – is added in excess

2nd VS – used to titrate the excess

Residual Titration - is used whenever the direct titration is not practicable

- for compounds which react too slowly with titrant

- poor solubility

- volatile substances are involved

Blank Determination – process of repeating the procedure but omitting the sample

Computation:

DIRECT TITRATION

% Purity = N x V X meq. wt./f x 1000 x100

wt of sample

RESIDUAL TITRATION

% Purity = N1V1 - N 2V2 X meq. wt. x100

wt of sample

Acids :

- # of replaceable Hydrogen

Ex: HCl = 1

H2 SO4 = 2

H3 PO 4 = 3

CH3 COOH = 1

Bases:

- # of replaceable OH

Ex: NaOH = 1

Mg(OH)2 = 2

Al(OH)3 = 3

Salts:

- total positive or negative charges/ cation or anion

Ex: NaCl = 1

MgO = 2

Ca3 (PO4)2 = 6

Results in drug assays are usually expressed in terms of %w/w, %w/v and %v/v

In the assay of conc. HCL:

samples are taken by weight

Difficult to measure small vol. of conc. acid

Results are express in % w/ w

In the assay dil. HCl:

Express in % w/ v

Standard Solution

is a solution of known normality or molarity

NORMALITY

eq/L or meq/ml

MOLARITY

moles/L or mmoles/mL

N = wt.

V x meq. wt.

= g

mL x g/mole

meq/mole

M = moles

L

= mmoles

mL

1. Find the molarity of HCl soln. which contains the volume of 2400 mL and also it contains 230 g of HCl. MW of HCl=36.46

2. Find the normality of 20 g NaOH diluted to a volume of 1L. MW of NaOH= 40

3. Compute for normality of HCl that make use of a primary standard (Na2CO3) that weighs 1.5 g. It consumed 30 mL of HCl after titration. MW of Na2CO3 = 106

4. Compute for normality of HCl using secondary standard (NaOH) having a concentration of 2 N. It consumed 35 mL of NaOH. The volume sample used was 25 mL.

5. A soln. with a final volume of 500 mL was prepared by dissolving 25mL of methanol (density= 0.7914) in chloroform. Calculate the molarity of methanol in the soln. MW = 32

6. Calculate the normality of a NaCl soln. prepared by dissolving 2.5 g of NaCl in water and then tapping it off with more water to a total volume of 500 mL. MW of NaCl= 58.44 g/mole

7. What mass of oxalic acid, H2C2O4, is required to make 250 mL of 0.05M soln.(MW=90)

8. Sulfamic acid is a primary standard that can be used to standardize NaOH. What is the molarity if 34.26 mL reacts with 0.3337 g of sulfamic acid. MW= 97

9. What is the normality of H2SO4 containing 73.5 g/ 500 mL of solution? MW of H2SO4= 98 g/mole

10. The molecular wt. of NaOH is 40. How many grams of NaOH pellets are needed to make 500 mL of 1.5 N soln?

11. Convert the following concentration as required:

0.5 M KOH to N

0.025 M H2SO4 to N

2 N NaCl to M

Express in decimal:

12. One twentieth N of HCl

13. One tenth M of H2SO4

Standardization

determination of normality or molarity of solution

accomplished by the use of another standard solution known as a SECONDARY STANDARD or by the use of known purity substance as PRIMARY STANDARD

The wt. of substance chemically equivalent to 1 mL of std. soln.

Express in mg/ mL

To get titer value: N x meq wt.

1 meq X 60 g x 1000 mg

mL 1000 meq 1 g

= 60 mg/mL

Ex.

1. What would the titer value be of the soln. in terms of CaCl2 of 0.05 N soln.? MW = 111

2. Compute for percent purity of 0.1 g sodium ascorbate if the titer value is 9.905 mg/mL. It consumed 10 mL thru direct titration with iodine.

Volumetric Apparatus

2 types:

to deliver: burets, pipets

to contain: volumetric flask, graduated cylinders

Burets

Graduated glass tubes of uniform bore throughout the whole length

Closed at the bottom by glass or stopcock

Volumes read at lower meniscus except highly colored liquid

BURETS

Calibration by means of:

1. Ostwald pipet

2. Kiehl buret

CHEMICAL REACTIONS USED IN TITRIMETRY

1. Neutralization (acid-base)

2. precipitation

3. complexation

4. redox

NEUTRALIZATION REACTION

a chemical process in which an acid (proton donor) reacts with a base (proton acceptor)

the products are : water and salt

Strong acid and Strong Base

HCL + NaOH → NaCl + H2O

INDICATORS

- are complex organic compounds used:

to determine end point

to determine ion concentration

indicators may be acids or bases

Mixed or combined indicators

are used to give sharp color change

Commonly used pH indicators

Color Change

Indicator pH Range Acid Base

Malachite green 0.0 – 2.0 yellow green

Methyl Yellow 2.9 – 4.0 red yellow

Bromophenol blue 3.0 - 4.6 yellow blue

Methyl Orange 3.2 – 4.4pink yellow

Bromocresol green 4.0 -5.4 yellow blue

Methyl red 4.2 – 6.2 red yellow

Bromocresol purple 5.2 – 6.8 yellow purple

Bromothymol blue 6.0 – 7.6 yellow blue

Phenol red 6.8 – 8.2 yellow red

Cresol red 7.2 – 8.8 yellow red

Thymol blue 8.0 – 9.2 yellow blue

Phenolphthalein 8.0 – 10.0 colorless red

Thymolphthalein 9.3 – 10.5 colorless blue

Indicators:

Aqueous : Phenolphthalein

Methyl orange

Methyl red

Strong Acid + Strong Base : php, MO, MR

Weak Acid + Strong base : php

Weak base + strong acid : MR

Strong Acids:

HI

HBr

HCl

H2SO4

HNO3

Weak Acid :

Acetic acid

Oxalic Acid

HF

HCN

Strong Bases:

NaOH

KOH

Weak Bases:

Alanine, C3H5O2NH2Ammonia, NH3Methylamine, CH3NH2Pyridine, C5H5N

Standard Acid Solutions used in Acidimetry and Alkalimetry

1. HCl – more preferable to sulfuric in the titration of compounds that yield precipitate

2, Sulfuric Acid – preferable in hot titrations since HCl will volatilize

Standard Alkali Solutions are:

1. NaOH

2. KOH

3. Ba(OH)2

All alkali solutions should be

restandardized frequently