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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
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
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
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
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
Standard Solution
is a solution of known normality or molarity
NORMALITY
eq/L or meq/ml
MOLARITY
moles/L or 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
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
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
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
INDICATORS
- are complex organic compounds used:
to determine end point
to determine ion concentration
indicators may be acids or bases
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
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