PHA6116 LEC: Neutralization Titration

NEUTRALIZATION TITRATIONS

• Widely used to determine the concentration of analytes that are themselves acids or bases or are convertible to such species by suitable treatment 

Water

• is the usual solvent: readily available, non-toxic, inexpensive, low temperature coefficient of expansion 

not titratable

Some analyte are _ in aqueous media; solubilities are too low, strengths of acids/bases are not sufficiently great to provide satisfactory endpoints

ACID-BASE INDICATOR

Is a chemical, or mixture of chemicals, that changes color over a known pH range.

Phthalein Indicators

Colorless in moderately acidic and exhibit a variety of colors in alkaline media; sparingly soluble in water but readily dissolve in ethanol

Phthalein Indicators

Used for strong bases

Phthalein Indicators

Phenolphthalein, thymolphthalein

Sulfonphthalein  Indicators

Exhibit 2 useful color-change ranges (One in acidic solution and the other in neutral or moderately basic media)

Sulfonphthalein  Indicators

Phenol Red

Azo Indicators

Exhibit a color change from red to yellow with increasing basicity; the transition ranges are generally on the acidic side of neutrality

Azo Indicators

Used for strong acids

Azo Indicators

Methyl orange, methyl red

Cresol purple

1.2 - 2.8

Red Yellow

Methyl Orange

3.1 - 4.4

Red Yellow

Bromocresol Green

4.0 - 5.6

Yellow Blue

Methyl Red

4.4 - 6.2

Red Yellow

Bromocresol Blue

6.2 - 7.6

Yellow Blue

Phenol Red

6.4 - 8.0

Yellow Red

Phenolphthalein

8.0 - 9.6

Colorless Red

Thymolphthalein

9.4 - 10.6

Colorless Blue

TITRATION CURVES

Plots of a concentration-related variable as a function of reagent volume

TITRATION CURVES

• Judge feasibility of a titration reaction 

• Selection of indicator

HCl

Widely used for titration of bases

HCl

Dilute solutions are stable indefinitely

HCl

Do not cause troublesome precipitation reactions with most cations

HCl, HClO_4, H₂SO₄

Restandardization is never required

HClO_{4 &} H₂SO₄

Solutions are also stable

HClO_{4 &} H₂SO₄

Useful for titrations where chloride ions can interfere by forming precipitates

HNO₃

Seldom used because of their oxidizing properties

STANDARD SOLUTIONS OF ACIDS

HCl
HClO₄
H₂SO₄
HNO₃

PRIMARY STANDARDS FOR ACIDS

Anhydrous Na₂CO₃
TRIS/THAM
Na₂B₄O₇ 10 H₂O
HgO

Anhydrous Na₂CO₃

Two end points are observed during the titration (first at pH 8.3; second at pH 3.8)

Anhydrous Na₂CO₃

Sharper end point can be achieved by boiling the solution briefly to eliminate H₂CO₃ and CO₂

TRIS/THAM

tris-(hydroxymethyl)aminomethane

TRIS/THAM

Greater mass per mole of protons consumed than Na₂CO₃

Na₂B₄O₇ 10 H₂O

used for HCl standardization

HgO

Indirect standardization of HCl solution

STANDARD SOLUTIONS OF BASES

NaOH, KOH, Ba(OH)₂

NaOH, KOH, Ba(OH)₂

None of these is obtainable in primary- standard purity

NaOH, KOH, Ba(OH)₂

Standardization is required after preparation

NaOH, KOH, Ba(OH)₂

React rapidly with atmospheric CO₂ to produce corresponding CO₃^2-

PRIMARY STANDARDS FOR BASES

KHC₈H₄O₄
HC₇H₅O₂

KHC₈H₄O₄

• Ideal primary standard

• Non-hygroscopic crystalline solid with high molar mass (204.2).

HC₇H₅O₂

• Its solubility in water is limited (non-polar), ordinarily dissolved in ethanol prior to dilution with water and titration 

HC₇H₅O₂

Blank test is required since commercial alcohol is sometimes slightly acidic

KH(IO₃)₂

Excellent primary standard with a high molecular mass per mole of protons.

KH(IO₃)₂

A strong acid that can be titrated using virtually any indicator with a transition range between pH 4 and 10.

Acidimetric

Base Organic or Inorganic

Standard Acid

Alkalimetric

Acid  Organic or inorganic

Standard Base

ELEMENTAL ANALYSIS

Several important elements that occur in organic and biological systems

ELEMENTAL ANALYSIS

Non-metallic (e.g. carbon, chlorine, bromine, fluorine and few other less common species)

KJELDAHL METHOD

Most common method for determining organic nitrogen which is based on a neutralization titration (Indirect)

KJELDAHL METHOD

• The standard means for determining the protein content of grains, meats, and other biological materials

KJELDAHL METHOD

Suitable factor for protein content computation: 6.25 for meats; 6.38 for dairy products; 5.70 for cereals

KJELDAHL METHOD

Steps followed in the analysis: decomposition, distillation, and titration

DETERMINATION OF INORGANIC SUBSTANCES

Numerous inorganic species can be determined by titration with strong acids or bases

Ammonium salts

are conveniently determined by conversion to ammonia with strong base followed by distillation; the ammonia collected and titrated as in the Kjeldahl method

Ammonium salts

Method described for _ can be extended to the determination of inorganic nitrate or nitrite; are first reduced to ammonium ion by Devarda’s alloy or Arnd’s alloy

CARBONATES AND CARBONATE MIXTURES

Qualitative and quantitative determination of the constituents in a solution containing NaOH, Na₂CO₃, and NaHCO₃, either alone or admixed

CARBONATES AND CARBONATE MIXTURES

• No more than 2 of these constituents can exist in an appreciable amount in any solution because the reaction will eliminate the third 

CARBONATES AND CARBONATE MIXTURES

• Analysis of such mixtures requires 2 titrations: one with an alkaline-range indicator (e.g. Pp) and the other with an acid-range indicator (e.g. MO)

Qualitative Determination

composition of the solution can then be deduced from the relative volumes of acid needed to titrate equal volumes of the sample

Quantitative Determination

once the composition of the solution has been established, the volume data can be used to determine the concentration of each component in the sample

Carboxylic and Sulfonic Acid Groups

Two most common structures that impart acidity to organic compounds; neutralization titrations are often employed to determine the equivalent weight of purified organic acids; equivalent weights serve as an aid in qualitative identification of organic acids

Amine Groups

Many amines that are too weak to be titrated as bases in water are readily titrated in non-aqueous solvents, such as anhydrous acetic acid, which enhance their basicity

Ester Groups

Are commonly determined by saponification with a measured quantity of standard base; excess base is then titrated with standard acid

Hydroxyl Groups

Can be determined by esterification with various carboxylic acid anhydrides or chlorides (e.g. acetic anhydride and phthalic anhydride)

Carbonyl Groups

Can be determined with a solution of hydroxylamine hydrochloride; the reaction produces an oxime and the liberated hydrochloric acid is titrated

DETERMINATION OF SALTS

Total salt content of a solution can be accurately and readily determined by an acid-base titration

Salt

is converted to an equivalent amount of an acid or base by passage through a column packed with an ion-exchange resin

Standard acid or base solutions

can also be prepared with ion exchange resins