Lab Manual Quantitative Analytical Method, Deepak Pant [epub ebook reader .TXT] 📗

calcium and magnesium cation concentration in a water sample. An excellent way to determine water hardness is to perform a complexometric titration using a standard ethylenediaminetetraacetic acid (EDTA) solution. EDTA is a chelating agent that can donate electrons (Lewis base) thereby forming a complex with metal ions (Lewis acid). The EDTA will complex first with the Ca2+ and then with the Mg2+ in a one-to-one molar ratio. But, in this experiment, it was assumed that only Ca ions contribute to the water hardness.
In this experiment, 20 drops of ammonia buffer and 10 drops of Eriochrome Black T indicator were added into 25 ml of tap water in Erlenmeyer flask. Then the tap water was titrated with EDTA solution two times to get the average volume of EDTA solution used. Ammonia buffer was used to adjust and keep the pH to 9.9-10.1. It is necessary to keep the pH at about 10 for two reasons. First, all reactions between metal ions and EDTA are pH dependent, and for divalent ions, solutions must be kept basic (and buffered) for the reaction to go to completion. Second, the Eriochrome black T indicator requires a pH of 8 to 10 for the desired color change.
Since both of EDTA solution and Ca ions are colorless, it is necessary to use Eriochrome black T indicator to detect the end point of titration. Eriochrome black T indicator (denoted as In) is a metal ion indicator whose color changes when it binds to a metal ion. When the indicator was added into tap water, the solution color changed from colorless into purplish red. It means the tap water which used in the experiment contained Ca ions. The reaction occurred:
Ca2+ + In2- → CaIn
(blue) (wine red)……………………equation 1
Then the solution in Erlenmeyer flask was titrated with the EDTA solution. When EDTA solution was added drops by drops into Erlenmeyer flask, the indicator must give up its metal ions, Ca2+ to the EDTA because EDTA can bind to the metal ions stronger than the indicator. When all metal ions bind with EDTA, the indicator is in its free form and it doesn’t bind to any metal. Then the solution color will turn to blue color which meant the end point was reached. The reaction occurred:
CaIn + EDTA4- → CaEDTA2- + In2-
(wine red) (blue)………………….equation 2
The calculation of the concentration of water hardness from tap water

The average volume of EDTA = 16.6 ml + 15.7 ml
2
= 16.15 ml
= 16.15 x 10-3 l
n EDTA = M EDTA x V EDTA
= 5 x 10-3 M x 16.15 x 10-3 l
= 80.75 x 10-7 mol
1 mol EDTA = 1 mol Ca
80.75 x 10-7 mol EDTA = 80.75 x 10-7 mol Ca
n Ca = 80.75 x 10-7 mol
gr Ca = n Ca x Ar Ca
= 80.75 x 10-7 mol x 40.078
= 3.24 x 10-4 gr
= 0.324 mg
[Ca] = 0.324 mg
25.10-3 l
= 12.96 mg/l
= 12.96 ppm
In this experiment, the sample water was not given so there are no much comparison could be made with the tap water. The distilled water did not need to be titrated with EDTA because when it was added with Eriochrome Black T Indicator the solution colour was dark blue. It means there were no metal ions in distilled water such as Ca2+ and Mg2+ which can bind to the indicator to give red color.
The [Ca] in tap water in this experiment were 12.96 ppm. Base on the classification of water hardness from Water Quality Association on www.ci.farmington.mn.us, it was said that between the 0 – 17.1 ppm, the water hardness can be categorized as soft so the tap water and distilled water used in this experiment could be categorized as soft.
III. Conclusion
- The distilled water sample did not contain Cl- and SO4- ions and may also contain ammonium chloride.
- The tap water sample did contain Cl- ions but did not contain SO4- ions. Ammonium chloride may be present.
- Distilled water did not contain any ions and can be considered as soft water.
- The concentrations of Ca ions in tap water were 12.96 ppm.
- Tap water used in this experiment is considered to be soft water.


5. Nonaqueous Titration
INTRODUCTION:
Non- aqueous titrations are those in which the titrations of too weakly acidic or basic substances are carried out using non –aqueous solvents so as to get sharp end point. Such titrations can also be used for the titration of the substances not soluble in water. The speed, precision and accuracy of the non –aqueous method are close to those of classical acidimetric and alkalimetric titrations. The apparatus involved are also same but moisture and carbon dioxide are to be avoided in non –aqueous methods because water, which is a weak base, can compete with the weak nitrogen base and the end point would not be sharp at all. It has been observed through experiments that the moisture content in non –aqueous titrations should not be more than 0.05%.Further, the temperatures during standardization in non –aqueous titrimetry should not be allowed.
TYPES OF SOLVENTS:
• Aprotic solvents
• Protogenic solvents
• Protophilic solvents
• Amphiprotic solvents
• Levelling solvents
1. Aprotic Solvents: Aprotic solvents include those substances, which may be considered chemically neutral, and virtually un-reactive under the conditions employed. Carbon tetrachloride and toluene come in this group; they possess low dielectric constants, do not cause ionization in solutes and do not undergo reactions with acids and bases. Aprotic solvents are frequently used to dilute reaction mixture.
2. Protophilic Solvents: Protophilic solvents are the substances that possess a high affinity for protons. The over all reaction can be represented as: -
HB+S <-> SH+ + B-
The equilibrium in this reversible reaction will be generally influenced by the nature of the acid and the solvent. Weak acids are normally used in the presence of strongly protophilic solvents as their acidic strengths are then enhanced and then become comparable to these of strong acids; this is known as the levelling effect.
3. Protogenic Solvents: Protogenic solvents are acidic in nature and readily donate protons. Anhydrous acids such as hydrogen fluoride and sulphuric acid fall in this category, because of their strength and ability to donate protons, they enhance the strength of weak bases.
4. Amphiprotic Solvents : Amphiprotic solvents consist of liquids, such as water, alcohols and weak organic acids, which are slightly ionized and combine both protogenic and protophillic properties in being able to donate protons and accept protons ethanoic acid displays acidic properties in dissociating to produce protons:
CH3COOH <-> CH3COO- + H+
But in the presence of perchloric acid, a far stronger acid, it will accept a proton:
CH3COOH + HClO4 ↔ CH3COOH2+ + ClO4-
The CH3COOH2+ ion can very readily give up its proton to react with a base, so basic properties of a base is enhanced, so titrations between weak base and perchloric acid can often be accurately carried out using ethanoic acid as solvent.
6. Levelling Solvents: In general, strongly protophilic solvents are important to force equilibrium equation to the right. This effect is so powerful that, in strongly protophillic solvents, all acids act as of similar strength. The converse occurs with strongly protogenic solvents, which cause all bases to act as they were of similar strength. Solvents, which act in this way, are known as Levelling Solvents.
INTERFERENCE DUE TO WATER IN NON –AQUEOUS TITRATIONS:
1. When a weakly basic drug is present, water (OH ̄) acts as stronger base as compared to the former one and preferentially accepts proton from an acid. Thus there is interference in the reaction of weak base with an acid.
2. Similarly when a weakly acidic drug is present, water (H +) behaves like a strong acid as compared to the former one and preferentially donates proton to the base. Thus there is interference in the reaction of weak acid with a base.
3. Hence in the presence of water, titration of either weakly acidic substances with stronger base or weakly basic substances with stronger acid is not possible.

BASIC CONCEPT OF NON –AQUEOUS TITRATIONS: The Bronsted Lowery theory of acid and bases can be applied equally well to reactions occurring during acid base titrations in non-aqueous solvents. This is because this approach considers an acid as any substance, which will tend to donate a proton, and a base as any substance, which will accept a proton. Substances which give poor end points due to being weak acids or bases in aqueous solution will frequently give far more satisfactory end point when titrations are carried out in non-aqueous media. An additional advantage is that many substances, which are insoluble in water, are sufficiently soluble in organic solvents to permit their titrations in these non-aqueous media. In the Bronsted Lowery theory, any acid, (HB) is considered to dissociate in solution to give a proton (H+) and a conjugate base (B-):- where as any base (B) will combine with a proton to produce a conjugate acid (HB+):
HB ↔ H+ + B-
B+H+↔ HB+
The ability of substances to act as acids or bases will very much depend on the choice of solvent system.
ADVANTAGES OF NON AQUEOUS SOLVENT OVER AQUEOUS SOLVENT:
1) Organic acids and bases that are insoluble in water are soluble in non-aqueous solvent.
2) Organic acid, which is of comparable strength to water, can be titrated easily in non-aqueous solvent. Bases also follow the same rules.
3) A non-aqueous solvent may help two are more acids in mixture. The individual acid can give separate end point in different solvent.
4) By the proper choice of the solvents or indicator, the biological ingredients of a substance whether acidic or basic can be selectively titrated.
5) Non aqueous titrations are simple and accurate, examples of non aqueous titration are: Ephedrine preparations, codeine phosphate in APC, tetracycline, teramycin, Antihistamines and various piprazine preparations.
SOME EXAMPLES OF NON-AQUEOUS SOLVENTS:
Glacial Ethanoic Acid: Glacial ethanoic acid is the most frequently used non-aqueous solvent. Before it is used it is advisable to check the water content. This may be between 0.1% and 1.0%.
Dimethylformamide: Dimethylformamide (DMF) is a protophillic solvent, which is frequently employed for titrations between, for instance, benzoic acid and amides, although end points may sometimes be difficult to obtain.
Acetonitrile: Acetonitrile (methyl cyanide, cyanomethane) is frequently used with other solvents such as chloroform and phenol and especially with ethanoic acid. It enables very sharp end points to be obtained in the titration of metal ethanoates when titrated with perchloric acid.
Dioxane: Dioxane is another popular solvent, which is often used in place of glacial ethanoic acid when mixtures of substances are to be quantified. Unlike ethanoic acid, dioxane is not a levelling solvent and separate end points are normally possible, corresponding to the individual components in the mixtures.
Alcohol: Salts of organic acids, especially of soaps are best determined in mixtures of glycols and alcohols or mixtures of glycols and hydrocarbons. The most common combinations are ethylene glycol (dihydroxyethane) with propan-2-ol or butan-1-ol. The combinations provide admirable solvents for both the polar and non-polar ends of the molecules.
APPLICATIONS OF NON –AQUEOUS TITRATIONS
Although indicators may be used to establish individual end points, as in traditional acid-base titrations, potentiometric methods of end point detection are also used extensively, especially for highly coloured solutions. Non aqueous titration have been used to quantify the mixtures of primary, secondary and tertiary amines, for studying sulphonamides, mixture of purines and for many other organic amino compounds and salts of organic acid. And also it is used for the titration of Halogen acid salts of weak bases.
EXPERIMENT: TITRATIONS OF WEAK BASES WITH PERCHLORIC ACID
Principle: weak bases are dissolved in acetic acid and are