Solubility: Solubility of slightly soluble and weak electrolyte and MCQs for NEET, GPAT, NIPER, Pharmacist and Drug Inspector exam

Solubility: Solubility of slightly soluble and weak electrolyte and MCQs for NEET, GPAT, NIPER, Pharmacist and Drug Inspector exam

Systems of solids in liquids include the most frequently encountered and probably the most important type of pharmaceutical solutions. Many important drugs belong to the class of weak acids and bases. They react with strong acids and bases and, within definite ranges of pH, exist as ions that are ordinarily soluble in water.

Most of these weak electrolytes are not very soluble in water but are soluble in dilute solutions of acids; such compounds as atropine sulfate and tetracaine hydrochloride are formed by reacting the basic compounds with acids. Addition of an alkali to a solution of the salt of these compounds precipitates the free base from solution if the solubility of the base in water is low.

The aliphatic nitrogen of the sulfonamides is sufficiently negative so that these drugs act as slightly soluble weak acids rather than as bases. They form water-soluble salts in alkaline solution by the following mechanism. The oxygens of the sulfonyl (—SO2—) group withdraw electrons, and the resulting electron deficiency of the sulfur atom results in the electrons of the N:H bond being held more closely to the nitrogen atom. The hydrogen therefore is bound less firmly, and, in alkaline solution, the soluble sulfonamide anion is readily formed. The sodium salts of the sulfonamides are precipitated from solution by the addition of a strong acid or by a salt of a strong acid and a weak base such as ephedrine hydrochloride.

The barbiturates, like the sulfonamides, are weak acids because the electronegative oxygen of each acidic carbonyl group tends to withdraw electrons and to create a positive carbon atom. The carbon in turn attracts electrons from the nitrogen group and causes the hydrogen to be held less firmly. Thus, in sodium hydroxide solution, the hydrogen is readily lost, and the molecule exists as a soluble anion of the weak acid. In highly alkaline solutions, the second hydrogen ionizes. The pK1 for phenobarbital is 7.41 and the pK2 is 11.77. Although the barbiturates are soluble in alkalies, they are precipitated as the free acids when a stronger acid is added and the pH of the solution is lowered.

Multiple choice questions

1.Many important drugs belong to the class of

a)weak acids

b)weak bases

c)both of these

d)none of these

2.Most of the weak electrolytes are not very soluble in water but are soluble in dilute solutions of acids.

a)true

b)false

3.____ and _____ are formed by reacting the basic compounds with acids.

a)atropine sulfate and tetracaine hydrochloride

b)atropine sucrallfate and tetracaine hydrochloride

c)atropine sulfate and tetracycline hydrochloride

d)all of the above

4.Addition of an alkali to a solution of the salt of the compounds ______the free base from solution if the solubility of the base in water is low.

a)sediments

b)precipitates

c)eliminates

d)none of the above

5.The aliphatic nitrogen of the sulfonamides is sufficiently negative so that these drugs act as slightly soluble weak acids rather than as bases.

a)true

b)false

6.The sodium salts of the sulfonamides are precipitated from solution by the addition of a strong acid or by a salt of a strong acid and a weak base such as

a)ephedrine hydrochloride

b)sulfonamide anion

c)sulfonyl (—SO2—) group

d)all of the above

7.The barbiturates, like the sulfonamides, are weak acids because the electronegative oxygen of each acidic carbonyl group tends to donate electrons and to create a positive carbon atom.

a)true

b)false

8.The pK1 for phenobarbital is

a)7.41

b)7.14

c)7.12

d)7.50

9.The pK2 for phenobarbital is

a)10.77

b)11.77

c)12.77

d)11.10

10.The barbiturates are soluble in

a)alkalies

b)acids

c)water

d)all of the above

11.The barbiturates are precipitated as the free acids when a stronger acid is added and the pH of the solution is

a)increased

b)kept constant

c)lowered

d)none of these

12.In sodium hydroxide solution, the hydrogen is readily lost, and the molecule exists as a soluble anion of the

a)weak base

b)weak acid

c)strong base

d)strong acid

13. Phenol is weakly basic and only slightly soluble in water but is quite soluble in dilute sodium hydroxide
solution.
a)true

b)false

14. Hydroxy acids, such as tartaric and citric acids, are quite soluble in water because they are solvated
through their

a) hydroxyl groups

b)carboxyl groups

c)amino group

d)all of the above

15. The potassium and ammonium bitartrates are not very soluble in water, although most alkali metal salts of tartaric acid are soluble.
a)true

b)false

Solution:

1.c)both of these

2.a)true

3.a)atropine sulfate and tetracaine hydrochloride

4.b)precipitates

5.a)true

6.a)ephedrine hydrochloride

7.b)false

8.a)7.41

9.b)11.77

10.a)alkalies

11.c)lowered

12.b)weak acid

13.b)false

14.a) hydroxyl groups

15.a)true

References:

  1. Martins Physical Pharmacy, 6th edition 2011, page no. 339-342.

 

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