70. in relation to sodium chloride and water mixture read the following statements :

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p) mixture is eutectic in nature

q) it has eutectic point - 1210 c

r) the composition of eutectic is 25.3 % by mass

s) the mixture is a true eutectoid and may exist as peritectic also

which of the set of statements is correct ?

  1. p and q
  2. q, r and s
  3. p, q and s
  4. p, r and s

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Saraswathi.B's picture

Answer: 

p and q

Eutectic mixture

These systems are prepared by the fusion method. Eutectic melts differ from the solid solutions in that the fused melt of the solute solvent shows complete miscibility but negligible solid solid solubility that is such systems are basically intimately blended physical mixture of two crystalline components, a phase diagram of the two component systems explains about the eutectic mixtures. When this mixtures is exposed to water the soluble carrier dissolves leaving the drug in a micro crystals line state which solubilises rapidly.

Examples of this mixtures includes paracetamol urea, griseoflavin urea, griseoflavin succinic acid NACL and water ( 1210C) etc. solid solutions and the eutectics which are basically melts are easy to prepare and economical with no solvents involved. The method however cannot be applied to

  1. Drugs which fail to crystallize from the mixed melt.
  2. Drugs which are thermolible.
  3. Carriers decompose at their melting point. This mixtures are product is often tacky, intractable or irregular crystal.

Solid dispersion

These are generally prepared by the solvent co precipitation method where by the guest solute and the solids carrier solvent are dissolved in a common volatile liquid solvent such as alcohol. The liquid solvent is removed by evaporation under reduced pressure or by freeze drying which results in the amorphous precipitation of guest in a crystalline carrier. Thus the basic difference between solid dispersions and solid solutions / eutectic is that the drug is precipitated out in an amorphous form in the former as opposed to crystalline form in the later. Example includes amorphous sulphathiazole in crystalline urea. Such dispersion are often called as co evaporates or co precipitates. This method is suitable for the thermo labile substances but has a number of disadvantages.

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