Aquametry in analysis, Slides of Pharmaceutical Analysis

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AQUAMETRY

Definition

• Aquametry is defined as the quantitative

determination of water present in any

material.

• The determination of water is one of the most

important and most widely practiced analysis in

pharmaceutical industry.

Importance of Aquametry / Moisture Content Determination

1. Many samples contain water as a solvent, as absorbed water or as water of crystallization. Hence, during quantitative analysis of such samples, one must take that into account and quantitative determination must be made on dry sample basis.
2. In expensive materials and large volume of less costly materials, water can be considered as an adulterant. To ensure true value and correct weight of the sample, moisture content should be determined.
3. Physical properties of a material may be modified by the presence of water (e.g. density). Hence, to determine true physical properties, moisture content determination is important.

Importance of Aquametry / Moisture Content Determination (cont.)

4. Many pharmaceutical procedures such as granulation, tablet formation, coating operation etc. are affected by water. For example, for proper compression, the granules must have water content of 2 - 6 %. So, water content of granules should be determined during and after drying stage.
5. Crude drugs contain a lot of moisture which over time decreases. So crude drugs must be sold on the basis of dry weight of the active ingredient. For this purpose, moisture content of the crude drugs must be determined.
6. Some sample needs a minimum amount of water for their stability, which should be determined by this process.
7. Water in substances can lead to microbial growth. So, quantification of water is necessary to ensure that water is not present in the sample above the acceptable limit.

Methods of

Determination

of Water

Methods to be Discussed

1. Physical methods a) Loss on drying (LOD)  With heat  Without heat b) Thermogravimetric analysis (TGA) c) Azeotropic distillation
2. Chemical methods a) Karl Fischer Titration

Thermal Determination of LOD: Procedure

1. A dried weighing bottle (with stopper) is taken and

weighed.

2. A known quantity of the sample (usually 1 - 2 g) is taken in

the weighing bottle (with stopper). It was weighed again.

We get the initial weight of the sample.

3. The bottle is placed in the drying chamber and stopper is

removed from the bottle but kept in the drying chamber.

4. The drying is continued at 105 ⁰C for 2 hours (or until the

weight becomes constant i.e. weight no longer decreases).

Disadvantages of Thermal Determination of LOD

1. Time consuming
2. Comparatively greater errors
3. Sample size is comparatively larger.
4. Water is slightly miscible with ether. So a small quantity of water will be present in the ether extract and is determined as “weight of other volatile constituents” giving a false reading. So, the result for sample with other volatile constituents is not satisfactory.

Determination of Moisture Without Heat:

Procedure

1. Weight of the dry desiccant/absorbent is determined.
2. The sample is placed in the drying chamber which is connected to the desiccant/absorbent.
3. Reduced heat is used and pressure is also reduced.
4. Water will evaporate. An inert gas is used to carry the evaporated water to the desiccant/absorbent.
5. The absorbent will gain weight. The difference in the weight is the moisture content of the sample.

Determination of Moisture Without Heat: Pros

and Cons

• Advantages:
  1. Heat application is not necessary or decreased.
  2. Interference by other volatile constituents is eliminated using a water specific absorbent.
• Disadvantage:
  1. Comparatively large sample is required.

Thermogravimetric Analysis (TGA): Apparatus

• The essential components are –
  1. A device for heating or cooling the sample according to given temperature program.
  2. A sample holder with controlled environment.

Thermogravimetric Analysis (TGA): Method

• The sample (generally 5 - 10 mg or less) is placed in the sample holder and heating rate is set. Then the machine is run, and temperature will increase. The temperature is allowed to increase to a point (say 230 ⁰C) and then the machine is stopped.
• A graph is obtained where the mass is placed on the y - axis and the temperature or time is placed on the x - axis. Time or Temperature Mass We can determine the moisture content from the loss of mass in the procedure.

Azeotropic Distillation

• There are many pairs of liquids identified which cannot be separated into pure components by distillation. Such pair is called an azeotrope.
• An azeotrope is a mixture of 2 liquids which boils at constant –
  1. temperature, regardless of the boiling points of each liquid
  2. ratio of composition
• For example: ▫ Water ( 100 ⁰C) and toluene ( 110. 6 ⁰C) forms an azeotrope comprising of 19. 6 % water and 80. 4 % toluene. This mixture boils at 84. 1 ⁰C. ▫ Water and carbon tetrachloride. This azeotrope contains 4. 1 % water and boils at 76. 8 ⁰C.

Azeotropic Distillation: Apparatus