In this article we will discuss about the preparation of preservatives which is capable of preventing microbial infection.
Formaldehyde Solution:
Formaldehyde is a volatile substance. The maximum concentration can be manufactured is 40%. In preservation, commercial grade formaldehyde is used. Strength of the solution is calculated on the basis of 40 ml reagent in 100 ml formaldehyde available in the market.
To prepare a 1% solution add 2.5 ml formaldehyde to about 50 ml distilled water. Make up the volume to 100 ml by adding distilled water. Museum specimens are usually preserved in 4 to 6% solution. For permanent preservation the solution should be changed after 4 to 5 days of first preservation. No further change is required.
Spirit Solution:
70% rectified spirit or 90% ethyl alcohol is a very good preservative. For prolonged preservation the solution should be changed after 4 to 5 days of the initial preservation. No further change is required. In prolonged preservation, however, the tissues become brittle.
Precaution:
Delicate organisms must not be put directly in 70% spirit after killing. Initially they should be kept in 50% spirit for a few days and then transferred to 70%.
Grades of Alcohol (Rectified Spirit):
Absolute ethyl alcohol being a very costly material, 90% ethyl alcohol (Rectified spirit) is used in the preparation of alcohol of different concentrations. The simplest method to prepare a solution of desired concentration, the volume of the spirit to be added to distilled water to make 100 ml solution is calculated by replacing the second digit of the concentration by first digit.
Examples:
a. To prepare a 30% solution 33.0 ml spirit is to be added to 67.0 ml distilled water.
b. For a 50% solution 55.0 ml spirit is to be added to 45.0 ml distilled water.
c. For a 70% solution 77-0 ml spirit is to be added to 23 0 ml to distilled water.
Glyceroalcohol:
Glyceroalcohol is an excellent preservative for soft bodied organisms. Not only the soft texture of the muscles are retained but the colour of the organisms too remains unchanged for quite a long time. It is of common use for preservation of specimens to be dissected later. To prepare glyceroalcohol, absolute glycerine (glycerol) and rectified spirit (90% ethyl alcohol) are mixed in 1:1 ratio.