Silkworm

Selection of the acid
For consistent good results, stronger inorganic acids have been found in general, better lhan organic
acids. Amongst the inorganic acids again, Nitric acid, Sulphuric acid and Aqua regia (a combination of
Nitric acid and Hydrochloric acid in a ratio of 1:3) have been found to be too strong for use and difficult to
handle. Comparative figures with regard lo concentration (percent by weight) and densities of different
inorganic acids are given in Table 2.
TABLE 2
Specilic gravity and concentration of different acids
Name of the acid
Concentration of acid at
saturation (%)
Density at 20"C
Hydrochloric acid (HCl)
Nitric acid (HNO")
Sulphuric acid (H,SO.)
40
100
100
1.1980
1.5129
1.8305
Evidently, Nitric acid and Sulphuric acid are distinctly stronger in nature and pose handling problems.
Thus, in practice, HCI alone is being exploited forthe acid treatment of silkworm eggs both at commercial
and laboratory levels. lt has been found to be the safest and best medium for treating diapausing eggs
to obtain maximum hatching.
Hydrochloric acid is a by-product obtained while manufacturing soda-ash and is subsequently
purif ied to the required levels. Pure HCI is colourless and emanates strong and obnoxious fumes. The
yellow colour of certain commercial HCI products is often caused by iron and other impurities. lt fumes
in moist air and is very easily soluble in water.' lt has strong acid taste with pungent irritating odour and
attacks the mucous membrane upon inhalation.
HCI is marketed as a solulion containing mostly 28 to 36% Hyirogen Chloride by weight and is
commonly known as concentrated HCl. The concentration and specific gravity of HCI are correlated as
indicated in Table 3.
TABLE 3
Relationship between Specific Gravity and Concentration of HCI
Specific gravity of acid at 20"C
1 .198
1.195
1.190
1.180
1.170
1.160
1.150
'1
.1 40
1.130
1.120
1.1 10
1.100
1.095
1.090
1.085
1.080
1.075
1.O70
Concentration of acid (%)
40
39
38
50
34
32
30
28
zo
24
22
20
19
18
17
16
15
14
9