DISSOLVED OXYGEN 349 
waters it often happens that the blue coloring matter is precipitated 
elther partly or completely through absorption by the clay which many 
rivers carry in suspension. True relative stabilities cannot be obtained 
for such waters except by determining the initial available oxygen at the 
start and the biochemical oxygen demand on incubation at 20° C. for 
ten days. Germicides, such as calcium hypochlorite, if present in suf- 
ficient quantity vitiate the results. If a sample contains free chlorine, 
therefore, store it about two hours, or until the chlorine is gone, and 
then add methylene blue. . 
Table XXXIV gives the relation between the time of reduction in 
days at 20° C. and the relative stability number. 
TABLE XXXIV 
RELATIVE STABILITY NUMBERS 
t20 8 | 120 s 
0 5 11 8 0 84 
10 21 90 87 
15 30 10 0 90 
20 30 11 0 92 
25 44 12 0 94. 
3 0 50 13 0 95 
40 60 14 0 96 
5 0 68 16 0 97 
6 0 75 18 0 98 
7 0 80 20 0 99 
at 
S=Relative stability or ratio of available oxygen to oxygen required for equilib- 
rium. Expressed in per cent. 
to = Time in days to decolorize methylene blue at 20° C. 
The theoretical relation is, 
The relation between the time of reduction at 20° C. and that at 37° 
C. is approximately two to one. It is desirable that each observer work 
out his own comparative 37° C. table or factor, but results should be 
reported in terms of 20° C. stability numbers. 
A relative stability of 75 signifies that the sample examined con- 
tains a supply of available oxygen equal to 75 per cent of the amount 
of oxygen which it requires in order to become perfectly stable. The 
available oxygen is approximately equivalent to the dissolved oxygen 
plus the available oxygen of nitrates and nitrites. The nitrites in 
sewage are usually so low as to be negligible. 
