June 11, 2015, 715004752 Rev. A
Page 144
To predict the miscibility of two liquids, subtract the smaller M-number value from the
larger M-number value.
•
If the difference between the two M-numbers is 15 or less, the two liquids are
miscible in all proportions at 15 °C (59 °F).
•
A difference of 16 indicates a critical solution temperature from 25 to 75 °C (77
to 167 °F), with 50 °C (122 °F) as the optimal temperature.
•
If the difference is 17 or greater, the liquids are immiscible or their critical
solution temperature is above 75 °C (167 °F).
Some solvents prove immiscible with solvents at both ends of the lipophilicity scale.
These solvents receive a dual M-number:
•
The first number, always lower than 16, indicates the degree of miscibility with
highly lipophilic solvents.
•
The second number applies to the opposite end of the scale. A large difference
between these two numbers indicates a limited range of miscibility.
For example, some fluorocarbons are immiscible with all the standard solvents and
have M-numbers of 0, 32. Two liquids with dual M-numbers are usually miscible with
each other.
A liquid is classified in the M-number system by testing for miscibility with a sequence
of standard solvents. A correction term of 15 units is then either added or subtracted
from the cutoff point for miscibility.
C.3 Buffered solvents
When using a buffer, use a good quality reagent and filter it through a 0.45-µm filter.
Do not leave the buffer stored in the system after use. Flush all fluid paths
with
HPLC-quality water before shutting the system down and leave distilled water in the
system (flush with 90% HPLC-quality water: 10% methanol for shutdowns scheduled
to be more than one day). Use a minimum of 15 mL for sparge-equipped units, and a
minimum of 45 mL for in-line vacuum degasser-equipped units.
C.4 Head height
Position the solvent reservoirs at a level above the HPLC equipment or on top of the
pump or detector (with adequate spill protection).
C.5 Solvent viscosity
Generally, viscosity is not important when you are operating with a single solvent or
under low pressure. However, when you are running a gradient, the viscosity changes
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