Hach DR/700 Procedures Manual page 33

b. If the number of digits required is less than 70, increase the sample
size to increase precision.
c. If the sample size is altered, adjust the amount of buffering or
indicating reagents by the same proportion.
4. When using the Digital Titrator for your buret method, note the
number of digits required for a sample titration. To convert the digits
required to the equivalent number of milliliters in the buret method was
used, calculate:
Equivalent Buret Milliliters = Digits Required x
If the sample size was changed, adjust the equivalent buret milliliters
accordingly. If the sample size was increased, reduce the equivalent
buret milliliters; if the sample size was reduced, increase the equivalent
buret milliliters. Multiply the equivalent milliliters by any normally
used factors to calculate concentration in oz/gal, g/L, etc.
Example: Adapt a buret procedure which normally requires about 20
mL of a 0.4 N titrant to the Digital Titrator. Try a 8.0 N titration
cartridge. The first equation above gives:
Digits Required =
Because this would use excessive titrant, reduce the sample size to one-
fourth its normal size to reduce the digits required to 200, well within
the recommended range.
Upon completion of the titration using the smaller sample size, calculate
the equivalent buret milliliters by the second equation above. If 205
were the digits required:
Equivalent Buret Milliliters =
Multiply the 5.13 mL by four to account for the reduction in sample size
to give the true equivalent buret milliliters of 20.5 mL. If the buret
method called for multiplying the number of milliliters of titrant by a
factor to calculate the concentration of a sample component, then
multiply 20.5 by that factor.
0.4 x 20 x 800
8.0
205 x 8.0
800 x 0.4
1-15
N
c
800 x N
t
= 800 digits
= 5.13 mL