Metrohm 754 Dialysis Unit User Manual
Page 28
3.3 The dialysis procedure
754 Dialysis Unit
25
3.3.3
Setting the rinsing time
A rinsing time of 2 min is normally sufficient to ensure thorough rinsing
of the sample and acceptor channels. This time can be increased if re-
quired.
3.3.4
Setting the transfer time
The time allowed for transferring the enriched acceptor solution to the
sample loop must be set to ensure that the part of the acceptor solution
with the highest concentration of ions is transferred to the sample loop.
With standard accessories the optimum transfer time depends on the
rate of operation of the pump; the following values can be used as a
guideline:
Mains frequency Optimum transfer time
50 Hz
0.5 min
60 Hz
0.4 min
For each analysis problem, determine the optimum transfer time by
taking measurements of the individual ion concentrations in relation to
the transfer time itself and verify your measurements from time to time.
Proceed as follows:
1 First measurement
•
Set the transfer time in the program to 0.2 min and the dialy-
sis time to 10 min (see section 3.3.2).
•
Immerse sample suction tubing 26
26 in the standard with 10
mg/L of the required anion or cation.
•
Start the program on the 732 IC Detector with the R/S> key. Wait until the LED in the flashing, signalling the end of the measuring process. 2 Additional measurements • Increase the transfer time in the program (see section 3.3.2) by increments of 0.1 min until the concentration measured falls again. • Take measurements as in point 1. 3 Determining the optimum transfer time • Record the measured peak area resp. concentration in rela- tion to the transfer time and determine the optimum transfer time. The Figure below shows an example of this determination process for anions using the 6.2714.010 dialysis membrane (cellulose acetate; thickness = 115 µ m; pore size = 0.2 µ m) available from Metrohm. The sample was a standard solution containing 10 mg/L of each of the indi- vidual anions, which was dialysed for 10 min (mains frequency 50 Hz).