Hach AutoCAT 9000 Instruction Manual page 168

AutoCAT 9000 Validation Study
Figure 3 Setup for chlorine vessel with inert gas headspace
AutoCAT 9000 Validation Study
Page 166
laboratory technique, preparation of chlorine standards in the low parts per billion range
can be achieved.
A distinction must be made regarding skill levels required to run the autotitrator to obtain
accurate and reproducible trace chlorine measurements, versus the skill level required to
accurately conduct MDL studies. For routine measurements, the operator must be able to
properly measure a 200-mL sample, observing correct sample handling techniques,
follow SOPs, and the menu-driven steps of instrument operation. Conducting valid MDL
studies however requires great care in conducting serial dilutions, scrupulous attention to
detail and accuracy during the multi-step standard preparation process. These analyses
require proper equipment, an environment free of airborne chlorine, and very careful
sample manipulation to assure reproducible replicate sample concentrations and sample
volumes.
General considerations:
1. Use organic-free 18.2 Mohm water.
2. All apparatus must be PET, Teflon, or glass.
3. Do not use amber glass.
4. Pre-treat all apparatus for chlorine demand.
5. Thoroughly rinse all apparatus after pre-treatment.
6. Use ultra-high purity nitrogen to pressurize bulk standard container to 5 psi.
7. Carefully fill volumetric ware and transfer without agitation.
8. Immediately analyze each 200-mL aliquot.
The schematic in Table 3 shows a setup for maintaining a 3-Liter reservoir of chlorine
standard solution under positive pressure inert gas headships. The constant 5-psi nitrogen
pressure in the reservoir headspace helps to maintain the chlorine standard at a constant
concentration.
Chlorine standard solutions, in sealed ampoules, with certificates of analysis, in the
range 25-75 mg/L are available. Dilution of aliquots of these standards in 1 to 3 liters of
reagent grade water will result in a stock solution at concentrations in the 1 to 2 mg/L
range, which will be stable for a period of days, if kept in a pre-treated sealed vessel,
under positive pressure inert gas. Spiking reagent grade water with this stock solution
will result in replicate 200-mL sample aliquots that will be consistent during the MDL
studies.
50081_AutoCAT 9000 Validation Study.fm