Hach AutoCAT 9000 Instruction Manual page 161

Appendix D Low Level Standards & Testing
50081_Low Level Standards & Testing.fm
The analysis of standard solutions for method verification is an essential part of
maintaining a thorough laboratory QA/QC program. Analyzing standards ensures
accuracy, maintains consistent performance, and provides documented proof of results.
In many cases, verification of a method will be performed by analyzing a standard at a
similar concentration to that of a typical sample. However, such a procedure is not
always feasible in every situation. One particular situation in which preparing a standard
at typical sample concentrations is not feasible is in the case of low range chlorine
analysis. Preparing an accurate low-level chlorine standard is difficult, as there are
multiple potential sources of error that can affect standard accuracy.
For verification of low range chlorine analysis, Hach recommends the preparation of
chlorine standard solutions at concentrations no lower than 0.10 mg/L. Preparation of
chlorine standard solutions at lower concentrations is not recommended for method
verification.
It is important to understand the practical limitations of preparing a low-level chlorine
standard. Some of the potential error sources in the preparation of a low concentration
chlorine standard solution include:
1. Dilution Water - The dilution water used to prepare a chlorine standard must be
chlorine-demand free. For accurate results, the water used to prepare a chlorine
dilution should be 18 megohm, organic free (< 20 g/L of total organic carbon), amine
free, and sterile (this can be achieved by filtration through a 0.2 µm filter). Dilution
water may also be tested for chlorine demand by following the procedure indicated
in Standard Methods for the Examination of Water and Wastewater 2350B.
2. Atmospheric Exposure - To avoid oxidation and loss of chlorine, low-level chlorine
standards must be prepared under a high-purity inert gas headspace. Nitrogen and
argon are acceptable gasses to use in this application. Contact with atmospheric
gasses, ammonia vapors, and exposure to ambient light must be avoided.
3. Containers - The containers in which chlorine standards are prepared and handled
will affect accuracy. It is important that all containers be chlorine-demand free.
Plastic containers should be avoided, as they can leach organics into water and be a
source of chlorine demand. Glass, Teflon, and PET are preferred materials for wetted
parts of the system. All parts should be pretreated for chlorine demand by soaking in
a dilute bleach solution (5 drops of commercial bleach per liter of water) followed by
rinsing with copious amounts of chlorine-demand-free deionized water. Amber glass
containers should be avoided.
4. pH - The pH of the solution also affects the stability of a prepared chlorine standard
solution. Chlorine is more stable as hypochlorite ion (at pH > 9) than as
hypochlorous acid. The pH of chlorine standard solutions prepared in deionized
water is typically < 9, meaning that the prepared standard solution is unstable and
must be prepared immediately prior to analysis.
5. Equivalent Standards - With the many potential sources of error in the preparation
of chlorine standard solutions, it may be tempting to prepare a chlorine equivalent
standard from potassium permanganate. However, the use of permanganate
standards for chlorine verification is not recommended. Although permanganate is
an oxidant, permanganate standards will not necessarily produce equivalent results
with DPD or amperometric methods. Dilute permanganate standards are also very
unstable and must be prepared immediately before use. Finally, the cleanliness of
glassware and dilution water used in the preparation of permanganate standards is
critical to accuracy.
Low Level Standards & Testing
Page 159