What concerns are there when working with chlorine standards?

Document ID

Document ID TE9340

Published Date

Published Date 03/07/2019
What concerns are there when working with chlorine standards?
Chlorine standard concerns
 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:
  • 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 megaohm, organic free (<20µg/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.  
  • 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.  
  • 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.  
  • 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.  
  • 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.  
In order to assess the accuracy of a chlorine analysis method, such as DPD, Hach recommends the preparation of a chlorine standard solution at a concentration greater than 0.10 mg/L. The actual concentration of the prepared standard must be verified, because the many potential error sources mean that the actual concentration of the standard solution may not match the theoretical calculated concentration of the standard. Also, because a chlorine standard solution is not prepared from a primary standard, the actual concentration of the prepared chlorine standard used for verification should be determined through a separate reference method, such as amperometric titration. Amperometric titration is a method that is traceable to a primary standard. The PAO titrant is traceable to arsenic trioxide, a primary standard. Standardization of PAO versus arsenic trioxide can be performed using an amperometric titrator.
Additionally, the calibration curves for colorimetric chlorine analyses that are programmed into Hach colorimeters and spectrophotometers are the result of multiple standards (referenced to amperometric titration) analyzed with multiple lots of reagent. Verification of the method with chlorine standards is recommended – however adjustment of the existing calibration curve or recalibration of the instrument is advised against. Verification can be performed by preparing a chlorine standard solution or through standard additions (sample spike). Procedures for standard additions can be found in the “Accuracy Check, Standard Additions Method” section of any Hach procedure or in the Hach Water Analysis Handbook. Many Hach spectrophotometers will automatically calculate and graph standard additions, as well. It is suggested that chlorinated wastewater or tap water be dechlorinated using UV prior to the addition of a chlorine spike.

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