Cyanide: The Molecule and Its Analysis A Simple Compound with Complex Problems Part 2: Cyanide Sample Collection, Pretreatment and Preservation

Transcription

1 Cyanide: The Molecule and Its Analysis A Simple Compound with Complex Problems Part 2: Cyanide Sample Collection, Pretreatment and Preservation Edward F. Askew PhD Askew Scientific Consulting

2 The first part of this series reviewed the types of cyanide compounds that exist in the environment and their general chemical bond characteristics. In this part, the pretreatment and preservation of the cyanide sample will be reviewed. Standard Methods for the Examination of Water and Wastewater, 4500 CN (B), 21 st Edition will provide the majority of the guidelines with additional information provided by Federal regulations[1], the EPA drinking water certification manual [2], and journal articles [3-7]. In this cyanide series, the definitions for pretreatment and preservation fall under the overall preliminary treatment of a cyanide sample. Pretreatment is any chemical/physical process that removes/neutralizes interactions that may bias the cyanide final concentration. Preservation is any chemical/physical process that prevents loss of the initial cyanide concentration. The cyanide sample pretreatment and preservation flow chart is summarized in Figure 1. Each process and decision block will remove a group of compounds that can react with the cyanide or other compounds in the sample to bias the final cyanide concentration. 1) Start with the sample collection in the appropriate bottle as per the regulatory requirements [1,2]. 2) Oxidizer treatment with a reducing agent. Oxidizing agents react with cyanide to irreversibly produce carbon dioxide and nitrogen gasses. As such, a reducing agent needs to be added to remove the oxidizing agent. The reducing agent should only be added in slight excess of the oxidizing agent concentration as a large excess of reducing agent can react with compounds in the sample to bias the final cyanide concentration. Typically, the oxidizing agent seen is either chlorine or hypochlorite. The three main reducing agents added for these chlorine oxidizers are: a) Arsenite in the form of the sodium salt or as an aqueous solution. This reducing agent has no know reactions that can bias the cyanide analysis, but arsenite is very poisonous and requires the sample collector to take appropriate precautions due to its toxicity. b) Thiosulfate in the form of the sodium salt or as an aqueous solution. This reducing agent has side reactions that bias the cyanide analysis if added in large excess. Thiosulfate should be added in small aliquots to a well mixed sample. The sample should be tested for chlorine oxidizer after each aliquot is added. c) Ascorbic acid either as a solid or as an aqueous solution. The drinking water laboratories are required to use this reducing agent for regulatory analysis as per the certification manual [2]. Excess amount of this reducing agent is suspected of reacting with nitrite and other nitro source compounds to produce cyanide during storage and distillation [3-5] 3) Sulfide removal by precipitation of lead sulfide. The removal of sulfide must be performed prior to the ph adjustment of the sample as sulfide reacts with cyanide at high ph to irreversibly form thiocyanate [7]. Lead acetate should be used to treat for sulfide unless the concentration of sulfide in the sample requires such excessive amounts of lead acetate that the ph of the sample is made acidic (ph < 7.0). Lead carbonate can be added to offset the lead acetate acidity and maintain the sample at neutral or slightly basic ph ( ). Care must be taken to not allow the ph to become too basic until all of the sulfide has been treated. If the addition of lead acetate or lead carbonate to the sample produces a precipitate, the sample must be filtered. The filtrate will continue to the next pretreatment step and the

3 precipitate will be transferred to a storage bottle and refrigerated as per regulatory requirements [1,2]. 4) Nitro source compounds, such as nitrite, inference. There is a literature article reporting that at a higher ph, nitrites react with carbonyl sources in the sample to produce cyanide or compounds that decompose to cyanide during distillation [3]. There are no regulatory requirements for sample pretreatment to prevent these reactions, but it is suggested that the addition of 2 grams of sulfamic acid per 500 milliliters of sample should prevent the nitrite/carbonyl compound reaction. Other nitro compound reactions, hydroxyl ammonium salts [4], have no literature referenced pretreatment recommendations. In all cases, the analyst should record in the sample bench sheet if nitro source compound interference is suspected. 5) Carbonyl compounds, such as a primary aldehyde, interference. The presence of a primary aldehyde or other organic carbonyl compounds has been shown to produce cyanide or compounds that decompose to cyanide during distillation. There are no regulatory requirements for sample pretreatment to prevent these reactions, but Standard Methods recommends the addition of 2 milliliters of a 3.5% solution of ethylenediamine per 100 milliliters of sample after the ph has been stabilized to between 12 and Caution should be used in treating for aldehydes as some literature articles show cyanide production from the acetate salts of ethylenediamine in the presence of nitro source compounds [4-6]. Treatment for nitro source compounds may address the presence of carbonyl compounds in the sample and reduce the number of steps required for sample pretreatment. In all cases, the analyst should record in the sample bench sheet if carbonyl compound interference is suspected. 6) Preservation by ph adjustment and refrigeration of the sample. The pretreated sample can now have the ph adjusted with sodium hydroxide solid or aqueous solution to a value of 12 to The ph should not exceed the upper ph limit value as additional reactions may occur in the sample. The sample may not reach ph equilibrium immediately if sodium hydroxide solid is used, so allow the well mixed sample to sit for ~ 30 minutes prior to measuring the ph. There is no accepted procedure to back adjust the ph if the upper value of 12.5 is exceeded. Care should be taken in adjusting the sample ph to prevent this from happening. In all cases, the analyst should record in the sample bench sheet if ph preservation limits have been exceeded. Once the ph has been adjusted, the sample will be refrigerated as per regulatory requirements [1,2] and stored correctly until distilled. Though the sample should now be ready for distillation, these pretreatment and preservation requirements do not guarantee that the final result obtained from distillation and colorimetric analysis is the actual free cyanide-available cyanide-total cyanide value. Part 3 of this series will focus on the reactions that occur during distillation and the products produced that can bias the final cyanide results.

4 Figure 1: Cyanide Sample Pretreatment Flow chart

5 References 1. EPA, Guidelines Establishing Test Procedures for the Analysis of Pollutants Under the Clean Water Act; National Primary Drinking Water Regulations; and National Secondary Drinking Water Regulations; Analysis and Sampling Procedures; Final Rule p EPA, Manual for the Certification of Laboratories Analyzing Drinking Water Carr, S.A., R.B. Baird, and B.T. Lin, Wastewater derived interferences in cyanide analysis. Water Res., (7): p Nonomura, M., Endogenous formation of hydrogen cyanide during distillation for the determination of total cyanide. Toxicol. Environ. Chem., (1): p Nonomura, M., Pretreatments for the determination of total cyanide - interferences, effects of reducing agents and additives. Int. J. Environ. Anal. Chem., (4): p Nonomura, M., et al., False detection of cyanide ion in photographic processing waste solutions using standardized reference methods. Int. J. Environ. Anal. Chem., (4): p Wilmot, J.C., et al., Formation of thiocyanate during removal of sulfide as lead sulfide prior to cyanide determination. Analyst (Cambridge, U. K.), (6): p