Technical Bulletin: Methodologies for Chloride Analysis
Does your Site show one or more of the following characteristics?
Analytical results show anomalous or elevated chloride concentrations in delineation samples that were not found in Source samples.
Background or backfill material samples with suspiciously high chloride concentrations.
Generally high chloride concentrations that do not seem to be analogous with EC and/or SAR values.
Located within an area of highly organic soils and/or decomposing organic matter.
If so, falsely biased high chloride analytical data may be of concern. This bulletin aims to assist you in identifying potentially erroneous chloride analytical data and to discuss the advantages of the ion chromatography method of chloride analysis compared to the default colorimetry method adopted by most environmental labs.
What is Chloride?
Figure 1: Chemical representation of the dissociation of sodium chloride in water. The sodium and chloride ions form weak counterion interactions with the partially charged hydrogen and oxygen atoms within the water molecule.
Chloride is the ionic form of the halogen, chlorine. It is formed when chlorine is chemically or electrochemically reduced and gains an electron. Typically in nature, however, chloride is formed when an ionic salt or acid of chloride dissolves in water. Common chloride salts include sodium, potassium, calcium, and magnesium chlorides. These alkali and alkaline earth metal salts are highly soluble in polar solvents such as water.
Chloride in Soil
Chloride is a crucial micronutrient for most crops and therefore directly affects soil fertility. Concentrations of chloride ranging from 6 to 30 mg/kg in soil are required to support the growth of most agricultural crops. However, the presence of elevated levels of chloride in soil due to anthropogenic activities can lead to crop toxicity. Anthropogenic releases of chloride are often associated with oil and gas production, road salting operations, salt mining, and industrial runoff. Chloride from produced water sources accounts for the majority of oil and gas‐related chloride releases in Western Canada. Produced water is a term used in the oil and gas industry to describe water produced as a byproduct of oil and natural gas extraction. The composition of produced water varies based on several factors including the formation of origin but typically most produced waters are highly saline with elevated chloride concentrations and contain varying amounts of oil residue, sand, mud, organic compounds, polyaromatic hydrocarbons (PAHs), and naturally occurring radioactive materials (NORMs). Due to the toxicity posed by excessive environmental chloride, guideline criteria or similar monitoring thresholds exist for chloride in soil and water within most Canadian jurisdictions.
Chloride Analysis
Various laboratory and in situ field analysis options are available for the analysis of chloride in soil samples. In situ testing options include chloride test strips (Quantabs) and portable variations of the ion-selective electrode. Chloride concentrations and general salinity can also be monitored in‐situ using electrical conductivity (EC) probes as a screening tool. EC probes are capable of measuring a sum of the ionic content of a sample and therefore may be useful if other ions are suspected as potential contaminants of concern. The use of EC probes as an in‐situ monitoring tool for chloride may not be suitable on sites that contain soils naturally elevated in sulfates or contain other pre‐existing salts (including fertilizers); these soils may have pre‐existing elevated EC values which could mask areas of elevated chloride content.
The main laboratory analytical methodologies are based on colorimetry via mercuric thiocyanate, ion chromatography, and titrations. Due to the relative ease of analysis, high sample throughput, and cost-effectiveness, most commercial environmental laboratories perform chloride analysis in soil by flow analyzer instruments using colorimetry via mercuric thiocyanate methodology.
Soil Extraction in Preparation for Analysis
Regardless of the methodology adopted for the analysis of chloride in soil, sample preparation is required in order to extract chloride into a liquid media. Most commonly, a saturated paste extraction is performed by hydrating a pre‐dried and ground soil sample using deionized water, which is later pressed through a filter while the filtrate is collected for analysis. The formed paste is typically allowed to rest for up to 16 hours prior to filtering to allow for the equilibration and transfer of chloride ions from the soil pore structure into the aqueous component of the saturated paste. The resulting aqueous extract is then ready for analysis.
Chloride Analysis by Colorimetry
The colorimetry method for chloride analysis aims to quantify a chloride concentration within a sample based on color intensity following a chemical reaction. The chemical reaction in question is based on the formation of the ferric thiocyanate complex. The main reagents used for this method are mercuric thiocyanate (Hg(SCN)2) and ferric nitrate (Fe(NO3)2). In the presence of chloride, thiocyanate is liberated and chloride binds to the mercuric cation to form mercury chloride. The liberated thiocyanate anions and ferric cations form the ferric thiocyanate (Fe(SCN)2+) complex which produces a red/orange color. The intensity of the color is directly proportional to the concentration of chloride within the soil extract.
The chloride concentration within the soil extract can be quantified using a spectrophotometer that measures the intensity of the red/orange color of the extract and compares the signal to a calibration curve.
Similar to most analytical methodologies, the colorimetric method for chloride analysis is prone to analytical interferences. Analytical interferences are defined as chemical species or external factors that can cause erroneous results during analysis. The colorimetric method for chloride analysis is susceptible to two main types of interferences:
Chemical interferences due to anions present within a sample that also has an affinity to bind to mercury can produce the colored ferric thiocyanate complex independent of chloride resulting in falsely elevated chloride results. Examples of this type of interference include high concentrations of anionic sulfur species, bromide, phosphate, cyanide, fluoride, nitrate, and nitrite.
Color interferences due to soil samples that produce naturally colored saturated paste extracts. A common example would be highly organic soil samples that contain humic and similar organic acids which can produce a dark-colored soil extract.
Based on the nature of your samples, the ion chromatography (IC) method may be a more suitable option for the analysis of chloride as this method is less susceptible to analytical interferences.
Chloride Analysis by Ion Chromatography
Unlike the colorimetry methodology, the IC method does not depend on quantifying an analyte concentration based on the intensity of a color change post-chemical reaction. Instead, the IC method relies on the direct separation of the anions within a sample and the quantification is conducted on each anion individually. Therefore, this method does not suffer from chemical or color interferences to the degree of the colorimetry method. For this reason, the IC method is considered more robust for chloride analysis and can offer lower analytical uncertainty and detection limits.
Figure 2: Schematic representation of ion chromatography. The sample is passed through an IC column under pressure from top to bottom. The anions within the sample interact with the cationic species lined within the column to varying degrees which promotes separation. Anions elute from the column at varying retention times and are quantified by a detector. The signal is plotted on a chromatogram of signal as a function of retention time.
Figure 3: An example IC chromatogram of anionic analysis. Fluoride is the first anion to elute and sulfate the last for this sample/column combination.
The main analytical interference of the IC method occurs when a sample contains an unidentified chemical species that elutes from the instrument column at the same retention time as chloride (or the analyte of interest). When this occurs, the unidentified species’ chromatogram peak can ‘mask’ the peak of the analyte of interest. This interference is usually easily identifiable by the lab analyst and can usually be resolved.
The default chloride in soil methodology used by most commercial environmental labs is based on colourimetry via mercuric thiocyanate. This method is prone to several analytical interferences that can produce erroneous results particularly for samples that are organic in nature, highly contaminated and/or contain anionic species that have a strong affinity to bind to the mercuric cation such as some anionic sulfur species, bromide, phosphate, cyanide, fluoride, nitrate and nitrite. Soil samples with suspicious chloride results should be requested to be re-analyzed by your laboratory using the IC method as a confirmatory measure. To ensure accuracy in the reported results, re-analysis of all chloride samples using the IC method is recommended on sites that have been confirmed to contain soils that cause interferences when using the colourimetry method.