Chlorine Analysis at Unique High Precision for a Large Ranges

In recent years, chlorine analysis has become increasingly important. The use of substitute fuel in cement plants, brickyards and other industries with high demand for energy requires easy, low maintenance chlorine analysis.

Conventionally, chlorine is determined using Schöninger digestion, which is very time consuming and is also difficult to automate. Hence, high-temperature combustion followed by coulometric detection of the formed HCl has been gaining interest of late. However, while the coulometric method is being used extensively, it does have a number of major disadvantages. Since the electrolyte solution has to be replaced very frequently, a considerable amount of maintenance effort is needed, and also the electrodes are vulnerable to signs of wear. With this approach, the dynamic range is limited to lower concentrations as well. On the other hand, a combination of conductivity detection and ion chromatography is a more stable approach that allows the individual determination of all halogens. A huge investment cost is the only disadvantage of this technique.

Elementar, a front runner of elemental analysis, has developed an easier alternative. The combination of advanced, well-established elemental analyzers with high-performance solid-state electrochemical detectors (ECD) makes the analysis of chlorine much simpler, more dependable and more accurate than ever before. This paves the way for chlorine measurements over a larger concentration range with minimal maintenance effort.

Chlorine Analysis

Measuring Principle

Through a blank-free ball valve, the sample is transferred from the sample carousel into the combustion tube. This does not require complex mechanics and/or compressed air to feed the samples into the combustion tube, unlike systems with a horizontal furnace.

The sample then enters the high-temperature combustion zone, which ensures 100% conversion of all chlorine to HCl. To avoid condensation and loss of HCl, water is efficiently removed from the carrier gas in the next step. The carrier gas finally passes the solid-state electrochemical detector to detect the HCl, as shown in Figure 1. This default arrangement is available for the following models: rapid CS cube, vario EL cube, vario MICRO cube and vario MACRO cube.

In a few instances, the ECD cell shows a slight cross sensitivity to CO2, which results in a small negative peak at the start of the integration. This does not considerably contribute to the resulting chlorine concentration for higher Cl concentrations (> 100 ppm). This small negative peak may influence the results for low concentrations such as in gasoline.

This is the reason why the setup of the chlorine option of the trace SN cube, which is optimized for low Cl contents, is slightly different (Figure 2). Once the moisture from the carrier gas is removed, the HCl is trapped on a special trap column. As soon as the combustion process and removal of interferences are completed, the special trap column heats to release HCl, which is then detected in the solid-state electrochemical detector. The Advanced Purge and Trap technology leads to peak focusing that dramatically improves the signal-to-noise ratio and to guarantee the highest sensitivity.

The detector signal is evaluated on a standard PC. This entire process is completely automated to enable easy use and overnight operation.

Functional principle of the chlorine detection option

Figure 1. Diagram of the chlorine detection option in the default configuration.

Functional principle of the trace SN cube with the chlorine detection option

Figure 2. Diagram of the trace SN cube with the chlorine detection option for low concentrations.

Chlorine Analysis

Electrochemical Detection

The following are the reactions that occur in the electrochemical cell:

Working electrode: Ag + HCl → AgCl + H+ + e
Reference electrode: O2 + 4e + 4H+ → 2H2O

Functional principle of the EC cell.

Figure 3. Diagram of the EC cell.

In this reaction, the generated electrons lower the current between the reference and working electrodes – a measure for the HCl concentration in the carrier gas. This type of detection method is termed as amperometric detection. The capillary diffusion barrier is fundamentally a hole whose diameter determines the cell sensitivity. The electrolyte is a concentrated, non-oxidizing acid. Figure 3 shows a schematic diagram of the electrochemical cell. The ECD cell can also be influenced by high concentrations of HBr, but this is seldom seen in practice.

HCl peak of a 1.4 mg NH4Cl sample

Figure 4. HCl peak of a 1.4 mg NH4Cl sample analyzed using the rapid CS cube with chlorine detection option and 5000 ppm measuring cell.

HCl peak of a diesel sample

Figure 5. HCl peak of a diesel sample (+ 5 ppm Cl) analyzed using the trace SN cube with chlorine detection option and 20 ppm measuring cell.

Calibration

There are many standards that can be employed for calibration. Ammonium chloride (Cl content 65.97%) is the preferred substance if no capsule press is available. Furthermore, the high solubility of chlorine in water is perfect for preparing standard solutions. One of the other possible calibration standards can be oil with a defined amount of Cl or if available matrix specific chlorine standards. Typically, these standards can be used without accelerants. For example, Figure 6 shows a typical calibration curve, achieved by a coal standard with 0.11% Cl on the rapid CS cube with Cl-option.

Calibration curve EC cell for chlorine.

Figure 6. Calibration curve EC cell for chlorine.

Available EC cells

Elementar provides three varied types of electrochemical cells based on the concentration range. The names of the cells denote the maximum HCl concentration in the gas stream which can be analyzed. It must be noted that this concentration cannot directly be transferred to the Cl content of the sample.

The following table presents the attributes of the different electrochemical cells.

CELL TYPE LOWER LIMIT
(µg Cl abs.)
UPPER LIMIT
(µg Cl abs.)
REL. SD Cl
(%)*
20 ppm† 0.05 2 8
200 ppm 0.5 20 5
5000 ppm 15 1200 5

*: obtained on Conostan standards in the middle of the measuring range
†: available for the trace SN cube only

Typical applications

The chlorine detection option is optimized for a wide range of applications in the area of biomass, fuel, residual fuel, soil, and waste, among others. It is not recommended to use the Cl-option for traditional elemental analysis (e.g. to determine the Cl content in pure chemicals) because of limitations in precision and accuracy. The following table shows some common applications for the Cl-option.

SUBSTANCE WEIGHT
(mg)
DETECTOR Cl
(W-%)
REL. SD
(%)
Anthracit-1 34 200 0.151 1.52
Anthracit-2 33 200 0.021 6.1
Wood coal 60 200 0.0045 8.9
Biomass-1 15 200 0.133 4.2
Biomass-2 30 200 0.0086 8.9
Organic waste 20 200 0.142 4.3
Residual fuel 31 5000 0.893 6.1
Plastic fiber 10 200 0.354 11.3
Diesel spiked with 1 ppm Cl 35 20 1.07 8.8

 

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This information has been sourced, reviewed and adapted from materials provided by Elementar Analysensysteme GmbH.

For more information on this source, please visit Elementar Analysensysteme GmbH.

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