How to Determine Total Bound Nitrogen (TNb) in Water

Parameter total bound nitrogen (TN_b) is a recognized limitation in the analysis of water for assessing and monitoring the quality of various types, with surface water and wastewater analysis being the main applications.

The new international standard DIN EN ISO 20236 (replacing the previous DIN EN 122602 for TN_b assessment in Europe) defines a catalytic high-temperature technique for the simultaneous determination of TOC (total organic carbon), TN_b, dissolved organic carbon (DOC), and dissolved bound nitrogen (DN_b).

The technique for verifying TN_b under the new standard is founded on the catalytically assisted high-temperature oxidation of the water sample at temperatures ≥ 720 °C in oxygen-rich environments.

The following detection of the NO created in this method was performed using a chemiluminescence detector (CLD) or a substitute device, for example, the electrochemical detector described in annex C of the standard (ChD/chemodetector).

Before verifying TN_b in a new water sample, the analyzer must be standardized with nitrogen standards of different concentrations. In line with the standard, nitrogen-mixed standard solutions (MSS) of potassium nitrate and ammonium sulfate were used.

First, two stock solutions of KNO₃ and (NH₄)₂SO₄, each with 1000 mg/L N, were prepared. Then, the MSS was primed by mixing equal volumes of the two different N stock solutions.

The MSS was diluted accordingly with ultra-pure water to make up the calibration solutions. The TN_b analyzers are usually standardized between 0 and 20 mg/L N (sometimes extended to 50 mg/L).

The new international standard recommends a daily system test using two or more niacin standards; the N concentrations should cover the working range. The niacin standards for the system test were primed from a stock solution containing 1,000 mg/L N via dilution with ultra-pure water.

A system test passes if the following conditions are met: the measured value does not diverge from the theoretical one by more than ± 5 % and/or ± 1 mg/L (whichever is higher); the repeatability coefficient of variation from at least two injections of the niacin standards does not exceed 5 % or ± 1 mg/L (whichever is higher); the individual values do not deviate from each other by more than 1 mg/L for concentrations < 10 mg/L N.

The new standard also defines interferences that may negatively affect the determination of TN_b.

High levels of TOC or DOC in the sample may reduce nitrogen recovery. This can be counteracted by measuring TN_b in different sample dilutions or via the standard addition method. Nitrogen compounds with double or triple bonds are not always entirely oxidized to NO.

The described standardization with the MSS of equal parts KNO₃ and (NH₄)₂SO₄ can lead to a positive bias for nitrate–N verifications in KNO₃ solutions and a negative bias for ammonium–N verifications in (NH₄)₂SO₄ solutions.

This article demonstrates how the multi N/C 3300 TOC/TN_b analyzer not only meets the conditions of the new international standard regarding nitrogen verification but also displays excellent performance characteristics. Both standards and samples with diverse compositions were examined.

Methods

The TN_b determinations were performed on the multi N/C 3300 TOC/TN_b analyzer. Although attention was focused on verifying TN_b, a combined NPOC/TN_b method was chosen because this technique (simultaneously verifying organic carbon and TN_b) is used for most samples measured in standard laboratories.

The acidification needed for this TOC/TN_b verification can be performed automatically using an auto-sampler directly before the actual measurement or manually (in advance, often during sampling).

The acidified samples were automatically purged with a partial flow of the carrier gas used, removing the total inorganic carbon (TIC) in the form of carbonates/hydrogen carbonates from the samples.

The comprehensiveness of the TIC removal can be determined automatically by activating the TIC control measurement in an NPOC method.

Post-TIC removal, the sample was directly injected into the analyzer’s combustion tube filled with catalyst. The sample’s nitrogen and organic carbon compounds are completely oxidized at high temperatures.

The NO formed in this process was fed into a CLD or a ChD, while the carbon dioxide formed was fed into a focus radiation non-dispersive infrared detector.

The AS vario auto-sampler, with a tray of 72 positions for 40 mL vials, was used to automatically verify TN_b and NPOC.

The samples and reagents used were the following:

• Nitrogen MSS in concentrations of 1 to 50 mg/L, produced from KNO₃ and (NH₄)₂SO₄;
• Niacin solutions for the system test in the concentrations 5, 10, 20, and 50 mg/L N;
• Control standard solutions of potassium nitrate and ammonium sulfate in the ratio NO₃–N to NH₄–N = 1:1 and concentrations of 5, 10, 20, and 50 mg/L N;
• Control standard solutions of potassium nitrate at 10 and 50 mg/L N;
• Control standard solutions of ammonium sulfate at 10 and 50 mg/L N;
• 2 mol/L HCl for the acidification of the samples and standard solutions;
• 5 water samples: 1 surface water, 2 process water, and 2 municipal wastewater, samples A to E.

Preparation of Samples

Before measurement, all samples were acidified with 2 mol/L HCl (0.5 mL per 100 mL sample) and were stored at approximately 4 °C in a refrigerator. After suitable warming to room temperature, the samples were decanted into 40 mL sample vials and placed on the auto-sampler tray.

On the day of assessment, all calibration and control standard solutions and the niacin solutions for the system test were prepared from their corresponding stock solutions.

Standardization

The calibration was performed with two device configurations: the multi N/C 3300 with a CLD and a ChD.

For both configurations, a multi-point standardization was performed in concentrations between 1 and 50 mg/L N. MSSs (potassium nitrate and ammonium sulfate in ultra-pure water, with equal proportions of nitrate–N and ammonium–N) were utilized for this purpose.

The calibration curves were assessed using linear regression and are shown alongside the correlation coefficients obtained in figures 1 and 2.

Figure 1. Calibration curve 1‒50 mg/L TN_b, R₂ = 0.99981, determined with chemiluminescence detector (CLD). Image Credit: Analytik Jena US

Figure 2. Calibration curve 1‒50 mg/L TN_b, R₂ = 0.99996, determined with chemodetector (ChD). Image Credit: Analytik Jena US

Table 1. Device and method settings for the standard and sample measurements. Source: Analytik Jena US

Results and Discussion

A distinct measurement sequence was administered alongside two different device configurations. Samples and various control niacin standards were alternately assessed.

Sixty-eight test vials were filled with four quantities per sample, system test, and control standard. At least one triple injection was made from each vial; Table 2 summarizes the results.

The measurement order does not correspond to the presentation in the table; it represents a summary of the control standards and samples measured across the sample sequence. The NPOC values of the samples were also noted and are shown in brackets beside the sample names for informative reasons.

Table 2. Measurement results. Source: Analytik Jena US

Figures 3 to 6 show the typical nitrogen measurement curves for the selected examples.

Fig 3. TN_b measuring curve sample A (surface water) with CLD. Image Credit: Analytik Jena US

Fig 4. TN_b measuring curve sample B (process water) with ChD. Image Credit: Analytik Jena US

Fig 5. TN_b measuring curve nicotinic acid standard 50 mg/L N with CLD. Image Credit: Analytik Jena US

Fig 6. TN_b measuring curve KNO₃ standard 10 mg/L N with ChD. Image Credit: Analytik Jena US

The results for the niacin solutions with the nitrogen concentrations 5, 10, 20, and 50 mg/L, measured as part of the system test, show that the test was fulfilled without exception over the entire range of concentrations.

The measured values satisfy the principle that deviation from the theoretical value must not exceed ± 5 %. The recovery rates, all between 97 and 104 %, verify this.

The superb reproducibility of the nitrogen measurement values of the system test solutions demonstrates the stability of the analysis system. Standard deviations from 0.02 to 0.5 mg/L (consistent with 0.4 % to 3.2 % relative standard deviation) were achieved.

These standard deviations were calculated from at least 12 sample injections (individual measured values) of a standard where the standards were distributed over the entire measurement sequence, more than fulfilling the new international standard’s requirements regarding the coefficient of variation for the niacin solutions.

The deviation of the individual values in the < 10 mg/L N range was also lower than 1 mg/L for all system test samples.

Highly reproducible sample results were also realized. The samples, including the system test solutions, were distributed in blocks over the entire series and were intended to expose the combustion tube to a certain matrix load.

To test calibration stability, control standards of potassium nitrate and ammonium sulfate were constantly assessed in the measurements. The recovery rates between 96 and 102 % show the stability and reliability of the calibration and analyzer.

The TN_b content was examined in control solutions containing either only nitrate-nitrogen or only ammonium-nitrogen. The new international standard notes that calibration with a NO₃/NH₄ MSS can lead to overestimation in nitrate-nitrogen determinations and underestimation in ammonium-nitrogen verifications.

The analyzed nitrate standard solutions with 10 and 50 mg/L N were determined with excellent recovery in the range of 97-104 %. The same can be said for the analyzed ammonium standards with 10 and 50 mg/L N, where recovery rates were between 99 and 101 %.

Following this, neither over- nor under-detections were found for the individual nitrate and ammonium standards, underlying the high performance of the analytical processes used.

The measurement data generated with two different device configurations demonstrate no significant differences in the quality of the measured values concerning the detection process used (CLD or ChD). Both methods comply fully with the nitrogen verification requirements of the new international standard

Conclusions

The multi N/C x300 series systems are exemplified by their superb performance in determining TN_b per the new international standard. Two equivalent detection methods, CLD and ChD, can be used for this purpose.

As well as the basic multi N/C 3300 device’s loop injection technology, an analyzer with direct injection expertise (the multi N/C 2300) can be used for this application. The multi N/C 2300 can be combined with one of the two detectors (CLD or ChD) to achieve comparable performance characteristics for nitrogen verification.

Various tests have also been performed via the multi N/C x300 series to determine the total organic carbon in water following the new international standard; conformity with standard methods has been established without restriction.

The analyzers have been characterized by long-term stable calibration, effective particle handling, and low consumable deterioration. The devices’ intuitive operation using modern software is a logical outcome.

The TOC and TN_b analyzers of the multi N/C x300 series guarantee reliable, economical procedural analysis for verifying both parameters in water samples under the new international standard at all times.

Figure 7. Multi N/C 3300 with AS vario (left) and multi N/C 2300 with AS 60 (right). Image Credit: Analytik Jena US

Table 3. Overview of devices, accessories, and consumables. Source: Analytik Jena US

References

1. ISO 20236:2018. Available at: https://www.iso.org/standard/67389.html.
2. iTeh Standards. (2024). iTeh Standards. [online] Available at: https://standards.iteh.ai/catalog/standards/cen/d44b69ac-9d8f-4e21-8c2c-054c6b6ae0a3/en-12260-2003?srsltid=AfmBOop-4QSq7Dhdl8nwOix-djTJq3p6pLSLJk2dZlWlvFdTy8Ve3V5Z [Accessed 8 Nov. 2024].

This information has been sourced, reviewed, and adapted from materials provided by Analytik Jena US.

For more information on this source, please visit Analytik Jena US.