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A New Method for the Determination of Total Nitrogen in Aqueous Samples

William Lipps Analytical & Measuring Instrument Division Shimadzu Scientific Instruments, Columbia, Md., USA

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Current EPA Definition for Total Nitrogen Requires Two Methods

Measure TKN

Measure NO3/NO2-N TN

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Must Use 40 CFR Part 136 Methods for CWA Reporting

•  Manual digestion!•  Manual distillation!•  Nessler, IC, titration, ISE, phenate!

TKN Methods!

•  Manual digestion!•  Automated distillation/diffusion!•  Direct colorimetry!

TKN Methods!

•  IC, reduction colorimetry!NO3/NO2 –N Methods!

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The “Classical” Kjeldahl Reactions Distilled and Titrated Ammonia

Digestion •  H2SO4 + 2(NH2)2CO -> (NH4)2SO4 +2 CO2

Distillation •  (NH4)2SO4 + NaOH -> NaHSO4 + H2O + 2NH3

Capture •  H2SO4 + 2NH3 -> (NH4)2SO4 + H2O

Titration •  H2SO4 + NaOH -> NaHSO4 + H2O

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Present Day TKN Methods Can Either Distill (Diffuse) or Measure NH4

+ in the Matrix

Manual Digestion •  H2SO4 + (NH2)2CO -> (NH4)2SO4 + 2CO2 •  Must use catalyst

Distillation (required for some methods) •  (NH)4SO4 + NaOH -> NaHSO4 +H2O + 2NH3

Detection •  Titration, ISE, manual colorimetry, automated

colorimetry

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Kjeldahl Method!

Image: Wikipedia

Substance + Conc. H2SO4 + CuSO4 + K2SO4

1 Liter RB Flask

Water out

Water in Conical Flask

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Operational Comparison of TKN Methods

Manual Digestion & Distillation Followed By:!

Require Final Digest Volume!

Require Final Distillate Volume!

Titration! no! no!Ion Selective

Electrode! no! yes!

Manual Phenate! no! yes!Automated

Phenate ! no! yes!

Block Digestion!Automated Salicylate! yes! N/A!

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Techniques for Analysis of Total Nitrogen (EPA Only Approves TKN)!

Manual Digestion & Distillation Followed By:! MDL (mg/L)!

Titration! 1.0 !

Manual Phenate! 0.05 ?!

Ion Selective Electrode! 0.03!

Automated Phenate! 0.01!

Block Digestion!

Automated Salicylate! 0.1!

Auto-GD/pH colorimetry! 0.2!

Auto-GD/Salicylate! 0.02!

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Some Problems With TKN Method for TN

Time consuming – 0.5 – 2 hour digestion •  Separate distillation and/or analysis

Uses hazardous reagents •  Boiling sulfuric acid •  Metal catalyst

Must run NO3 + NO2 separately Not applicable to all N compounds

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TKN Recovery in Various N Compounds

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New Method is Needed That Measures Total Nitrogen As Single Result

Organic Nitrogen! NH3-N! NOx-N! TN!

This method can be a lab or online method

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Any New Method for TN Should…

Obtain equivalent results to TKN and other TN methods Be “rugged” Use few reagents Have a large dynamic range Sample/”handle” a complex matrix

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Total Nitrogen Can Be Easily Measured on TOC Analyzer"

Catalytic Combustion (CC) •  2(NH2)2CO +5O2 + catalyst à 4NO + 4H2O + CO2

Ozonation

•  NO + O3 à NO2 + O2!•  NO + O3 à NO2* + O2!

Detection

NO2* à NO2 + light !

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TOC + TN Analyzer!

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Schematic of TOC + TN Analyzer

TN Analysis

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Total Nitrogen Measurement is Rugged and Uses Few Reagents

Measure the sample (dilute)

Decomposition : 720℃ with Pt catalyst

Formation of NO

Formation of NO2* , -> NO2, Photon

Emission

Obtain the TN Result (every 5 minutes)

Ozone

Electricity

O2 from air

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TN TKN

Measurement contains

- Ammonia - Nitrate - Nitrite - Organic nitrogen

compounds

- Ammonia - Organic nitrogen

compounds

Consumables Catalyst hydrochloric acid

Mercury or selenium, sulphuric

acid

Analysis time 3-7 min (together

with TOC measurement)

Minimum 60 min

Online realization Easy Not possible

Comparison of TN to TKN!

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TKN and Catalytic Oxidation Recovery in Various N Compounds!

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Comparison of TN to TKN (QC Samples)!

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HTCO Methods Obtain Near Equal Results with Persulfate TN on Unknown Samples

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Recovery (%) of Various Nitrogen Compounds

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Recovery of TN with Concentration !150 microliter sample!"

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Recovery of TN with Concentration!300 microliter sample !"

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Total Nitrogen 300 Microliter Sample!"

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WK46665 Standard Test Method for Total Nitrogen in Water by High-Temperature Catalytic Combustion and Chemiluminescence Detection This test method is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.06 on Methods for Analysis for Organic Substances in Water. Current edition approved [date to come]. Published [date to come]. DOI:10.1520/Dxxx-xx.

Standard Test Method

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If Kjeldahl Was Alive Today, He Would Have a Shimadzu"

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