Kjeldahl nitrogen is a typical titration application, following digestion and ammonia. The ASTM D8001 standard now provides an alternative approach involving persulfate digestion and subsequent IC determination without the need for distillation.
Moreover, total phosphorus and total nitrogen can be determined using this method. This article shows the results of control samples consisting of organic substances. Since these organic substances are dissolvable in ultrapure water, it is possible to determine the nitrogen concentration corresponding to Kjeldahl Nitrogen and Total Nitrogen.
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Sample
Nitrogen and phosphorous containing compounds in ultrapure water.
Sample preparation
Alkaline persulfate digestion as outlined in the ASTM D8001 standard before Metrohm inline ultrafiltration and IC analysis.
Columns
| . |
. |
| Metrosep A Supp 16 - 250/4.0 |
6.1031.430 |
| Metrosep A Supp 16 Guard/4.0 |
6.1031.500 |
Solutions
| Eluent |
5.5 mmol/L sodium carbonate
2.5 mmol/L sodium hydrogen carbonate |
| Suppressor regenerant |
500 mmol/L sulfuric acid |
| Rinsing solution |
STREAM |
Analysis
Conductivity detection following the sequential suppression
Instrumentation
| 930 Compact IC Flex Oven/SeS/PP/Deg |
2.930.2560 |
| IC Conductivity Detector |
2.850.9010 |
| 858 Professional Sample Processor |
2.858.0020 |
| 800 Dosino (suppression) |
2.800.0010 |
| IC equipment: Inline Ultrafiltration |
6.5330.110 |
| MSM Rotor A |
6.2832.000 |
| Adapter sleeve for Suppressor Vario |
6.2842.020 |
Parameters
| Flow rate |
0.8 mL/minutes |
| Injection volume |
20 μL |
| Pmax 15 |
MPa |
| Recording time |
42 minutes |
| Column temperature |
45 °C |
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Method detection limit (MDL)
Calculated from the standard deviation of 7 injections
| |
Nitrate-N [μg/L] |
Phosphate-P [μg/L] |
| MDL-1 |
14.9 |
17.2 |
| MDL-2 |
13.5 |
14.6 |
| MDL-3 |
15.6 |
14.6 |
| MDL-4 |
14.4 |
16.9 |
| MDL-5 |
14.5 |
18.0 |
| MDL-6 |
13.2 |
15.6 |
| MDL-7 |
15.8 |
16.2 |
| Mean |
14.6 |
16.4 |
| Std. dev. |
0.98 |
1.11 |
| MDL (3.14 x SD) |
3.1 |
3.5 |
Nitrate-N: nitrate as nitrogen
Phosphate: phosphate as phosphorus
Results
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| Sample |
Nitrate as nitrogen |
Phosphate as phosphorus |
| |
expected [mg/L] |
actual [mg/L] |
recovery [%] |
expected [mg/L] |
actual [mg/L] |
recovery [%] |
| NH4Cl |
1.0 |
1.06 |
106 |
- |
- |
- |
| Glycine |
2.5 |
2.63 |
105 |
- |
- |
- |
| Tripolyphosphate |
- |
- |
- |
10.0 |
9.10 |
91 |
| Phytic acid |
- |
- |
- |
5.0 |
4.22 |
84* |
* Phytic acid recovery is improved when using slightly higher persulfate concentration
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This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.
For more information on this source, please visit Metrohm AG.