NECi Standard Operating Procedure

AMD / Total Nitrogen Analysis by Alkaline Persulfate Digestion Using Nitrate Reductase for Nitrate Determination / 11-13-13
Revision # / Date / Initials / Revision # / Date / Initials
Original / 11-13-13 / AMD / 1
2 / 3
4 / 5
Chemicals
Item / Manufacturer / Catalog #
Sodium Hydroxide / Sigma Aldrich / S8045-1KG
Potassium Persulfate / Sigma Aldrich / 37,982-4
100ppm Nitrate Standard / NECi
Labware
Item / Manufacturer / Catalog #
15mL Centrifuge Tubes with Flat Top Cap / Biologix Research Company / 10-9151
Ultrasonic Water Bath / Fisher Scientific / FS15
Autoclave
NECi Standard Range Nitrate Test Kit / NECi / L-NTK-201

I.Purpose

This method is intended to be used in conjunction with NECi Standard Range Nitrate Test Kit to provide a means of assessing the total amount of nitrogen in a water sample. Total nitrogen=organic nitrogen + ammonium + nitrate + nitrite. Alternatively, Nitrate + Nitrite can be determined by the NECi Discrete Analyzer Nitrate Analysis Method.

II.Stock Solutions

1. 500 mL 1.5M Sodium Hydroxide

30 g NaOH

500mL DI H2O

2. 50mL Alkaline Persulfate Solution

2 g K2S2O8

50mL DI H2O

III. Sample composition – 1.5mL total volume

1 mL Standard or sample

500 µL Persulfate solution

IV.Reagent Preparation

1. Sodium Hydroxide Solution (1.5M):

a. Dissolve 30 g of sodium hydroxide (NaOH, MW: 40.0) in approximately 350mL of DI H2O. Be cautious as NaOH releases heat when dissolved in water.

b. After complete dissolution, bring the volume to 500 mL with additional DI H2O. The solution has a shelf life of 6 months at room temperature.

2. Alkaline Persulfate Solution:

a. Add 2 g of potassium persulfate (K2S2O8, MW: 270.33) and 5 mL 1.5M NaOH in approximately 35mL of DI H2O.

b. Place into ultrasonic bath to dissolve the K2S2O8(~30 minutes)

c. Bring volume to 50mL with additional DI H2O. Prepare fresh solution before every set of assays. This is sufficient reagent for ~70 Total Nitrogen Assays.

3. Potassium Nitrate Standard:

a. Prepare stock solution (100mg N/L) by dissolving 16.31 mg of potassium nitrate (KNO3, MW: 101.1) in 100 mL of HPLC water. The stock solution is stable for 6 months when stored at 4°C. To create a working solution (10 mg N/L), dilute stock solution 10x with HPLC water. Further dilute the working solution to create standard solutions, using the volumes listed in the chart below:

NO3—N mg N/L / 10 ppm Nitrate Standard / HPLC water
0 / 0 / 5mL
2.5 / 1.25mL / 3.75mL
5 / 2.5mL / 2.5mL
7.5 / 3.75mL / 1.25mL
10 / 5mL / 0mL

Table 1. Preparation of Nitrate Calibrants

V.Procedure

1. Mix the samples and persulfate in a 2:1 ratio (1mL Standard or sample: 500µL persulfate solution) in autoclavable culture tubes with caps.

2. Autoclave the mixture at 250°F (121°C) and 17lb/in2 (117.2 kPa) for one hour. Set the autoclave for slow release of pressure, to ensure that no samples rupture due to a drastic change in pressure.

3. Remove samples from the autoclave and allow samples to cool to room temperature. These samples can be stored at room temperature for several days.

4. Analyze samples for total nitrate concentration using one of these methods:

a. NECi Standard Range Nitrate Test Kit: follow the test kit instructions using the Calibrants shown in Table 1.

b. Discrete Analyzer (DA) Nitrate Analysis Method: Use the NECi DA Reagent Pack for your instrument and apply standard QC/QA practices for Nitrate analysis.

VI. Calculations

1. Calculate the Total Nitrogen in the sample using a Standard Curve and the linear regression fit to the line. The values for the Total Nitrogen in the samples will be in units of mg N/L.

2. The Kjeldahl Method reports Total Nitrogen as mg N/L. Since the NECi Nitrate Test Kit is calibrated as mg N/L, the results of the analysis done here will be in the same units as the traditional Kjeldahl Method. Thus, there is no need to convert the results to any other form.

VII.Alternate Procedure to Correct for Nitrate and Nitrate in the Samples:

If the initial Nitrate + Nitrite, as mg H/L, is determined before the sample is digested, then the Total Nitrogen in the form of Organic Nitrogen and Ammonium can be calculated.

1. In a portion of the sample, analyze for Nitrate + Nitrite using either the NECi Nitrate Test Kit or the NECi Discrete Analyzer Analysis Method. This will yield the Total Nitrate+ Nitrite in mg N/L.

2. In the other portion of the sample, carry out the persulfate digestion as described above. Determine the Nitrate in the digested sample by either the NECi Nitrate Test Kit or the NECi Discrete Analyzer Nitrate Analysis Method. This will yield the Total Nitrogen (Organic Nitrogen + Ammonium + Nitrate + Nitrite) in mg N/L.

3. To calculate the NET Total Nitrogen as Organic Nitrogen + Ammonium in mg N/L, subtract the Total Nitrate + Nitrite in mg N/L in the sample from the Total Nitrogen + Ammonium + Nitrate + Nitrite) in mg N/L in the sample.

VIII.Waste Handling:

Follow all local regulations with regard to disposal of all toxic waste generated in using this method.