[ttl8792]

I have a question about Nitrate-Nitrogen determination in soil based on the following SOP.

"
This method involves the quantitative extraction of nitrate nitrogen (NO3-N) from soils using 2.0 M KCl.  Nitrate is determined by reduction to nitrite (NO2-N) via a cadmium reactor, diazotized with sulfanilamide and is coupled to N-(1-Napthyl)-ethylenediamine dihydrochloride to form an azochromophore (red-purple in color) measured spectrophotometrically at 520 nm.  The method is readily adapted to manual or automated techniques.  The procedure follows that of Keeney and Nelson (1982) for determining nitrate nitrogen with a modification in which 25 mL of KCl and 5.0 g of soil are used instead of 100 mL and 10 g soil.  Extending the shaking period to thirty minutes with 2.0 M KCl (Bremner and Keeney, 1965) permits the simultaneous extraction of ammonium and nitrate . . . .

Procedure

   1.   Weigh or scoop 5.0 ± 0.05 g of air-dried soil pulverized to pass 10 mesh sieve (< 2.0 mm) into extraction vessel.  Add 25.0 mL of 2.0 M KCl extraction solution using repipette dispenser (See Comment #2).  Include a method blank.

   2.   Place extraction vessel(s) on reciprocating mechanical shaker and shake for thirty (30) minutes.

3.   Filter extract (See Comment #3), refilter if filtrate is cloudy (comment #4). 

   4.   Nitrate-N content of the extract is determined using a spectrophotometer, automated flow analyzer (Technicon Method No. 329-74W/A) or FIA instrument.  Calibrate using standard calibration solutions and operate instrument in accordance with manufacturer instructions.  Determine nitrate concentration of KCl extract, method blank, unknown samples and record results as mg L-1 of nitrate in extract solution (See Comment #5).


Calculation

1.   NO3-N mg kg-1 in soil = (NO3-N mg L-1 in filtrate - method blank) × 5   

2.   Report soil nitrate nitrogen concentration to the nearest 0.1 mg kg-1 (See Comment #6).

"

I for the life of me can't figure out why you would multiply the results by 5, as instructed in the calculation section.

What am I missing?

[mjc123]

Look at the units on both sides of the equation.

[ttl8792]

so, we're using:

mg/kg = mg/L (ppm) * volume of solution added / mass of soil sample

This makes sense, and was what I was assuming just purely based on the math.

So, it this is true, why do we not assume, like with most dilute aq. solutions, that mg/kg = mg/L = ppm?

[Borek]

Compare mass of the sample with mass of the solution.

[ttl8792]

mass of sample is 5 g, and mass of solution is ~ 25 g.

[Borek]

And you still don't see what is going on?

[mjc123]

So, it this is true, why do we not assume, like with most dilute aq. solutions, that mg/kg = mg/L = ppm?

mg/(kg OF WHAT?)
mg/(L OF WHAT?)

[ttl8792]

Ya, I see it now.

I thought it was something silly I was overlooking.

Thanks, all.