Reagents:

A. 4 g phenol/ 500 ml of ethanol (stable)
B. 0.375 g Sodium nitroprusside (sodium nitroferricyanide)/ 500 ml distilled water. (Store in dark plastic bottle, stable for 1 week)
C. 15 g trisodium citrate and 0.8 g NaOH/ Liter distilled water (stable)
D. Oxidizing solution: 4 ml sodium hypochlorite (NaOCl, 5%)/ 100ml of alkaline solution C. (Prepare just before use)

Ammonia Standard: Dissolve 2.6745 grams of NH₄Cl, dried overnight at 100 ^oC, in one liter of distilled water (5OmM stock solution). Dilute to 25, 50, 100, 150 and 250 mM standards or as required.

Approximate sample size:

• Pore water - 0. 1 to 2 ml
• Overlying water - <1 ml to 3-5ml
• Standards - 1 ml standard

1. Pipet sample or standard into scintillation vial using an automatic pipet.
2. Add 1 ml of solution A (phenol solution). The samples may be refrigerated at this stage and analysis completed up to at least two weeks later.
3. Dilute samples to uniform volume (from 1 to 3 ml)
4. Add 1ml of solution B (Nitroprusside) using a repipet bottle.
5. Add 2 ml of solution D (oxidizing solution) using a repipet bottle.
6. Allow color to develop for 4 to 6 hours.
7. Read absorbance at 640nm.

Notes: Blanks should not be higher than 0.010 absorbance units.

References:
  Presley, B.J. (1971) Appendix: Techniques for Analyzing Interstitial Water Samples. Part I: Determination of Selected Minor and Major Inorganic Constituents. In Winterer et al., 1971, Initial Reports of the Deep Sea Drilling Project, Volume VII. Washington, U.S. Government Printing Office, pp. 1749-1755.
  Solórzano, L. (1969) Determination of Ammonia in Natural Waters by the Phenolhypochlorite Method. Limnol. Oceanogr., 14: 799-801.