Samples are digested and oxidized with potassium persulfate and 3.75 N NaOH to release phosphorus from combination with organic matter. Unfiltered samples measure total phosphorus, both dissolved and particulate, organic and inorganic. Samples filtered through a 1.0 um glass fiber filter or a 1.0 um Nuclepore 47 mm membrane measure total dissolved phosphorus, organic and inorganic.
In a 1000 mL volumetric flask, dissolve 4.394 g anhydrous potassium dihydrogen phosphate (KH2PO4) to the mark with deionized water. This standard stock solution is 1000 mg P/L as PO43-.
III) Preparation of Reagents
A) 3.75 N NaOH
Add 135 g sodium hydroxide (NaOH) slowly to 900 mL of deionized water. Stir until dissolved. Store in a plastic container.
In a 500 mL volumetric flask, dissolve 25 g potassium persulfate (K2S2O8) and 75 mL 3.75 N sodium hydroxide (NaOH) to the mark with deionized water.
C) 0.6 N HCl
In a 500 mL volumetric flask, dilute 25 mL concentrated hydrochloric acid (HCl) to the mark with deionized water.
D) 0.035 N NaHCO3
In a 500 mL volumetric flask, dissolve 1.5 g Sodium Bicarbonate (NaHCO3) to the mark with deionized water.
IV) Sample Preparation
Store samples under refrigeration and analyze as soon as possible after collection. If samples are to be stored for a long period, store frozen.
B) Add 0.2 mL oxidizing reagent.
C) Cap tubes tightly, shake and autoclave (liquid cycle) for 1 hour. Remove from autoclave and allow to cool to room temperature.
D) Add 0.1 mL 0.6 N HCl. Shake well.
E) Add 0.8 mL 0.035 N NaHCO3. Shake.
F) Samples can be stored several days refrigerated.
G) Analyze total and total dissolved phosphorus with the Lachat Quikchem 8500 Flow Injection Analyzer orthophosphate method.
H) Calibrate with standards that bracket the sample concentration. Correlation coefficient should be greater than or equal to 0.950. Check the calibration curve for drift, accuracy and precision with standards and controls every 20 samples.
KIOWA CHEMISTRY LABORATORY Procedure Manual
Last Rev. June 14, 2013 by: Chris Seibold