Total Phosphorus/Total Dissolved Phosphorus:

Lachat QuikChem 8500 Flow Injection Analyzer

I) Principle

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.

II) Preparation of Stock Standard

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.


B) Oxidant


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.

V) Analysis Procedure

A) Add 12 mL of standard, control or sample to appropriate glass culture tube with a teflon-lined screw cap.


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.


Revised 24 February 24, 2012 Chris Seibold


Mark Williams at
Chris Seibold at seiboldc@
with questions or comments.

Last Rev. June 14, 2013 by: Chris Seibold