Malachite Green Method for Determining Phosphorus Concentration in Diverse Matrices

ABSTRACT

Sequential phosphorus (P) fractionation is a procedure often used to assess P dynamics in riparian soils and stream sediments. The most commonly used method to quantify P in fractionation extracts is the molybdate blue-ascorbic acid (MB-AA) colorimetric method. Because sequential P fractionation generates a large number of samples, it would be advantageous to use a single, rapid analytical technique to determine P concentrations. Thus, we investigated the malachite green (MG) colorimetric method for determining P concentrations in the diverse sequential fractionation matrices. For the MG method, a 96-well standard microplate was used, and absorbance was read on a universal microplate reader. We found that the MG and MB-AA methods agreed for determining P concentration in extracts of water, 1 M hydrochloric acid (HCl), and concentrated sulfuric acid (H₂SO₄). Small differences between the two methods were found for extracts of 0.5 M sodium bicarbonate (NaHCO₃) and 0.1 M sodium hydroxide (NaOH), and concentrated HCl, most likely related to differing characteristics of the sediments that were analyzed. Despite this discrepancy for some extracts in the sequential extraction, the microplate system-MG method can be valuable because of its simplicity, sensitivity, and rapidity compared to the MB-AA method.

KEYWORDS:

• Phosphorus fractionation
• sequential extraction
• malachite green colorimetric method
• molybdate blue-ascorbic acid colorimetric method

Acknowledgments

This research was partially funded by the USDA-NIFA Agriculture and Food Research Initiative Competitive Grant No. 2013-67019-21393. Mention of trade names does not imply recommendation or endorsement by Iowa State University or the USDA Agricultural Research Service.

Additional information

Funding

This work was supported by the USDA-NIFA Agriculture and Food Research Initiative [2013-67019-21393].