In recent years, humic acid has garnered significant attention in the market, with a growing variety of products and a steady shift toward industry standardization. Different types of humic acid require distinct testing methods, and the same product may yield significantly different results under varying standards.
This article provides an in-depth overview of the various testing methods for humic acid, including applicable standards and characteristics, to aid in understanding quality indicators within humic acid products.
I. Humic Acid Content Testing Methods
Humic acid content is the primary measure of product quality and raw material standards. The main testing approaches focus on two categories: soluble humic acid (for finished products) and free humic acid (for raw materials).
1. Testing Soluble Humic Acid Content
Soluble humic acid mainly refers to ionic forms of humic acid present in mineral-derived humic acid fertilizers and humic salts, serving as a critical quality indicator. The primary methods for testing soluble humic acid content include volumetric and gravimetric analyses.
Volumetric Method
Principle: Humic acid samples are dissolved in water and treated with sulfuric acid. Potassium dichromate is then used to oxidize the carbon content in the sample, and the consumption of potassium dichromate is measured by titration to calculate humic acid content.
Advantages: This mature method is highly reliable, reproducible, and stable, making it one of the most commonly used testing methods in both laboratories and production environments. Most third-party testing laboratories adopt this method as a default standard.
Limitations: The method does not distinguish between fulvic acid and other organic matter, potentially leading to an overestimation of humic acid content, especially when the sample has a high fulvic acid concentration.
Common Standards: HG/T 3278-2011 "Humic Acid Sodium for Agriculture" and GB 11957-2001 "Determination of Humic Acid in Coal."
Gravimetric Method
Principle: Soluble humic acid salts in the sample are dissolved, the solution’s pH is adjusted to 1 with acid, filtered with a constant-weight filter paper, and the precipitate is washed with hydrochloric acid until colorless. The ash content is subtracted to determine the soluble humic acid content.
Advantages: Compared to the volumetric method, the gravimetric method removes interference from fulvic acid and other acid-soluble organic matter, providing a more accurate measurement of humic acid.
Limitations: This method is more complex, time-consuming, and has lower reproducibility, with the same sample often yielding varied results.
Common Standards: GB/T 33804-2017 "Humic Acid Potassium for Agriculture," HG/T 3278-2018 "Humic Acid Sodium," and NY 1106-2010 "Humic Acid-Containing Water-Soluble Fertilizers."
2. Testing Free Humic Acid and Total Humic Acid in Raw Materials
· Total humic acid consists of free humic acid and complexed humic acid, representing all humic acids present in coal. Within coal, humic acids are classified as readily extractable free humic acid and complexed humic acid, which binds with metal ions like calcium, magnesium, and iron, forming humates that are more challenging to extract. Free humic acid can be extracted with NaOH, while complexed humic acid requires extraction with sodium pyrophosphate.
· In Leonardite, testing focuses primarily on free humic acid, as this is the alkaline-extractable component that determines the soluble humic acid content in potassium or sodium humate products. Many products marketed as “raw humic acid powder” are essentially unactivated Leonardite powder, with low utilization of free humic acid.
· Testing Methods for Total and Free Humic Acid: GB 11975-2001 "Determination of Humic Acid Yield in Coal."
II. Methods for Testing Fulvic Acid Content
Fulvic acid testing within the industry mainly follows three standards:
HG/T 5334-2018: "Potassium Fulvic Acid"
GB/T 34765-2017: "Determination of Fulvic Acid Content in Mineral Sources"
NY/T 3162-2017: "Determination of Fulvic Acid in Fertilizers by Volumetric Titration"
The most widely used standards are HG/T 5334-2018 and GB/T 34765-2017.
1. Comparison of HG/T 5334-2018 and GB/T 34765-2017
Standard | Reaction Principle | Solution Differences | Method Distinctions | Distinguishes Biogenic Fulvic Acid | Results Comparison |
HG/T 5334-2018 | Same as below | - | Uses trisodium phosphate to remove cations | Yes, via fluorescence wavelength | Tends to be higher, widely applicable |
GB/T 34765-2017 | Same | Adds chloride ion solution | Removes both cations and chloride ions | No | Tends to be lower, more accurate for mineral sources |
2. NY/T 3162-2017 Standard
NY/T 3162-2017 uses a 0.2 mol/L sulfuric acid solution to extract fulvic acid, followed by oxidation with potassium dichromate and sulfuric acid. This method applies a unique carbon factor for fulvic acid, which often leads to slightly lower results compared to other standards.
In practice, HG/T 5334-2018 and GB/T 34765-2017 are generally preferred due to their broader application and higher accuracy for mineral-derived fulvic acids.
III. International Standards
Different countries utilize various methods to test humic acid, with several internationally recognized standards listed below:
CDFA Method (California Method)
Developed by the California Department of Food and Agriculture (CDFA), this method separates humic and fulvic acids, removing fulvic acid to measure the remaining humic acid. This test includes organic ash content, which may lead to higher reported humic acid levels, especially in products with high ash content. Additionally, as California does not recognize fulvic acid, samples with fulvic acid content often show lower results under this method.
ISO 19822-2018
In 2014, Richard T. Lamar introduced a new humic acid testing method, which the International Humic Substances Society promoted and was eventually adopted as an ISO standard. This global standard uses DAX-8 resin to adsorb hydrophobic fulvic acid, providing accurate measurements of hydrophobic fulvic acid content. However, it does not detect hydrophilic fulvic acid components such as polysaccharides, amino sugars, and amino acids. Lignosulfonate products can also degrade the DAX-8 resin, necessitating prior differentiation of lignin and fulvic acid using infrared spectroscopy. This method does not apply to biogenic fulvic acid products.
IV. Differences in Standards
In the market, some humic acid products report high content levels due to the use of “inflated” testing methods, such as the volumetric method (HG/T 3278-2011). Given the variety of standards, the same product may yield different results under different testing protocols, and some vendors may even label products with high content levels without proper testing.
Consumers are advised to pay close attention to the testing standards used when selecting humic acid products with high reported content and to verify with manufacturers regarding the applied testing methods. High-quality humic acid products undergo rigorous testing, and awareness of testing standards can help avoid misleading claims.
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