Fulvic acid (FA) and humic acid (HA) are key components of humic substances, widely used in agriculture, environmental protection and other fields. However, their detection results often show great differences, which brings troubles to quality control and application. The main reasons for these differences lie in the diversity of detection methods and the poor stability of the substances themselves, which are detailed as follows.
The lack of unified standardized detection methods is the primary factor leading to large result differences. At present, there is no globally recognized uniform detection standard for FA and HA, and different laboratories and industries adopt different detection technologies^{(6)}. For humic acid, common methods include colorimetric method, CDFA method and HPTA method. The colorimetric method is a simple qualitative test, which is widely used for product registration and labeling, but it cannot distinguish FA from HA and is easily interfered by additives such as lignosulfonate, resulting in artificially high results. The CDFA method is more accurate and effective in identifying non-humic additives, but it cannot fully account for ash content, which may cause deviations. The HPTA method, as the most accurate quantitative method, can effectively remove interference, but it is only used in a few regions at present. For fulvic acid, the HPTA method and gravimetric method are commonly used, and the differences between these methods also lead to inconsistent results. Additionally, different extraction methods for FA and HA, such as IHSS method and NAGOYA method, can also cause deviations in detection results^{(7)}.
The poor stability of FA and HA themselves further amplifies the detection differences. Both substances are complex organic mixtures with dynamic structures, and their properties are easily affected by environmental conditions^{(2)}. Their colloidal stability varies with pH value and electrolyte concentration: higher pH value enhances their stability through electrostatic repulsion, while increased electrolyte concentration leads to aggregation and destabilization. Compared with HA, FA has stronger dispersion stability due to more surface negative charges, but it is more prone to structural changes under the influence of external factors such as UV radiation, which can alter their functional groups and stability^{(5)}. In addition, the degree of humification, molecular weight distribution and functional group content of FA and HA from different sources (such as soil, peat and lignite) are quite different, which makes their response signals in detection inconsistent. Long-term application of organic materials can also change the structure and stability of FA and HA, further affecting detection results^{(2)}.
Other factors also contribute to detection differences. Sample pretreatment is crucial: improper operation such as high temperature or strong acid during pretreatment will destroy the structure of FA and HA, leading to inaccurate extraction of effective components. In addition, the accuracy of detection instruments and the calibration of standard substances will also affect the results. For example, uncalibrated spectrophotometers or non-standard reference substances will lead to deviations in absorbance measurement and result calculation.
In conclusion, the differences in detection methods, the poor stability of FA and HA, and improper sample pretreatment are the main reasons for the large differences in their detection results. Establishing a unified international detection standard, standardizing pretreatment operations and improving instrument accuracy are effective ways to solve this problem and ensure the reliability and comparability of detection results.
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[5] Environmental Science Editorial Department. Effect of UV Radiation on Chemical Stability of Humic Acid[J]. Environmental Science, 2013, 34(10): 4027-4033.
[6] Li S X. Humic Acid Product Analysis and Standards[M]. 7-122 Press, 2007.
[7] Kuwatsuka S, Watanabe A, Itoh K, et al. Comparison of two methods of preparation of humic and fulvic acids, IHSS method and NAGOYA method[J]. Soil Science and Plant Nutrition, 1992, 38(1): 23-30. DOI:10.1080/00380768.1992.10416948.
Q1: Why is there no globally unified detection standard for fulvic acid (FA) and humic acid (HA)?
A1: FA and HA are complex organic mixtures with diverse sources (e.g., soil, peat, lignite) and variable structures. Different countries and industries have different application scenarios and quality requirements for them, leading to the lack of a unified international standard. At present, various regions adopt their own common methods, such as HPTA, CDFA and colorimetric methods, which further causes inconsistent detection results.
Q2: Which detection method is the most reliable for FA and HA?
A2: The HPTA method is generally recognized as the most accurate quantitative method. It can effectively remove interference from additives (e.g., lignosulfonate) and ash, ensuring the accuracy of detection results. However, due to its complex operation and high cost, it is only used in a few regions and professional laboratories. For daily rapid detection, the colorimetric method is more commonly used, but it should be noted that it may have deviations caused by interference.
Q3: Can the structural changes of FA and HA be avoided during detection?
A3: It is difficult to completely avoid structural changes, but they can be minimized by standardizing sample pretreatment. For example, avoiding high temperature, strong acid and strong alkali during pretreatment, storing samples in a dark and low-temperature environment, and completing detection as soon as possible after sample collection. These measures can reduce the impact of external factors on the stability of FA and HA, thereby improving detection accuracy.
Q4: Do different extraction methods affect the detection results of FA and HA?
A4: Yes. Different extraction methods (such as IHSS method and NAGOYA method) have different extraction efficiencies for FA and HA. Some methods may not fully extract the effective components, while others may extract impurities together, leading to deviations in the final detection results. Therefore, standardizing the extraction method is an important measure to reduce detection differences.
