tailieunhanh - Humic Matter in Soil and the Environment: Principles and Controversies - Chapter 7

Điện TÍNH CỦA VẤN ĐỀ Humic 7,1 xứ, LOẠI PHÍ ĐIỆN Humic và axit fulvic được coi là hợp chất lưỡng tính, nhưng Stevenson (1994) giả định chúng là axit yếu. Tuy nhiên, nó cũng biết rằng các chất humic có thể, trên thực tế, phản ứng với cả hai cơ sở và các axít, do đó thực hiện cả hai điện tích dương và âm. Những đặc tính và hành vi được coi là đặc điểm khác biệt của các chất lưỡng tính. Các điện tích âm thường được nghiên cứu sâu hơn và do đó là tốt hơn được biết. | CHAPTER ELECTROCHEMICAL PROPERTIES OF HUMIC MATTER ORIGIN AND TYPES OF ELECTRIC CHARGES Humic and fulvic acids are considered amphoteric compounds but Stevenson 1994 assumes them to be weak acids. However it is well known that humic substances can in fact react with both bases and acids hence carry both positive and negative charges. These properties and behavior are regarded as distinctive characteristics of amphoteric substances. The negative charges are usually studied more intensively and consequently are better known than the positive charges. All these charges are developed by the ionization or dissociation of various functional groups. . Negative Charges The negative charges are attributed to dissociation of protons from the functional groups in the humic molecule. The two most Copyright n 2003 by Marcel Dekker Inc. All Rights Reserved. important functional groups in this respect are the carboxyl and phenolic-OH groups. In general these two functional groups control the electrochemical behavior of humic matter and are the main reasons for adsorption cation exchange complex and chelation reactions. The carboxyl COOH groups start to dissociate their protons at pH Posner 1964 and the humic molecule becomes negatively charged Figure . At pH the charge is very small or even zero. At pH the phenolic-OH groups also dissociate their protons and the humic molecule attains a high negative charge. Since the development of the negative charge is pH dependent this charge is called pH dependent charge or variable charge Tan 1998 . At low pH the charge is expected to be low whereas at high pH the negative charge is high which corresponds to low cation exchange capacity CEC at low pH and high cation exchange capacity at high pH. According to the theory of CEC the negative charge will eventually reach a maximum value at pH . This will be explained further in Section on cation exchange capacity. The Significance of the Henderson -Hasselbalch .