Laws of soil chemistry.pptx

Important Laws of soil chemistry in Soil Science. #Agriculture #Soil Science #laws in soil chemistry #goldschmidt's laws #ostwalddilutionlaw #lawofmassaction #schofiledratiolaw #inverratiolaw

1. By – ANJALI RAJ 21412SAC005 M.Sc. Ag. (Final Year) Department of Soil Science and Agricultural Chemistry IMPORTANT LAWS OF SOIL CHEMISTRY

2. The laws governing structural characteristics of the phyllosilicates are known as Goldschmidt’s Laws – Law I and Law II. GOLDSCHMIDT’s LAW I In an ionic crystalline compound, Isomorphous replacement of one cation by another, without incurring any change in the order of the crystal pattern, is permitted provided that the radii of the cation replaced and the cation substituting it agree with in 15%. GOLDSCHMIDT’s LAW II Also known as “ Radius Ratio Law”. When the number of anions surrounded a central cation such that they satisfy the charge completely, the number of anions that can be so accommodated around the cation depends on the radius the central cation to that the surrounding anions. Thus, this radio governs is known as “Coordination Number”. CONCLUSIONS In Tetrahedral site, a sphere of radius up to 0.41 times and in Octahedral site a sphere of radius up to 0.73 times can be accommodated. Si 2+ remain only in four – fold coordination state in tetrahedral sheet while Al 3+ have both four- and six – fold coordination state in tetrahedral and octahedral sheets of phyllosilicates. Cations such as Mg 2+ , Fe 3+ and Fe 2+ can be accommodated only in Octahedral sites. K + ion can be accommodated in a Dodecahedral arrangement of oxide ions, in between the two hexagonal arrays of oxide ions. Victor Moritz Goldschmidt (1888-1947) Nationality – Norwegian Fields - Geochemist, Mineralogist, Scientist Father of Modern Geochemistry and Crystal Chemistry Awards – Foreign Member of the Royal Society Elliott Cresson Medal  (1903) Wollaston Medal  (1944) GOLDSCHMIDT’s LAWS

3. This law states that “ the rate of any  chemical reaction   is proportional to the product of the masses of the reacting substances, with each mass raised to a power equal to the coefficient that occurs in the  chemical equation .” Where, K = equilibrium constant α , β = stoichiometric coefficients for the reaction [A’] = concentration of reactant A [B’] = concentration of reactant B [A] = concentration of product A [B] = concentration of product B Applications Different ion exchange equations for Homovalent ions given by Glen ( 1913 ), Kerr ( 1928 ) . and Vanselow ( 1932 ) . Ion exchange equations for Heterovalent ions given by Gapon ( 1933 ) . Cato Maximilian Guldberg ( 1836-1902 ) & Peter Waage ( 1833-1900 ) Nationality – Norwegian Institutions – Royal Frederick University Fields – Chemistry (both) Mathematics (Guldberg) Awards- Guldberg - Order of St. Olav Order of the Dannebrog Order of Vasa Order of the Polar Star Order of Charles XIII Waage – Order of St. Olav LAW OF MASS ACTION

4. This law states that “When cations in a dilute solution are in equilibrium with a larger number of exchangeable ions, a change in the concentration of the solution will not disturb the equilibrium if the activities of all the monovalent cations are changed in one ratio, those of all the divalent cations in the square of that ratio, and those of all the trivalent cations in the cube of that ratio.” This law valid if – Soil solution is fairly dilute There are no appreciable negative charges on the soil colloid Non – exchangeable ions are not released from the clay lattice. Applications In Donnan membrane equilibrium to describe the description of distribution of ions between the exchange phase and the soil solution phase in equilibrium with each other. Reginald Keith Schofield (1913-1979) Nationality – British (England) Alma Meter - University of Reading Known for – Research in soil cations and Schofield ratio law Awards – Waksman Medal from the Soil Science Society of America in 1963. SCHOFIELD RATIO LAW

5. The disturbance of the position of equilibrium between the exchange and the solution phase due to change in concentrations (or activities) of the ions in the exchange phase. This is dealt by the inverse ratio law . This law ensues that – The monovalent cations are more compatible with the similar (but oppositely charged) ionic fields of the low CEC clays. The divalent cations are more compatible with the similar (but oppositely charged) ionic fields of the high CEC clays. Applications In nutrient uptake by plants – Monocotyledons have a lower root CEC so they exhibit a preference for the monovalent cations. Dicotyledons have a higher CEC so they exhibit a preference for the divalent cations. INVERSE RATIO LAW Elmer Clarence Matson (1900 - 1988) Nationality – American Alma meter - University of Minnesota , University of Wisconsin Institutions - University of Wisconsin Known for – Inverse ratio law, research on availability of nutrients Awards – Soil Science Award, in 1967

6. Ostwald’s dilution law states that “ The degree of dissociation of a weak electrolyte is inversely proportional to the square root of molar concentration or directly proportional to the square root of volume holding one mole of the solute for a weak electrolyte.” O stwald dilution law equation – Limitations The law only applies to weak electrolytes. T he behavior of ions in solution is ideal, which means that the ions are evenly distributed and that there are no interactions between them. In reality, the behavior of ions in solution can be influenced by various factors, such as the presence of other ions, the temperature, and the pressure. It assumes that the degree of dissociation of a weak electrolyte remains constant at all dilutions. does not account for the effect of activity coefficients on the behavior of ions in solution OSTWALD DILUTION LAW K a = α ²C/(1-a) Friedrich Wilhelm Ostwald (1852-1932) Nationality – Russian Fields – Physical Chemistry Alma mater - Imperial University of Dorpat Awards – Faraday Lectureship Prize  (1904) Nobel Prize for Chemistry  (1909) Wilhelm Exner Medal  (1923)