A New Buffer that Mimics the SMP Buffer for Determining Lime Requirement of Soil.
Frank Sikora, Univ of Kentucky, 103 Regulatory Service Bldg., Lexington, KY 40546-0275
The SMP buffer used for determining lime requirement on soil contains chromium and p-nitrophenol which classifies the solution as a hazardous waste. A buffer without hazardous chemicals producing the same pH as SMP buffer would eliminate hazardous waste and have no affect on agronomic interpretation. Chemicals chosen to replace chromium and p-nitrophenol were 2-(N-morpholino)ethanesulfonic acid monohydrate (MES) and imidazole. Acid titrations of SMP buffer and various mixtures of a new buffer were performed to duplicate the acid-base characteristics of the SMP buffer. The new buffer is adjusted to pH 7.7 and contains 69.6, 13.7, 31.4, 89.3, and 2000 mM of triethanolamine, imidazole, MES monohydrate, acetic acid, and KCl, respectively. Coefficient of determinations (r2) for linear regressions of soil-buffer pH with the new buffer versus SMP buffer on 255 Kentucky and 90 North American Proficiency Testing (NAPT) soils were 0.974 and 0.963, respectively. Repeated soil-buffer pH measurements were made using 14 internal quality control soils and 10 NAPT soils over 150 days. Only one of the soil samples showed a significant drift in soil-buffer pH with time using the new buffer, and the drift was also observed using SMP buffer. With 112 measurements of soil-buffer pH using the new buffer on 14 NAPT soils, only 1 measurement fell outside NAPT warning limits for SMP soil-buffer pH. The linear regression equation for new buffer versus SMP buffer lime recommendations was y=0.119 + 0.994 (x) with an r2 of 0.971 for 158 Kentucky soils requiring lime and y=0.800 + 0.849 (x) with an r2 of 0.845 for 33 NAPT soils requiring lime. The new buffer without hazardous chemicals mimicked the soil-buffer pH obtained with SMP buffer, provided lime recommendations similar to that with SMP buffer, and had a serviceable life of at least 150 days.