Evaluation of a Rapid, Automated Titration System for Evaluating Lime Requirements for Peat-based Substrates

Determining lime requirements for peat-based substrates is a critical yet time-consuming process for substrate formulators.  Typically, the substrate is incubated with lime over a period of days to allow the base sufficient time to react.  Automatic acid-base titration offers a quicker solution that requires only hours to generate a multipoint, smooth curve that allows accurate interpolation for determining the needed base to achieve a target substrate pH.  However, the titration parameters that give the best fit to lime-incubation data have yet to be fully evaluated.  Samples of screened sphagnum peat moss (1.5 g dry weight) were titrated using 0.1 N HCl and 0.1 N NaOH by means of an automatic titrator (Titralab®856, Radiometer Analytical SAS, Villeurbane Cedex, France).  Degassed, deionized water was used to raise peat samples to 50 g.  Samples were then taken through two pH cycles between pH 3 and 11.  The first cycle promoted hydration of the peat, and the second cycle was used to determine the pH buffer capacity.  We varied titration parameters to run samples under eight regimes: titrating at fast, moderate, and slow rates (maximum 7.00, 2.00, and 0.33 mL acid/base per minute, respectively); fast rate with slowed addition near endpoints; changing the forward titration endpoints from pH 11 to pH 8 for one or both cycles; and titrating at a moderate rate after shaking for 2 hours or 24 hours at approximately pH 11.  The volume of acid or base added was then converted into the moles of hydroxyls or protons bound per gram peat.  Hysteresis and pH drift were observed.  To determine which parameters were most closely aligned with actual incubation pH data, rates for CaCO₃ were calculated from automatic titrator curves, and samples of 20 g screened peat were incubated with reactive CaCO₃ for 7 days.  The curve generated by shaking for 2 hours at pH 11 (maximum 2.00 mL acid/base per minute) was closest to the actual incubation pH data.  All curves underestimated the amount of CaCO₃ required; however, parameters that allowed more exposure time to high pH (slowing addition near endpoint, shaking) came closer to the actual incubation data.  Thus, for substrate formulators wishing to use automatic titrators to determine lime requirements, a conversion factor will be required.  These parameters offer a quicker and more accurate approach.  Additionally, our methodology describes a protocol for evaluating parameters and comparing them with incubations of novel substrates.