Determination of bicarbonate in substrate extracts using back-titration and the Gran Plot approach

The back-titration procedure, associated with the Gran Plot method to detect the process end point, was compared with other volumetric procedures for the potentiometric acid-base titration determination of bicarbonate and was explored as a tool for the determination of the bicarbonate content in substrates used as growing media. This procedure was compared with the second derivative approach as well as with continuous titration with standard HCl on the extract solution ("direct measurement"), until reaching pH 4.0. By using known standard samples, it was noted that the results obtained with the second derivative method were both lower and inconsistent, because of the high dilution of both the sample solution and the titrant. On the other hand, the continuous direct titration (until pH 4.0) presents positive errors due to the presence of dissolved CO2 in the titrated solution. Although comparable in precision, the best performance and accuracy were observed for the back-titration with the Gran Plot approach. These aspects, associated with its low cost and inherent operational simplicity, permit us to suggest the Gran Plot method as a reliable option for routine determination of bicarbonate in substrate extracts and related materials.