A preliminary study to measure total water in lint cotton demonstrated that volumetric Karl Fischer Titration of moisture transported by a carrier gas from an attached small oven is a more accurate method than standard oven drying in air. The objective of the present study was to assess the measurement reproducibility of this new reference method as a function of sample type, sample sequence and condition of the working medium in the titration cell. Sensor response was linear and accurate over the range of 5.5 to 8.5 mg water, which corresponds to 5.5 to 8.5 % water in 100 mg cotton (recommended sample size). Optimal conditions were: oven temperature, 150oC; 270 sec of drying, and nitrogen gas. The actual temperature inside the sealed glass sample vial was 15 to 25oC less than the oven temperature. Karl Fischer selectivity to water over interfering substances increased with extent of cleaning: raw, mechanical cleaning, and scoured and bleached. In sequential analysis of cotton using the same working medium in the titration cell, the non-aqueous volatiles caused a negative bias (trend) in measured water contents. This was prevented by alternating six replicates of cotton and one replicate of: a blank vial, a water standard vial or adding fresh working medium to replace spent medium in the titration cell. Method validation involved checking for residual water in cotton by NIR spectra taken through the bottom of the vial and comparing results from a second reference procedure developed in this laboratory – Low Temperature Distillation.
