Location: Cotton Structure and Quality Research
Title: Reference method for total water in lint cotton by automated oven drying combined with volumetric Karl Fischer titration Authors
Submitted to: Journal of Cotton Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 2, 2011
Publication Date: July 13, 2011
Citation: Montalvo Jr, J.G., Von Hoven, T.M., Cheuk, S.Y. 2011. Reference method for total water in lint cotton by automated oven drying combined with volumetric Karl Fischer titration. Journal of Cotton Science. 15:189-205. Interpretive Summary: Water in cotton is important because the fiber properties (mechanical, dimensional and electrical) are dependent on the water content. The standard test methods for water in lint cotton are based on oven drying at 105oC; all of the loss in weight is attributable to moisture. A more accurate reference method was developed based on oven drying in combination with Karl Fischer Titration of the water vapor from the dried cotton. Karl Fischer Titration, used in standard test methods worldwide, involves titrating water with an iodine reagent. Our approach in these studies was to conduct a series of probing experiments to help understand the critical variables in the Karl Fischer method, such as, response to cotton and sample size; oven temperature, drying time and carrier gas to transport moisture vapor into the titration cell; temperature variability in the sample container; selectivity to water; and effect of the non-aqueous volatiles in cotton on results. From the information gathered, all of the variables could be optimized and this resulted in excellent water content values in cottons that were validated by other techniques. For the first time, it is possible to calibrate, package and distribute to collaborators standard reference cottons certified for water content.
Technical Abstract: In a preliminary study to measure total water in lint cotton we demonstrated that volumetric Karl Fischer Titration of moisture transported by a carrier gas from an attached small oven is more accurate 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 from 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 in vitro NIR and comparing results from a second reference procedure developed in this laboratory – Low Temperature Distillation.