Validation of an in vitro method for the determination of cyanide release from ferric-hexacyanoferrate: Prussian blue

Prussian blue (PB), ferric hexacyanoferrate, Fe4[Fe(CN)6]3 is indicated for the treatment of known or suspected internal contamination with radioactive cesium, radioactive thallium, or non-radioactive thallium. Owing to the molecular properties, cyanide is likely dissociated from PB under physiologically relevant pH conditions, thus raising a concern for the safety of the product. The objective of this study was to calibrate and validate a cyanide assay over a wide pH range (from 0.5 to 12) on the basis of Spectroquant cyanide test method (Merck). Merck's photometric method requires that the measurement solution be within pH 5.5–6.0, hence samples and standards need to be adjusted to this pH range. Since the process of pH adjustment may have significant impact on the determination of cyanide, the analysis method needs to be optimized, calibrated and validated under each pH condition in the study. The validation characteristics included accuracy, precision, quantification limit, linearity, and stability. The intra-day accuracy ranged from 90% to 109% for the deionized water and solutions of pH 0.5–12. The intra-day precision (R.S.D.) ranged from 2.4% to 8.1% for the deionized water and solutions of pH 0.5–12. The analytical range was linear from 0.05 to 0.5 ppm (mg/L). The R2 ranged from 0.9925 to 0.9998. This validated method was successfully implemented to determine cyanide release from PB under various pH conditions (from 1.0 to 12) at different time-points (from 1 to 24 h).