The effect of organic acids and amines in Cr VI determination

Abstract CSIRO Leather Research Centre, Australia, were so interested in the paper presented at the IULTCS Congress in Cancun by Kallenberger and Hernandez that they decided to carry out their own research on the effect of organic acids and amines in chromium VI determination since the original authors felt that they had no time to carry out additional work. The authors are Cameron Simpson, Mark Hickey and Catherine Money. None of the organic acids or amines they tested generated false positives using the IUC 18 standard method. Tests did show that the diphenylcarbazide solution reacted with the organic acids and amines resulting in colour development. However, the colour was pH dependent and when the extraction solution or phosphoric acid solution was added, the pink/red colour dissipated in all cases. Introduction There has been much debate over results obtained from the analysis for hexavalent chromium (Cr VI) in soils and leathers¹. Questions concerning the accuracy of the current test method have again been raised, this time regarding potential false positives generated from some compounds found in leather and soil extracts. A paper titled 'Organic Acids and Amines Produce Colour with Diphenylcarbazide Determinations' by Kallenberger and Hernandez was presented at the IULTCS Congress in Cancun in 2003². The paper suggested that organic acids and amines may give false positives in Cr VI determinations. There was considerable interest in the presentation and it was concluded that further investigation was required³. The work presented in this report further examines the effect of simple organic acids and amines on Cr VI determinations obtained using the IUC 18 method⁴. The diphenylcarbazide Cr VI method used by Bartlett and James in 19795 was also tested for potential false positive results from spikes of organic acid and amines. Experimental The IUC 18 chromium VI determination method consists of an extraction phase where the soluble chromium VI is leached from the sample at pH7.5 to 8. The Cr VI in solution oxidises 1,5-diphenylcarbazide to l,5-diphenylcarbazone to give a red/violet complex with chromium. The colour development is highly dependent on pH and so conditions must be monitored carefully throughout the analysis. Various organic acids and amines were added to the IUC 18 extraction solution (5.1)⁴ and any colour development was monitored throughout the Cr VI determination procedure. No actual leaching of leather was performed. The test compounds were spiked into the extraction solution and the analysis was completed. The compounds tested to determine their potential to generate false positive were: * Organic acids * Acetic acid * Propionic acid * Butyric acid * Amines * Methylamine * Ethylamine Summary of tests In this work, the blank (7.4 IUC 18) refers to a 25ml volume containing 0.5ml of diphenylcarbazide solution (5.2), and 0.5ml of phosphoric acid (5.3) and extraction solution (5.1) to make 25ml volume. IUC 18 Method Blank Blank with 0.2ml glacial acetic acid substituted for phosphoric acid Blank with 0.5g potassium propionate substituted for phosphoric acid Blank with 0.4ml 2,2 dimethyl butyric acid (98%) acid substituted for phosphoric acid Blank with 0.5ml butyric acid substituted for phosphoric acid Blank with 5g rancid butter (butyric acid) 0.5ml diphenylcarbazide solution + 0.5ml butyric acid (99%) 0.5ml diphenylcarbazide solution + 0.5g potassium propionate 0.5ml diphenylcarbazide solution + 0.5ml glacial acetic acid 0.5ml diphenylcarbazide solution + 0.5ml methylamine (36% solution in water) 0.5ml diphenylcarbazide solution + 0.1g ethylamine hydrochloride (98%) Blank Blank containing 20ppm Cr VI Blank containing 20ppm Cr VI with 0.2ml butyric acid Blank containing 20ppm Cr VI with 0.2g potassium propionate Blank containing 20ppm Cr VI with 0.2ml methylamine (36% solution in water) Blank containing 20ppm Cr VI with 0.2g ethylamine hydrochloride (98%) Blank containing 20ppm Cr VI with 0.2ml glacial acetic acid Bartlett and James chromium oxidation test method⁵ Diphenylcarbazide Cr VI test5 + 0.2ml glacial acetic acid Diphenylcarbazide Cr VI test + 0.2g potassium propionate Diphenylcarbazide Cr VI test + 0.2ml butyric acid (99%) Diphenylcarbazide Cr VI test + 0.2ml methylamine (36% solution in water) Diphenylcarbazide Cr VI test + 0.2g ethylamine hydrochloride (98%) Results Test 3 showed that potassium propionate forms a pink colour with 1,5-diphenylcarbazide, at pH8, but when the pH is lowered to 4.4 with the IUC 18 standard amount of phosphoric acid, the colour dissipates. Tests 7-11 showed that in the presence of the diphenylcarbazide solution alone, all of the organic acids and amines tested give a pink colour reaction with the 1,5-diphenylcarbazide. However with the addition of the extraction solution and phosphoric acid, as per the IUC18 method, the colour dissipates. The tests were repeated with a 1ml spike of 1,000ppm chromium VI standard solution (K₂Cr₂O₇) in the extraction solutions added prior to the addition of the diphenylcarbazide and phosphoric acid solutions. The spike gave a final concentration of 20ppm in the test solution. The absorbance at 540nm was no greater than the control in any of the samples with organic acid or amine spikes; hence no oxidation of the 1,5-diphenylcarbazide occurred as a result of the organic acids or amines. In all cases, the pH of the solutions was adjusted with phosphoric acid to match the pH of the blank solution containing 20ppm of Cr VI (test 13). Tests 19-23 used the Bartlett and James chromium oxidation test5: no false positives were observed with any of the tested reagents. Conclusion and discussion None of the organic acids or amines tested generated false positives using the IUC 18 standard method. Tests did show that the diphenylcarbazide solution (5.2) reacted with the organic acids and amines resulting in colour development. However, the colour was pH dependent and when the extraction solution (5.1) or phosphoric acid solution (5.3) was added, the pink/red colour dissipated in all cases. The Kallenberger and Hernandez paper is correct in stating that simple organic acids and amines react with diphenylcarbazide to produce a pink colour. However, it has been found that if the IUC 18 standard method is adhered to, organic acids and amines will not produce false positives in Cr VI determinations. These findings do not alter the fact that concerns about Cr VI content in leather are unjustified as discussed by Cory⁶. As recently stated by Long⁷, the criteria for setting limits in materials should be based on the risk to the manufacturer and consumer, and CEN BT 132 is currently considering the effect of Cr VI on the human body. Reports of Health-Based Soil Action Levels⁸ and case studies involving human exposure to Cr VI in soil and ground water9 put the relative toxicity of Cr III and VI into perspective. Cr VI is more toxic than Cr III but low levels can be tolerated and are not carcinogenic. The human studies showed that the gastrointestinal tract can reduce ingested Cr VI to Cr III at concentrations up to 10ppm Cr VI and soil concentrations of 1240ppm Cr VI do not elicit allergic contact dermatitis in over 99.9% of the general population. It is acknowledged that about 0.5% of the population is sensitive to chromium but chrome tanned leather has been worn for over 100 years and this sensitivity has been managed.