The analysis of total organic carbon (TOC) by the American Public Health Association (APHA) closed-tube reflux colorimetric method requires potassium dichromate (K2Cr2O7), silver sulfate (AgSO4), and mercury (HgSO4) sulfate in addition to large volumes of both reagents and samples. The method relies on the release of oxygen from dichromate on heating which is consumed by carbon associated with organic compounds. The method risks environmental pollution by discharging large amounts of chromium (VI) and silver and mercury sulfates. The present method used potassium monochromate (K2CrO4) to generate the K2Cr2O7 on demand in the first phase. In addition, miniaturizing the procedure to semi microanalysis decreased the consumption of reagents and samples. In the second phase, mercury sulfate was eliminated as part of the digestion mixture through the introduction of sodium bismuthate (NaBiO3) for the removal of chlorides from the sample. The modified method, the potassium monochromate closed-tube colorimetry with sodium bismuthate chloride removal (KMCC-Bi), generates the potassium dichromate on demand and eliminates mercury sulfate. The semi microanalysis procedure leads to a 60% reduction in sample volume and ≈ 33.33 and 60% reduction in monochromate and silver sulfate consumption respectively. The LOD and LOQ were 10.17 and 33.90 mg L-1 for APHA, and 4.95 and 16.95 mg L-1 for KMCC-Bi. Recovery was between 83 to 98% APHA and 92 to 104% KMCC-Bi, while the RSD (%) ranged between 0.8 to 5.0% APHA and 0.00 to 0.62% KMCC-Bi. The method was applied for the UV-Vis spectrometry determination of COD in water and wastewater. Statistics was done by MINITAB 17 or MS Excel 2016. ᅟ Graphical abstract.
In this study, a simple sample preparation method was developed for the determination of tri-and hexavalent chromium in water samples. It utilizes a pre-heated customized glass tube (CGT), to supply the heat energy required for the reaction of Cr(III) with ammonium pyrrolidinedithiocarbamate (APDC). The products of the Cr complexes, tris(1-pyrrolidinecarbodithioato)chromium(III) and bis(1-pyrrolidinecarbodithioato)[1-pyrrolidinecarbodithio(thioperoxoato)]chromium(III) were chromatographed with Shimadzu LC-20AT and Zobax Eclipse C18 (150mm × 4.6mm, 5µm) column using ACN: Water, (7:3, v/v) as the mobile phase. The concentration of Cr(III) ranged from 0.06mgL-1to 0.09mgL-1and that of Cr(VI) was between 0.02mgL-1to 0.04mgL-1in the samples. Percentage recoveries from spiked real samples were between 87% (tap water) to 110% (wastewater) for Cr(III) and 92% (pond water) to 117% (tap water) for Cr(VI). The limits of detection (LODs) were 0.0029mgL-1and 0.0014mg/L-1for Cr(III) Cr(VI) respectively. While the limits of quantitation (LOQs), were 0.0098mgL-1and 0.0047mgL-1for Cr(III) and Cr(VI) respectively. Method precision (RSD (%)) was 3.3% and 3.5% for Cr(III) and Cr(VI) respectively. The developed method was applied for the speciation analysis of chromium in drinking water, tap water, wastewater, river water, and pond water samples. Our findings proved the method is simple and inexpensive. The method was validated by the analysis of a certified reference material (CRM) SLRS-4. The percentage recovery and RSD(%) from the spiked CRM were 91% and 115% and 0.32% and 1.4% for Cr(III) and Cr(VI) respectively.