Spectrophotometric Determination of Small Amounts of Lead(II) with 5, 10, 15, 20-Tetrakis[4-(trimethylammonio)phenyl]porphyrin

Abstract

A highly sensitive method for spectrophotometric determination of lead(II) by use of a water-soluble porphyrin was developed.5, 10, 15, 20-Tetrakis[4-(trimethylammonio)phenyl]porphyrin (ttmapp) formed a lead(II) complex by heating for 5 min in an alkaline solution.. A high and constant absorbance of the complex was obtained in a pH range 9.1-.11.8. The soret band maximum of the complex (ε=3×10⁵) lies at 463 nm, separated substantially from that of free ttmapp (411nm). The tolerance limits of 22 metal ions are given. Lead(II) was separated from many metal ions by extraction with dithizone-carbon tetrachloride solution from alkaline solution followed by back extraction with 1 mol⋅dm^-3 nitric acid. The recommended procedure is as follows. Take 20-40 ml of a sample solution containing 0-20 μg of lead (II) into a 100 ml beaker and adjust the pH to 9.5 with a borate buffer. Heat the solution to 90°C and add 1 ml of a 4×10^-4 mol⋅dm^-3 ttmapp solution, then allow it to stand for 5 min at 90°C. After cooling to room temperature dilute the solution to 50 ml. Measure the absorvance at 463 nm against water. A straight calibration line was obtained up to at least 400 ppb lead, and the sensitivity (Sandell index) was 0.835 ngPb cm^-2. The relative standard deviation (n =10) was 1.7% for the spectrophotometry and 4.3% for the extractionspectrophotometry. This method was applied to the determination of lead in sewage water.