Lecture 5 : Methods (Contd.)

The paragraph introduces the 5th class of analytical chemistry, focusing on the role of analytical chemists in identifying toxins responsible for deer killings. It discusses the importance of toxicology research and how certain toxins can be lethal to humans as well. The sampling process is highlighted, with a specific focus on the deer kidneys as a source of samples due to the rapid elimination of toxins through the urinary tract. The discussion also touches on the identification of a particular toxin, AsO43-, and the two key aspects of analytical chemistry: qualitative identification and quantitative measurement, which are crucial in understanding arsenic poisoning and its effects on living tissues.

This paragraph delves into the methodology of handling deer kidney samples for arsenic analysis. It describes the process of homogenizing 10 grams of kidney tissue and placing it in porcelain crucibles for further analysis. The importance of sample weight (W) and the limitations of the analytical technique are mentioned. The paragraph explains the process of dry ashing, where samples are heated at high temperatures to destroy organic matter and leave behind inorganic species, such as arsenic. The goal is to convert the arsenic present in the sample into a form that can be easily handled and analyzed, such as As2O5, which is achieved through the burning process in the presence of air.

The paragraph discusses the transformation of arsenic species during the dry ashing process. It explains that the inorganic arsenic species, whether in the form of arsenate (As5+) or arsenite (As3+), will be converted to arsenic oxide (As2O5) through the burning process. The presence of oxygen during dry ashing ensures the stabilization of the higher oxidation state, leading to the formation of arsenic pentoxide. The paragraph also touches on the potential conversion of this oxide back to arsenite using a reducing agent like stannous chloride, which is a crucial step in preparing the sample for further analysis.

This section explains how to handle arsenate and arsenite ions in solution. It describes the use of mineral acids to dissolve arsenic pentoxide and the subsequent reduction of arsenate to arsenite using stannous chloride. The paragraph also introduces the concept of converting arsenite ion to arsine gas (AsH3) using a strong reducing agent like zinc metal. The focus is on the safe handling of arsine gas, which is toxic and lethal, and the importance of using a conical flask and a cuvette for the safe collection and analysis of this gas.

The paragraph discusses the use of spectrophotometry for the estimation of arsenic in samples. It explains how arsine gas is bubbled into a solution and reacts with a reagent, silver diethyldithiocarbamate, to form a colored complex. This complex can then be quantitatively measured using a spectrophotometer, with the absorbance being proportional to the concentration of arsenic. The paragraph emphasizes the importance of using the correct wavelength (lambda max) for maximum absorbance and the application of Lambert Beer's law to determine the concentration of arsenic in the sample.

The final paragraph summarizes the analytical results and their implications. It describes how the technique can determine low concentrations of arsenic poisoning in deer samples, as well as higher concentrations in grass samples. The paragraph highlights the practical application of the analytical method, including the need for dilution of samples with high arsenic concentrations. It concludes with a recommendation based on the investigation's findings, suggesting that the use of arsenical herbicides should be avoided in wildlife areas to protect deer and other animals that might consume the grass.