Dr. Ananya Baksi in the lab studying the dissolution of silver in glucose water. A new silver foil used for the experiment (right).

In a finding that may have many implications, IIT Madras researchers have found that silver can slowly dissolve in water if heated to about 70 degree C in the presence of glucose.  As much as 0.5 weight per cent of a silver plate can get dissolved in glucose water within a week.  The results of a study were published recently in the journal Angewandte Chemie.

Like gold, silver is a noble metal and is therefore supposed to be inert (resistant to chemical corrosion, especially to chemical reagents used in daily life). However, Prof. T. Pradeep from the Department of Chemistry, IIT Madras and his team found that silver atoms gets released from a plate in a simple, two-step mechanism — silver ions are first formed at the metal surface, which later form specific metal complexes with sugar.

“Atoms are highly reactive on the surface of the metal as they less connected and less bound and this allows the atoms to be released,” said Prof. Pradeep.

Metal dissolution leads to corrosion of the plate and nanoscale pits get formed on the plate. Further dissolution occurs at the pits and as a result the pits get bigger, making a polished silvery metal appear black. Under favourable conditions, up to 10 per cent of the metal can get dissolved in 90 days.

“We have been studying the effect of metals in food and how toxic metals get into our food chain from soil, water and fertilizers,” he said. Silver foils are used to decorate sweets and often such foils are eaten along with the sweets. In the past, silver vessels were used for cooking and even today silver plates are used. “So we were curious to know the interaction of silver with foods especially sweets. This prompted us to study the interaction of sugars with silver,” Prof. Pradeep said.

Dissolution of silver by glucose directly from the metallic state gets enhanced in the presence of ions such as carbonate and phosphates. The study found that enhancement of silver dissolution in glucose was about 10 and 7 times in the presence of 50 ppm of phosphate and carbonate respectively. But in the absence of glucose, phosphate and carbonate were found to have no significant effect on silver dissolution.

“We store water in vessels made of different metals. We don’t know what happens to the water.  When we cook food, especially using lots of spices that are reactive, we may be consuming some metal too. We are damaging the health of our population by using poor quality vessels,” he warned.

“Chemistry of sugars at metal surfaces can have tremendous impact on our population if ingredients of steel, copper and brass can dissolve in water and get accumulated in our food. The extent to which dissolution occurs is much larger than the permissible limits of many metals in water,” he stressed. While the presence of certain metals at specific concentrations may be beneficial, it can be extremely toxic in the case of others.

An offshoot of the study is that the method can be used for developing novel and green extraction processes for noble metals. In general, toxic chemicals, such as cyanide, are used for extracting silver.

“This newly developed method can extract silver effectively by a simple and green method. It does not require any harmful chemicals or high temperature or expensive set-up,” Prof. Pradeep said.

The study was performed by Dr. Ananya Baksi, a Postdoctoral Fellow under the direction of Prof. Pradeep.

Published in The Hindu on June 6, 2016

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