Alternative Units Why have them? Which are used?

Different strokes … Not all chemists deal with the same concentrations Some deal with huge amounts of sample Others deal with minute amounts of sample

Dangers! Certain chemicals are dangerous in very small amounts – lead, arsenic, PCBs, The monitoring of industrial and environmental sites for contaminants and/or pollutants

Environmental chemists perhaps? Chemists often have to deal with contaminants that are very low in concentration and difficult to detect, although many can be very harmful to humans and other life forms even in these low concentrations. Chemists must use a different set of units for these minute concentrations – commonly used are parts per million (ppm), parts per billion (ppb) and parts per trillion (ppt).

How much is a ppm? It sounds very small. One part of contaminant / chemical / pollutant per 1 million parts of water.

But, what’s that mean? It is like saying 1mg (milligram for the lost) of chemical in a kilogram of water (assuming an aqueous environment). But since the density of water = 1g/mL, then its 1 mg of chemical in 1L of water.

Then why ppt? So weaker concentrations of ppm and ppt, can be formalized as: 1ppb = 1  g (microgram) of chemical per litre of water, AND 1ppt = 1ng (nanogram) of chemical per litre of water.

Nasty PCB’s. Polychlorinated biphenyls (PCB’s) are mixtures of chlorinated compounds They have been used as coolants and lubricants in transformers, capacitors, and other electrical equipment PCBs do not readily break down in the environment and thus may remain there for very long periods of time

So what’s a safe limit? In the US, the EPA (Environmental Protection Agency) has set a permissible level of PCB’s in drinking water at 4ppt! Consequently the analysis apparatus must be able to accurately measure at that level. The numbers are large and we are almost talking the chances of winning the lottery as equal to the chances of finding the particular chemical.