From: "Elson T. Elizaga" > On a related topic, I just got information from a chemist that the blue > thing in silica -- the silica that we use for our photographic equipment > -- is cobalt chloride which, he claims, is highly toxic. <clip> it is, but not all silica gel contains cobalt salts, only the blue/pink stuff.. > BUT cobalt chloride (which is the indicator compound found in > silica gel) is highly toxic. the LD-50 of cobalt chloride in > rats is 770 mg/kg, compare that with malathion with an LD-50 of > 1375 mg/kg orally. of course, comparison like this is > meaningless, not really, they're a good guide but have a look below for more LD50's to put a perspective on things.. > cobalt in silica is not dust. you're right. but it's also not > fused. and it is soluble in water (or the sweat of you hands). > and that's how you get the cobalt into your system (if you > touch the silica gel with your bare hands, hint hint...use > gloves :). > > Any comments from other chemists in the house? Ethanol LD50 is debatably around 1000 Asprin has a LD50 of 750 Caffeine (orally) has an LD50 of 192mg/kg BW potassium cyanide is around 6mg/kg thimet, ( a commonly used insecticide in Oz) has an LD50 of between 1-4mg depending on what source the data comes from - that places it between 2-6 times more toxic than cyanide yet little old ladies have been known to fling this stuff about with gay abandon in the attempt to present the perfect garden! incidentally, silica gel manufactured without cobalt chloride still poses risks from the dust which can cause allergic reactions both acute and chronic. The main thing in using ANY chemical agent is risk management - exposing yourself to unneccessary risks is foolish, but then shying away from sensible applications to a product can be counter productive too. Silica Gel and Cobalt are both efficient dessicants and are safe to use as long as people are aware of the risks and approach the products with a suitable respect. One last thing about cobalt chloride and it's history with photography - it's the component in woods glass filters which make these filters suitable for UV work, and similar filters have been made in the past by using a saturated solution of cobalt chloride in thin square boxes - that's 760grams of the stuff per litre - enough to kill around 14 scientific photographers in one go. karl
