Q: While on holiday at Scotnish, a tiny village near Tayvallich in Strathclyde, I noticed that ordinary toilet soap turned the local tap water bright blue. Emptying the basin left a small amount of blue scum behind. Water left without the addition of soap would remain clear for hours. Can anyone suggest an explanation?
A: The blue scum is probably copper stearate 鈥� a blue-green, waxy and insoluble compound. The reaction mechanism is akin to calcium stearate formation in hard water but in this case is: Cu2+ + 2CH3(CH2)16COO鈭� 鈫� Cu(CH3(CH2)16COO)2
Assuming that this private water supply is typical of those that are found in the region 鈥� coloured, soft and acidic 鈥� it will dissolve copper from the piping in standard domestic water systems. The use of a detergent-based hand cleanser would avoid the formation of scums. Many public water suppliers in this area make an adjustment to the pH to prevent the copper dissolving.
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A: I was very interested in this question as I observed 鈥淟owland blues鈥� when I retired to Moffat. Some soft waters are aggressive to copper and fewer than two parts per million of copper in the water is enough to give a noticeable blue colour when soap is added because soap is quite a sensitive reagent for detecting Cu2+ ions.
However, I only notice the blue colour when soap is mixed with water from the hot tap. Water from the cold tap does not give any colour, even if it is subsequently heated in an electric kettle.
I took samples to the local water authority鈥檚 laboratory and they discovered that the water from the cold tap contained 70 micrograms of copper per litre whereas the water collected from the hot tap (which had been stored in a copper cylinder overnight) contained 1880 micrograms.