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My rainwater is colourless when collected, but if I add a small amount of sodium hydroxide to increase the pH to about 7, after a few days it goes yellow. The colour change happens more quickly if I increase the temperature. This change to yellow is accompanied by the disappearance of any dinoflagellates or cryptophytes which were living in the water (determined by 300X microscope.) The rainwater appears to contain a significant amount of dissolved organic carbon. The rainwater is collected from the PVC roof of my shed in a covered plastic container which I have recently cleaned. The pH of the rainwater is between 5.4 and 6 when collected (the best I can do with bromothymol blue indicator solution and narrow-range indicator paper.) I am located in the south-west of England about one mile from the sea.

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    $\begingroup$ Rain water shouldn't contain dinoflagellates and cryptophytes, where did you collect it from? what is the pH when you collect it? Where are you located? $\endgroup$ – com.prehensible May 5 at 7:57
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    $\begingroup$ In addition to the comment of @com.prehensible, in my experience adding NaOH to water in order to increase the pH to 7 is very close to impossible unless there is some buffering species present, which again suggests something else is present in the water. At a guess I would say iron, or a chelate of iron (often coloured), is contaminating the water sample? $\endgroup$ – tomd May 5 at 10:15
  • $\begingroup$ com.prehensible, thank you for replying, please see amendments to the text of my question. I am under the impression that small marine organisms less than 10 microns in size can find their way into the clouds somehow and based on previous experiments mine seem to prefer a pH > 7. $\endgroup$ – Shaun Ross May 5 at 12:26
  • $\begingroup$ user1136, thank you for replying, surely all rainwater contains acid so that will serve to buffer pH against NaOH won't it, and if the yellow colour is due to the presence of an iron compound, why isn't the water yellow to begin with? $\endgroup$ – Shaun Ross May 5 at 12:38
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    $\begingroup$ tomd, thank you for replying. I failed to mention that the water doesn't go back to being colourless if the pH is lowered. I feel increasingly that the yellow colour is due to a water mould growing on sugar in the rainwater. I am currently doing an experiment and will post results when I'm sure. I asked this question on biology.stackexchange! $\endgroup$ – Shaun Ross May 9 at 15:43

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