I just wondered what was in Universal Indicator paper,  I have some really old stuff here at home.   It doesn't seem to work very well.  however soaking some in a beaker of water, turned it blue.

I tried to investigate further and see if this could actually be used as indicator,  but my results seem rather inconclusive.    It did turn citric acid a pale pink colour, but it didn't change colour when added to bicarbonate of soda, which given that is alkali, it would turn towards the blue / purple anyway depending on the pH.

So just asking what is in this and if it has a shelf life,  I think mine could be 20+ years old,  so probably not much good any more.

1 hour ago, paulsutton said:

I just wondered what was in Universal Indicator paper,  I have some really old stuff here at home.   It doesn't seem to work very well.  however soaking some in a beaker of water, turned it blue.

I tried to investigate further and see if this could actually be used as indicator,  but my results seem rather inconclusive.    It did turn citric acid a pale pink colour, but it didn't change colour when added to bicarbonate of soda, which given that is alkali, it would turn towards the blue / purple anyway depending on the pH.

So just asking what is in this and if it has a shelf life,  I think mine could be 20+ years old,  so probably not much good any more.

Sodium bicarbonate is a very weak alkali - a saturated solution of it only has a pH of 8-9 or so.

But I imagine some of the dyes may eventually oxidise over time and cease to function, so it may be better to replace it. 

Universal indicator contains several dyes that change colour at different pH values. There is a Wiki article that is quite helpful:https://en.wikipedia.org/wiki/Universal_indicator

 

13 hours ago, exchemist said:

Sodium bicarbonate is a very weak alkali - a saturated solution of it only has a pH of 8-9 or so.

But I imagine some of the dyes may eventually oxidise over time and cease to function, so it may be better to replace it. 

Universal indicator contains several dyes that change colour at different pH values. There is a Wiki article that is quite helpful:https://en.wikipedia.org/wiki/Universal_indicator

 

Thanks for this, will replace them,    I suspect that soaking the paper in water resulted the dyes leeching out. 

It is all part of the learning process eh

 

Paul