Red Cabbage indicator

[paulsutton]

I am using some Red Cabbage indicator to investigate the pH of seawater collected locally.  I seem to be getting results that imply that local seawater is acidic.

I have attached a photo and also attached a pH scale portion of the CompoundChem website

https://www.compoundchem.com/2017/05/18/red-cabbage/

According to :  https://centerforsurfresearch.org/chemical-composition-of-seawater/

"Seawater has a slightly alkaline composition, ranging between 7.5 and 8.4".

Granted,  seawater pH probably varies around the world.   But this just seems way off.

In which case the colour should surely be more towards Purple / Blue

The pH indicator does work as I used so know that adding acids or alkalis produces the expected colours.

Has anyone else tried this ?  Could there be a reason for the acidification, granted the sea was a bit rough when this was collected so there is a chance the colour and acidity could be due to something else being mixed in with the water.

I will try and collect some more seawater on a calm day see if that is any different.

Just asking as it may help my investigations further.

Thanks

Paul

 

 

"

Edited April 16, 2023 by paulsutton
Just a quickj tidy up of the text.

[John Cuthber]

Seawater is typically slightly alkaline, but poorly buffered.
There isn't much in it (apart from salt).
So adding small amounts of acids or bases to it will change its pH by a large amount.

And, if your red cabbage indicator has much vinegar in it, you will overwhelm the small amount of alkali in the seawater and the mixture will end up acidic.

[exchemist]

1 hour ago, paulsutton said:

I am using some Red Cabbage indicator to investigate the pH of seawater collected locally.  I seem to be getting results that imply that local seawater is acidic.

I have attached a photo and also attached a pH scale portion of the CompoundChem website

https://www.compoundchem.com/2017/05/18/red-cabbage/

According to :  https://centerforsurfresearch.org/chemical-composition-of-seawater/

"Seawater has a slightly alkaline composition, ranging between 7.5 and 8.4".

Granted,  seawater pH probably varies around the world.   But this just seems way off.

In which case the colour should surely be more towards Purple / Blue

The pH indicator does work as I used so know that adding acids or alkalis produces the expected colours.

Has anyone else tried this ?  Could there be a reason for the acidification, granted the sea was a bit rough when this was collected so there is a chance the colour and acidity could be due to something else being mixed in with the water.

I will try and collect some more seawater on a calm day see if that is any different.

Just asking as it may help my investigations further.

Thanks

Paul

 

 

"

Paul, when I cook red cabbage the problem is to stop it turning blue when I sweat it with butter or add any water (I live in a hard water area). The normal way to do this in cookery is by adding acid, e.g slices of apple, or vinegar. How did you prepare the indicator? Did you buy a fresh red cabbage and extract the anthocyanin yourself, or does it come from something in a jar. If the latter, it will almost certainly have some sort of acidifier in it, to keep the colour.   

[Sensei]

16 hours ago, paulsutton said:

Granted,  seawater pH probably varies around the world.   But this just seems way off.

Have you seen the ocean world map?

https://www.google.com/search?q=seawater+world+map+ph

 

[exchemist]

21 minutes ago, Sensei said:

Have you seen the ocean world map?

https://www.google.com/search?q=seawater+world+map+ph

 

These show different ranges in detail but in all cases the pH is 8 +/- 0.2or so.

So red cabbage should be blueish, if there are no acids around to distort it. But as @John Cuthber points out, at such a neutral pH, very small changes in  H+ concentration have have a big effect on the value, so any traces of acid contamination can alter it quite a bit. 

[paulsutton]

3 hours ago, exchemist said:

These show different ranges in detail but in all cases the pH is 8 +/- 0.2or so.

So red cabbage should be blueish, if there are no acids around to distort it. But as @John Cuthber points out, at such a neutral pH, very small changes in  H+ concentration have have a big effect on the value, so any traces of acid contamination can alter it quite a bit. 

This could explain why I seem to be getting my results.  I didn't make the indicator one of the my local library staff made some up for the STEM group we run at the library

I think I have a few options

Firstly determine what colour red cabbage indicator is normally, as according to

https://www.youtube.com/watch?v=OMXMlWybv8A

 

It is more of a purple colour,  so would suggest it is neutral

1.  Make up a fresh batch,  would using De-ionized water rather than tap water be better?

2.  Repeat experiments and see if there is a change in result

I will keep trying, 

Thank you again

Regards

 

Paul

 

[exchemist]

45 minutes ago, paulsutton said:

This could explain why I seem to be getting my results.  I didn't make the indicator one of the my local library staff made some up for the STEM group we run at the library

I think I have a few options

Firstly determine what colour red cabbage indicator is normally, as according to

https://www.youtube.com/watch?v=OMXMlWybv8A

 

It is more of a purple colour,  so would suggest it is neutral

1.  Make up a fresh batch,  would using De-ionized water rather than tap water be better?

2.  Repeat experiments and see if there is a change in result

I will keep trying, 

Thank you again

Regards

 

Paul

 

Yes I would definitely use deionised water and a fresh, untreated cabbage. But I've never done this myself, I should stress. My experience is only in the kitchen, where I find the juice from red fruits goes distinctly blue when I rinse dishes. Almost all of them get the purple colour from anthocyanins, I believe, from blackcurrants to aubergines and red cabbage, so the same behaviour is expected, modified only by whatever acidity there may be in the fruit or vegetable involved.)

The reason for the colour change is quite interesting. These are conjugated ring systems with extensively delocalised π-orbitals. Protonation and deprotonation alters the bonding and thereby changes the energy gap between ground and first excited state, so that the molecule absorbs a different chunk of the visible spectrum.    

[paulsutton]

1 hour ago, exchemist said:

Yes I would definitely use deionised water and a fresh, untreated cabbage. But I've never done this myself, I should stress. My experience is only in the kitchen, where I find the juice from red fruits goes distinctly blue when I rinse dishes. Almost all of them get the purple colour from anthocyanins, I believe, from blackcurrants to aubergines and red cabbage, so the same behaviour is expected, modified only by whatever acidity there may be in the fruit or vegetable involved.)

The reason for the colour change is quite interesting. These are conjugated ring systems with extensively delocalised π-orbitals. Protonation and deprotonation alters the bonding and thereby changes the energy gap between ground and first excited state, so that the molecule absorbs a different chunk of the visible spectrum.    

Cool, thanks will do that,  yeah the infographic I have here explains the science behind it,  pretty clever stuff,   shows how nature seems to have an answer for all sorts of things.   I would guess the washing up water goes blue as the liquid is usually alkaline.

I will see what I can make this week,   the other stuff still works ok,  just from  a science view point it should be done properly and any anomalies removed as not to either impact results of experiments or the interpretation of experiments.

 

Paul

[exchemist]

2 minutes ago, paulsutton said:

Cool, thanks will do that,  yeah the infographic I have here explains the science behind it,  pretty clever stuff,   shows how nature seems to have an answer for all sorts of things.   I would guess the washing up water goes blue as the liquid is usually alkaline.

I will see what I can make this week,   the other stuff still works ok,  just from  a science view point it should be done properly and any anomalies removed as not to either impact results of experiments or the interpretation of experiments.

 

Paul

OK. By the way the anthocyanins in my red fruit turn blue just rinsed with water - before any detergent is added. So the water itself is sufficiently alkaline to do it. From looking it up, I see the pH of London water is in the range 8.0-8.5. 

[paulsutton]

15 minutes ago, exchemist said:

OK. By the way the anthocyanins in my red fruit turn blue just rinsed with water - before any detergent is added. So the water itself is sufficiently alkaline to do it. From looking it up, I see the pH of London water is in the range 8.0-8.5. 

It may be a hard water area,   I will include a link to the wikipedia page as it may be helpful to others here too

https://en.wikipedia.org/wiki/Hard_water

I would guess you know what it is anyway,  

Paul

17 minutes ago, exchemist said:

OK. By the way the anthocyanins in my red fruit turn blue just rinsed with water - before any detergent is added. So the water itself is sufficiently alkaline to do it. From looking it up, I see the pH of London water is in the range 8.0-8.5. 

 

[exchemist]

3 minutes ago, paulsutton said:

It may be a hard water area,   I will include a link to the wikipedia page as it may be helpful to others here too

https://en.wikipedia.org/wiki/Hard_water

I would guess you know what it is anyway,  

Paul

 

Yup. Chalk. CaCO3.

[paulsutton]

Just made up a new batch and it is the right colour.  The new batch is in the beaker on the left,  the old batch is on the right. 

 

Thanks to everyone for their help with this.

 

Paul

 

 

[exchemist]

1 hour ago, paulsutton said:

Just made up a new batch and it is the right colour.  The new batch is in the beaker on the left,  the old batch is on the right. 

 

Thanks to everyone for their help with this.

 

Paul

 

 

That looks a lot better, certainly.