How does lemon juice affect the color intensity of henna?

[L²Cc]

For my IB chemistry extended essay, I'm investigating the effect lemon juice has on the color intensity of henna, an olive-green, tooth-paste like substance we apply to the body for either decoration or health reasons (image google it). I know that the lemon juice darkens the henna, but I have to carry out an experiment that validates this so that I can then explain the molecular reactions that happen when these two substances come together. Do you have any ideas of an experiment that's fairly accurate and gives concrete data values for the color change that henna may undergo after the lemon juice has been added. I was thinking of simply 'painting' henna across some animal skin and leave it dry and see the color change may happen, but this is fairly bias...I would rather have proper data values. Any ideas?

 

[eli64]

I would start with trying to find out what is in henna, what kind of organic compound(s) determine its composition. If lemon juice (weak acid) darkens henna, it might be some kind of oxidation, or acid/base reaction that gives such a colored product. If you can find the active ingredient in henna that gives such a color change with the lemon juice- there are spectrophotometric measurements you could take with the reaction mixture that would give you concrete data values

Also does it just darken the henna alone or when the henna is on the skin? Those are two different experiments.

 

[symbolipoint]

Try also other acids, like maybe acetic acid, carbonic acid, maybe oxalic acid. Also, is the color change reversible? When you neutralize the acidity of the treated henna, does the color go back to its original?

If acidity/alkalinity alone affects the henna color, what concentration of henna is needed for seeing or assessing its color? Could you perform a pH adjustment in a small container of henna?

 

[L²Cc]

Thank you. Eli64, what we do here, in Morocco, is we mix the lemon juice with henna while preparing the mixture. This causes the henna to be darker when on the skin, so, to answer your question, there is no visible color change when the henna is still a 'fluid'. Hence, how can I obtain spectrophotometric measurements when the henna is dry?

Symbolipoint, I like your idea of observing the different color changes that happen with other acids, but then how can I experimentally and analytically compare the color changes?

 

[symbolipoint]

Symbolipoint, I like your idea of observing the different color changes that happen with other acids, but then how can I experimentally and analytically compare the color changes?

Pick some standard henna concentration, replicate this, and treat most of them with different acids, and measure pH's of each. Pick ONE replicate to use as a blank. Exactly what is your test standard must be for you to decide, since you are familiar with this henna. Also, you will need to determine how many moles of acid formula unit are needed for an given unit of pH change. Your color assessment could be taken as precise visual judgements, but if your henna solutions can be dilute enough, maybe a spectrophotometer could be used.

 

[chemisttree]

The active ingredient in henna is 2-hydroxy-1,4-naphthoquinone. I believe that the acid in the lemon juice helps to extract the dye and that the dye is oxidized on the skin from atmospheric oxygen, causing it to darken.