I am trying to understand the reaction that takes place in in the old "Lemon Juice invisible ink" trick, and use it for a modern application.
1) Lemon Juice is used as ink to write a message on a piece of paper. The ink is allowed to dry.
2) Later, the paper is heated the writing appears in brown.
NB: I am an engineer by training, not a chemistry student, so bear with me. :-)
My understanding is as follows, (please correct any aspects if this which I have got wrong):
1) The citric acid in the lemon juice causes acid catalysed dehydration of the cellulose in the paper.
2) As cellulose is a polymer of glucose, so this means the paper where the lemon juice was now contains glucose.
3) The glucose has a caramelization temperature of 160 degrees, and the amount of caramelization that occurs on heating is increased by the acidity.
4) The caramelization of glucose is a pyrolysis process, and the products of this pyrolysis cause the paper to go brown where the Lemon juice was.
I would like to treat a piece of paper evenly with citric acid, and then draw on it with an infra-red heating laser, locally heating the paper enough to do pyrolysis of the glucose without coming anywhere near the ignition temperature of the paper.
However, I think that the whole sheet of paper will turn brown over the next few days due to the increased acidity of the paper.
I would therefore like to neutralise the effects of the acid after the laser has been used, to stabilise the reaction.
Preferably this should take the form of adding a chemical that reacts with the citric acid to form a neutral-pH buffer solution, rather than simply adding an alkali.
Please can you tell me what this chemical or chemicals might be and what the necessary mixing proportions or formula is?
PS: Nothing toxic please!