[NicholasLee]

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?
Thank you

PS: Nothing toxic please!

[Borek]

Experiment with sodium bicarbonate. Cheap and safe.

[orgopete]

I think this is an interesting premise. Because lemon juice probably contains a lot of chemicals, the color could come from another reaction, but this is being covered by the proposed experiment. By treating paper with citric acid, we'll know if this is a simple acid catalyzed reaction. Keep us informed.

Beside bicarbonate, you might also try ammonia. Ammonia would be easy, but probably not as permanent as bicarbonate. You might also research how archival quality paper is created.