citric acid and Metal - What's the Reaction?

A. In the simplest case, the case of a paper clip, the metals are likely iron with a thin zinc based coating. There are several things going on in the juice-metal interaction, but mainly the metals are reacting with the acids (citric, ascorbic, etc.) to form soluble and insoluble metal oxides (hydroxides), metal citrates and other acid pairs with the metals, and there are likely reactions with the limonenes, terpenes and other constituents in the juice.

This is kind of hard to place into a simple frame of reference for the standard fourth grade level, but for your edification, the two common "citrus fruit" acids are:

citric acid , known to chemists as
2-hydroxyl-1,2,3-propanetricarboxolic acid
HOOCCH2C(OH)(COOH)CH2COOH*H2O

and

ascorbic acid (vitamin C), known to chemists as
l-ascorbic acid, C6H8O6

The colors of the reactions will likely be rust on the paper clip (orange or red or brown). Keep in mind, left at room temperature, fruit juices mold, ferment and spoil rapidly; the spoiling actions introduce other acids to the experiment such as acetic, malic and lactic acids, not to mention the thiols (thiols are "rotting" smells--acetic acid ⇦ this on eBay or Amazon [affil links] is a vinegar ⇦in bulk on eBay or Amazon [affil links] y smell in itself); the spoilage will undoubtedly add weird colors to the mix. Be careful!

The action of the acids on metals is the reason you don't store fruit juices in metal (non-stainless steel)containers, and why cans of fruit or fruit juice must be lined with materials which resist acids.

A. Dear Yvonne,

You have one of the premier Metallurgical resources right there in Tucson at the University of Arizona's College of Engineering and Mines. Drs. Hiskey, Poirier and others would probably love to give this question to one of their students for extra credit to help you.

Sincerely, A former Wildcat

Hi Seun. It isn't obvious to we readers whether you are an elementary school student or a post-graduate researcher, which makes it difficult to say anything helpful. Please get back to us.

Regards,

A. Hi, Jordan -- you are from my old home town, nice to hear from EO!

Stainless steel is an alloy of nickel, chromium, and iron that is specially designed to be corrosion resistant. So it is able to withstand immersion in orange juice (a very mild acid) without corrosion.

Steel wool is made of steel (the element iron) and steel rusts readily in water or in juice. Also note that the surface area of steel wool is huge, so the reaction between the liquid and the solid can proceed very quickly compared to the other materials you used.

Aluminum is a very active metal. It immediately forms a thin tarnish of aluminum oxide on it. This tarnish is largely waterproof so the water ceases reacting with the aluminum, whereas the orange juice, being a mild acid, can dissolve that tarnish and cause the aluminum to continue reacting with the liquid.

The surface of pennies is copper, and copper does not substantially corrode either in water or in mild acids. However, pennies made after 1982 have a zinc core with copper plating and that plating may be porous so the liquid can react with the zinc. Try to consistently use pennies dated before 1982 or after 1983 for these experiments.

The most important part of your experiment is to accurately record what happened rather than to explain it. Newton explained the law of gravity in 1686, and science and astronomy made 225 years of astounding progress based on Newton's observations before Einstein explained them completely differently in 1915. So, write your conclusions any way you wish, but never be tempted to erase a single word of what you observed in your experiment. Your "observations" are truths, and the heart of "the scientific method", and far more important than your explanation for them. Good luck with your grade!

! We did a similar experiment for a science project testing various metal fittings (including hot dipped galvanized) in vinegar and acetic acid. Just a word of warning. These experiments need to be done in a well ventilated area to avoid any potential for metal fume fever.

A. Hi Maddie. vinegar is basically acetic acid . Lemon juice & orange juice are complex materials but citric acid is probably the most abundant acid in them.

Regards,

A. Janine,
Perhaps you have already heard of citric acid passivation, then.

However, I'm not sure of what you are trying to accomplish. Preventing corrosion and removing corrosion are two different operations. Though the proper mix of chemicals can do both, for a school project you ought to focus on one or the other. citric acid serves more on the prevention side (passivation) and is not as effective as other things at removing existing corrosion.

Also, from an orange peel you'll mostly be getting orange oil, or D-limonene ⇦ this on eBay or Amazon [affil links] , which is a good general cleaner but is not citric acid . The actual juice of the fruit would have to be your acid source. (Though industrially, citric acid is made by fermenting corn starch with a particular species of mold.)

Also, for the purposes of a school project, you will need a reliable method of causing stainless steel to rust, in order to have samples to remove rust from, or as a control sample to demonstrate that your prevention treatment was effective. (Industrially there are other ways this confirmation can be accomplished, but relying on such sources rather than first-hand demonstration will most likely defeat the purpose of a school project.)

We usually hear more about school projects on the corrosion of carbon steel, because getting stainless steel to rust in a controlled manner, on demand, can actually be a very difficult thing.

A. Hi FJ. If we told you whether or not pineapple peelings can remove rust from metal you would be learning "junk science" -- knowing the result you want or expect, so you tend to dismiss contrary evidence as anomalies while giving excess weight to anything that agrees -- rather than science, which rests on observations.

So, do some experimenting with rust and pineapple peelings, carefully record your observations in a "lab book", then tell us about those observations, and then we can jointly try to come up with possible explanations for what happens or doesn't happen.

Regards,