# Under which criteria HNO3 is a concentrated or a diluted acid? [closed]

I would like to buy nitric acid to remove impurities of copper on an aluminum plate. I read that a 10% nitric acid will do the job. However I've also read that concentrated nitric acid is very corrosive to human skin.

So is a 10% nitric acid solution is still very corrosive to human skin. What am I risking if one drop falls on my arm?

Is there any tables that shows the reactivity of acids under various concentrations to human skin?

According to the European regulation on classification, labelling and packaging of substances and mixtures, the classification of nitric acid depends on concentration.

At a concentration of 20 % ≤ c, nitric acid is classified as skin corrosive category 1A. (For a pure substance on the basis of the results of animal testing, this category corresponds to a substance that produces destruction of skin tissue, namely, visible necrosis through the epidermis and into the dermis, in at least 1 of 3 tested animals where responses are noted following up to 3 minutes exposure and up to 1 hour observation.)

At a concentration of 5 % ≤ c < 20 %, nitric acid is classified as skin corrosive category 1B. (This category corresponds to a substance that produces destruction of skin tissue in at least 1 of 3 tested animals where responses are described following exposure between 3 minutes and 1 hour and observations up to 14 days.)

Furthermore, at a concentration of 65 % ≤ c, nitric acid is classified as oxidising liquid category 3.

Therefore, nitric acid is labelled in accordance with the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) as follows:

Concentration 65 % ≤ c

H272: May intensify fire; oxidiser
H314: Causes severe skin burns and eye damage

Concentration 20 % ≤ c < 65 %

H314: Causes severe skin burns and eye damage

Concentration 5 % ≤ c < 20 %

H314: Causes severe skin burns and eye damage

I am unaware of the table you are looking for, but I can explain "concentrated". I can also remind you that the better strategy is to prevent exposure.

"Concentrated" means that the reagent is present in its highest concentration possible, which happens in one of three ways:

1. The solution is saturated
2. The reagent is essentially pure
3. The reagent is unstable at higher concentrations

This website from Sigma-Aldrich lists the concentrated reagents, the percent by mass concentration, and the molar concentration. Here are some of the acids:

$$\begin{array}{|c|c|c|c|}\hline \mathrm{acid} & \mathrm{formula} & \mathrm{\%\ w/w} & \mathrm{molarity}\\ \hline \mathrm{Acetic} & \ce{CH3CO2H} & 99.8 & 17.4 \\ \mathrm{Formic} & \ce{HCO2H} & 90.0 & 23.6 \\ \mathrm{Hydrochloric} & \ce{HCl} & 37.2 & 12.1 \\ \mathrm{Hydrofluoric} & \ce{HF} & 49.0 & 28.9 \\ \mathrm{nitric} & \ce{HNO3} & 70.4 & 15.9 \\ \mathrm{phosphoric} & \ce{H3PO4} & 85.5 & 14.8 \\ \mathrm{sulfuric} & \ce{H2SO4} & 96.0 & 18.0 \\ \hline \end{array}$$

So, 10% nitric acid is one seventh as concnetrated as concentrated nitric acid. Is it corrosive to skin? Yes. Is one drop going to be terrible? Not if you wash it off quickly and thoroughly.

What should you do? Use the appropriate personal protective equipment! Obtain a pair of butyl rubber gloves. There are available in sizes that go all the way up to your elbow.