# Does firing a proton beam on water make it acidic?

Lets say I fire a proton beam upon the surface of water. Lets assume I do the experiment in an air-free environment and that the proton beam doesn't reflect off the surface of water.

Now, protons, which are simply $\ce{H+}$, should enter the bulk of water and probably combine with a $\ce{H2O}$ molecule and form $\ce{H3O+}$. As per the definition of acidity and pH:

$$\ce{pH}=-\log [H^+]$$

If the pH reduces, the solution would become acidic as seen here. But at the same time, the water becomes charged, so I am not sure whether this is applicable.

So, in short, does water become acidic due to a proton beam?

Edit: I do not have access to proton guns and it's certainly not on sale on eBay. I only attempt to theorize on what may happen.

• sigh What would happen would depend on energy of protons. Also sth separating water from vacuum would be needed. – Mithoron Apr 23 '17 at 16:51
• Put water in a stainless container. Place Mylar foil for a beam entrance (easy and the standard for in-air ion beams. Irradiate with MeV protons. Ground the chamber for charge neutralization. Now it is a balance of solvated protons vs formation of H2. Not hard to do, if you have an ion accelerator handy. – Jon Custer Apr 23 '17 at 17:19
• I think you've answered your own question. The water would become acidic but the positive charge would cause reactions with the surroundings. – gsurfer04 Apr 23 '17 at 23:12
• Now, I have two conflicting answers, and no way to choose which is correct. Quite a dilemma indeed! – Pritt says Reinstate Monica Apr 24 '17 at 17:12

## 2 Answers

No, probably not.

pH represents the concentration of hydrogen (/hydronium) ions in water, requiring a negative ion complement. pH could be thought of roughly as a response to things other than water or it's components that are also dissolved in the water. If you try to artificially add protons to the water, the water will reestablish the original equilibrium concentration quickly.

Protons stopping would likely acquire electrons most of the time, find each other, and combine to molecular hydrogen. As the amount of "inserted" hydrogen increased, bubbles of hydrogen gas would tend to form in order to establish a partial pressure of hydrogen 'above' the liquid in order to maintain a finite dissolved fraction.

I found a table of numbers and a form of Henry's law here that can be used to estimate the partial pressure of H2 gas 'above' the water that would be in equilibrium with the amount of dissolved H2.

If you wanted to perform the experiment however, this could be done in a fairly straight-forward way with a low energy proton accelerator and its associated machine shop, instrument builders, and graduate students.

Proton beams from a Tandem or other MeV range accelerator can be focused to much smaller than 1 millimeter in diameter, and slowed by absorbers so that they stop within a spread in range of less than 1 millimeter as well.

A cubic millimeter of water has $\frac{1}{18}10^{-3}N_A$ molecules, or about $3.3 \times 10^{19}$.

A beam current of 10 pna (particle nanoamperes) for an hour (just for an example) would represent $10 \times 10^{-9} \times 3600 sec \times 1 coul$ or about $2.2 \times 10^{14}$.

Numerically that suggests a resulting pH of around 5.2 if the protons remained as H+ ions. However, this is not going to happen because of the reasons stated above.

This might be measured in situ with micro titration for example by introducing a small amount of indicator at the end of the experiment and collecting transmitted light with a microscope objective. There may be other ways as well.

Assuming the last 5 MeV of energy of the beam were dissipated in the water, the heating is $5 \times 10^6 volts \ \times \ 1 \times 10^{-8} amperes$ or about 50 milliwatts, which should be possible to dissipate across six square millimeters of surface area without catastrophic results.

Practically, you'd have to use a clever window to isolate the water from the beam line vacuum without chemical contamination by reaction or sputtering. Ice might be an excellent choice, except that you'd have to use a lower current for a longer time to handle the heating. Diamond-like carbon (DLC) might perhaps be another option, both for chemical as well as heat and charge conductivity reasons.

• While you have answered my question of whether proton beam can penetrate water or not, you haven't answered whether as a result water becomes acidic or not? – Pritt says Reinstate Monica Apr 24 '17 at 5:20
• @PrittBalagopal OK I've completely rewritten the first paragraph of my answer... yikes this is an interesting question! – uhoh Apr 24 '17 at 5:37
• @PrittBalagopal let me know if you'd like further clarification. – uhoh May 3 '17 at 10:22

Yes, the concentration of $\ce{H3O+}$ will increase which will lower the pH value of the water. This is theorie based.

In reality you won't be able to measure the change. The mount of protons you need to significantly change the pH will also effect the charge of the water as you mentioned. The charged water will affect your method of measuring AND the protons will leave the water because of it's positive charge.

To neutralize the water you could add elections, which will cause the protons to form hydrogen gas. Or you could add a negative ion like chloride, which would have been the same as adding $\ce{HCl}$ to the water.