# Can 32% hydrochloric acid be stored in a milk jug (HDPE)?

Can $32\%$ ($20$-degree Baume) hydrochloric acid (typically found as 'muriatic acid' in hardware stores) be stored in a milk jug?

I have read that acids can be safely stored in polyethylene bottles. The milk jug I have has the HDPE resin ID code on it. However looking at Wikipedia, it is clear that HDPE is used to make a wide range of products with different "strengths". Given the "squishiness" of milk jugs, I would assume it is on the lower end of the "strength" scale. Does this make it any less suitable for holding diluted hydrochloric acid?

Context: I plan on etching some circuit boards as per this tutorial. And I am looking for places to store the resulting copper (II) chloride solution.

$$\ce{Cu + 2HCl + H2O2 -> CuCl_2 + 2H2O}$$

I am assuming that if the container can hold hydrochloric acid, it can hold the resulting copper(II) chloride solution.

There is more than just material composition to container selection. Yes HDPE is compatible with hydrochloric acid, but you have to consider rigidity and failure probability. Think: would you want to store Concentrated Hydrochloric acid in a 2 mil bag? Probably not because the probability of puncture and resulting loss/cost from the spill would be sufficiently high. Though not as extreme your milk jug is along those lines as it is softer and less rigid as the original container.

There is also the issue of legal considerations. In the US I'm sure that milk jug would not meet the DOT requirements for transport, and OSHA prohibits using containers typically used for food as being used for chemical or waste storage.

That said for home use only, you will probably be okay to store it, certainly not recommended, but this is the real world. I would at least recommend that if you do use a milk jug, you at least use a basin that can hold all of the contents for secondary containment in case of a leak which milk jugs occasionally do AND certainly label the contents.

• I like the reminder about secondary containment. You could also put the milk jug in a plastic bag and tie the bag top to help contain vapors of HCl. Milk bottle tops are not really snug, especially after use, and HCl tends to corrode things before dissipating. – James Gaidis Sep 8 '18 at 13:01

According to this chart from a manufacturer of plastics, HDPE is resistant to concentrated $\ce{HCl}$ acid as well as to copper salts (other manufacturers report the same - Google if needed). As mentioned in the comment thread, do be sure to label and store the containers properly.

I prepared the very same etching solution you mentioned to make a custom shield for an Arduino project (it worked great by the way) and still have the etching solution sitting in my garage. It's in a small HDPE tote and has been for over 9 months without any problems. However, the tote has thicker sidewalls than a milk jug. I wouldn't worry about the chemical compatibility and would worry mostly about structural integrity of the container. It might be wise to buy a thicker walled HDPE or Polypropylene (PP) container at the dollar store.

• Thanks for the reply! Does your tote have a cover (liquid and air tight)? Doing a google image search for "HDPE tote" and can't quite picture what you have. Also did you experience any problems with the muriatic acid and rust? It seems to be a common problem with storing the acid... and people resort to storing it outdoors =/? Glad to hear the etching process works! – Jet Blue Sep 7 '18 at 22:52
• I looked for a photo of the kind of box/tote I bought and couldn't find one that matches well. If I find a good image of the box/tote I used, I'll post it. It does have a lid, but I wouldn't trust it to be waterproof. Now that I think more about it, the Muriatic acid I used is this one. osh.com/Osh-Categories/Outdoor/Outdoor-Living/Pool/… It comes in two one-gallon milk jug like containers. The walls are thicker than you would find on a normal milk jug. – Veluminum Sep 7 '18 at 23:01
• It looks like a tiny version of this. zamza.info/plastic-boxes-with-lids/… – Veluminum Sep 7 '18 at 23:06