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I have an area of artificial grass at home in the back garden and during the summer months, the area emits a smell which I can only think is from the dog's urine when he pees all year long. The smell isn't a problem in the winter or during wet days.

I read a lot of posts online about using white vinegar and baking/bicarbonate of soda

I don't have any of that at home, but I do have caustic soda from unblocking drains/pipes that I use once in a while. I wonder if caustic soda can be used in a Pump Sprayer with Water or some other cleaning agent to help break down the smell from urine?

I'm a complete noob with chemistry but I thought this is the right forum to ask the question.

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    $\begingroup$ Caustic soda, if anything, will make the stench worse. $\endgroup$ – Ivan Neretin May 19 at 8:39
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    $\begingroup$ And possibly dissolve your artifical grass $\endgroup$ – Waylander May 19 at 8:41
  • $\begingroup$ Is the odor ammoniacal or a different stench? I think NH3 is not so objectionable at low concentrations, but bacterial stench would be. $\endgroup$ – James Gaidis May 19 at 14:03
  • $\begingroup$ I wouldn't know. It doesn't stink of the usual piss odour you'd find in an underground passage or a NYC subway. It stinks somewhat different. $\endgroup$ – kkudi May 19 at 14:08
  • $\begingroup$ Maybe you could go to a nearby lab and have a whiff of ammonia, and compare the smells. $\endgroup$ – William R. Ebenezer May 19 at 18:58
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Well, it would actually make things a little awkward.

Why do Vinegar and baking soda work?

The reason for the usage of vinegar or sodium bicarbonate must first be understood.

  • Vinegar ($\ce{CH3COOH}$) is an acid, and hence protonates the carbonyl oxygen present in urea[1].

urea protonation

  • $\ce{NaHCO3}$ (baking soda) is amphoteric, meaning it can react with both acids and bases. Urea is basic, and hence forms a salt. This answer on our site talks exclusively about the deodorizing property of baking soda.

Both actions are quite much the same, and prevent the hydrolysis of urea (which happens to release ammonia ($\ce{NH3}$) which is the "foul" smelling substance)

urea hydrolysis

Why wouldn't sodium hydroxide work?

Not that $\ce{NaOH}$ wouldn't react, but because it would accelerate the hydrolysis of urea[2].

NaOH and urea

So if you're looking to stop the hydrolysis of urea (and the inevitable generation of ammonia) $\ce{NaOH}$ might not be the right guy.

What I wouldn't advise

But, hey, if you're looking to remove all the urea once and for all, you could pour a few buckets of $\ce{NaOH}$ and let the ammonia create a riot in your unsuspecting neighborhood.

What I would advise

Urea hydrolysis is catalysed by the enzyme urease (so it is dead slow without it)

From reference [3] (emphasis mine)

Urea is a stable compound with a decomposition half-life in aqueous media of 3.6 years. However, hydrolysis of urea is 104 times faster when the urease enzyme is present.

If we could inhibhit its action, we'd have got what we needed.

If you're wondering where urease comes from and why the dog urine stench is greatly enhanced in summer months, you're looking at the same answer: microorganisms.

It has been published that Metals and fluorides could be used to inhibhit urease. I quote further from source [3].

2.3. Inhibiting chemicals

Seven different chemicals, silver nitrate, zinc nitrate, sodium fluoride, glacial acetic acid, vinegar, citric acid, and sulfuric acid, were used to inhibit urea hydrolysis.

The list is by no means exhaustive.

From source 4, different classes of compounds which might be used:

The classes of urease inhibitors include: amides and esters of phosphoric acid, thiols, hydroxamic acids, phosphinic and thiophosphinic acids, boric and boronic acids, phosphate, heavy metal ions, bismuth compounds, quinones, and fluoride.

If you're not keen on purchasing baking soda, I'm sure you'll find any one of the above an interesting buy.


References:

  1. Interaction of urea with weak acids and water, Prabhat K. Das Gupta and S. P. Moulik, J. Phys. Chem. 1987, 91, 5826-5832 DOI:10.1021/j100306a061

  2. What is the reaction between NaOH and urea?

  3. Characterization of urea hydrolysis in fresh human urine and inhibition by chemical addition, Hannah Ray, Daniella Saetta, Treavor H. Boyer, Environ. Sci.: Water Res. Technol., 2018, 4, 87-98 DOI: 10.1039/C7EW00271H

  4. A combined temperature-pH study of urease kinetics. Assigning pKa values to ionizable groups of the active site involved in the catalytic reaction, Barbara Krajewska, J. Mol. Cat. B: Enzymatic Volume 124, Feb. 2016, 70-76 DOI: 10.1016/j.molcatb.2015.11.021


EDIT

There has been some discussion on if the stench is truly ammoniacal or is just bacterial odor. In that case, I would suggest sprinkling of significantly higher concentration of acid. Extreme pH is known to disrupt bacterial life forms and the acid will also prevent to a great extent the possibility of urea hydrolysis, thus preventing the odor anyway.

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  • $\begingroup$ Hi - thanks so much for the detailed analysis. So baking soda + vinegar in a pump sprayer is what we're saying? and how effective is this? how often should it be repeated ? Also what sort of quantities do we need? e.g for 5 L pump spray - how many grams of bicarbonate soda, how much white vinegar and how much water? Excuse my naiveness, but I know nothing about chemistry. $\endgroup$ – kkudi May 19 at 14:16
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    $\begingroup$ Either baking soda or vinegar. not both(that would give you a volcano of bubbles!). As for quantities, its difficult to speculate. How much area do you have to cover? $\endgroup$ – William R. Ebenezer May 19 at 19:01
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    $\begingroup$ @Mithoron I haven't tried it really, but I wrote it under the assumption that the smell was from ammonia being released by bacteria hydrolysing the urea. In that case we could ask the OP to spray caustic soda and see if it would work? I would suggest acid instead of base, as we'd still be inhibhiting urea hydrolysis and might also kill the bacteria causing the stench. $\endgroup$ – William R. Ebenezer May 21 at 4:03
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    $\begingroup$ @kkudi Well I meant vinegar without water. soda will definitely need water. And I found another way: pine oil should be a good disinfectant too. $\endgroup$ – William R. Ebenezer May 21 at 20:25
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    $\begingroup$ thanks so much! will be spraying next weekend. Will probably have to wait a week or two and a hot day to report on results, but will definitely come back and report my findings! thanks once more! $\endgroup$ – kkudi May 21 at 20:29
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Hydrogen peroxide (3%) is recommended for cleaning up animal pee inside a house. It could work outside as well, perhaps by killing bacteria, or maybe oxidizing the stuff that bacteria feed on. It should be safe on plastic.

Chlorox Multi-Surface cleaner is very useful. It contains quaternary ammonium surfactants, not bleach. These are bacterial killers too. It comes in a spray container

And the last resort would be bleach, but you might want to test it on a less visible area. It could be diluted quite a bit. And you might need to rinse the area (after a an hour?) to remove the solids that would otherwise crystallize out when the water evaporates. The solids might be noticeable as a white spot, and if they still contain hypochlorite, there might be continued action which weakens the plastic.

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