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I am encountering an issue in my laboratory. My supervisor and I are currently faced with the task of neutralizing a volume of 60 liters of Aqua Regia, which contains suspended metals, at a concentration of 2% or lower. Our current approach involves using Granular Sodium Hydroxide (NaOH) as a neutralizing agent. However, we have observed that we often end up using excessive amounts of NaOH, resulting in an overly basic solution. Therefore, I seek guidance on the appropriate quantity of NaOH required to neutralize the Aqua Regia effectively.

Furthermore, I would like to inquire about the viability of preparing an aqueous solution of NaOH prior to its addition to the 60-liter drum. Will this method enhance the neutralization process?

Any advice is greatly appreciated.

EDIT: At my job we digest silver alloys and we use 10 mL of 30% HNO3 and 5 mL Concentrated HCl. When we filter to collect AgCl we use DI H2O and the amount that we use is not the same through out all of the filtering. So the concentration of the Aqua Regia is most like below 2%. I just used 2% as a guess really.

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    $\begingroup$ Seems curious that you do not have an environmental health and safety (EHS) outfit, or the like, that can tell you what you can and cannot do with hazardous waste, particularly when you have substantial volumes. Where I was, there were lots of rules and regulations, yearly online safety training and processing hazardous waste was a no-no. Everything went into properly labeled and appropriate containers, for later pick up by the EHS people. $\endgroup$
    – Ed V
    Commented Jun 15, 2023 at 17:13
  • $\begingroup$ At my job we like to do things our selves because I work for a family run business and It can get costly to get EHS to come and pick up all of our waste. We use alot of acid if you can see my edit and we would need them to come almost every other day. We do follow OSHA Guidelines and do have the correct PPE. (safety glasses, lab coats, gloves etc...) $\endgroup$
    – Matt
    Commented Jun 15, 2023 at 17:41
  • $\begingroup$ Thanks for the clarification! I know from experience that the EHS folks can be weirdly pedantic: one time I saw them bring a big plastic tub, on wheels and loaded with baking soda (just in case of a spill in transit), to pick up a single bottle of chemical. That chemical was sodium bicarbonate. ;-(. I have not voted, by the way. $\endgroup$
    – Ed V
    Commented Jun 15, 2023 at 17:47
  • $\begingroup$ Wow that's just a bit over the top haha. I started my job last September and I remember my supervisor and I talked to the President of our company for EHS and he told us it was some ridiculous price to get our waste picked up every week. We would need it more than once a week as well. $\endgroup$
    – Matt
    Commented Jun 15, 2023 at 17:54
  • $\begingroup$ There's no "2% Aqua Regia", you can have aqua regia (small letters!) diluted with water. $\endgroup$
    – Mithoron
    Commented Jun 15, 2023 at 23:00

2 Answers 2

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Instead of adding sodium hydroxyde $\ce{NaOH}$ to your solution, it is advisable to replace it by sodium bicarbonate ($\ce{NaHCO3}$) which is also named sodium hydrogen carbonate. This powder is not corrosive (like $\ce{NaOH}$). But it will produce plenty of gas ($\ce{CO2}$) when in the contact with acidic solutions. As aqua regia is made by mixing $\ce{HCl}$ and $\ce{HNO3}$, their reactions with $\ce{NaHCO3}$ are : $$\ce{NaHCO3 + HCl -> NaCl + H2O + CO2}$$ $$\ce{NaHCO3 + HNO3 -> NaNO3 + H2O + CO2}$$ And $\ce{NaHCO3}$ will also react similarly with $\ce{NOCl + Cl2}$ which are the products of the reaction of $\ce{HCl}$ plus $\ce{HNO3}$

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  • $\begingroup$ I do not have access to Sodium Bicarbonate, we could buy it but my facility already has large amounts of NaOH. $\endgroup$
    – Matt
    Commented Jun 15, 2023 at 17:29
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You do not say, what exactly you mean by 2% aqua regia ($\pu{1.2 L}$ diluted to $\ce{60 L}$?), nor its state after reaction with metals, so no precise numbers are possible.

Take small aliquote and realize how much hydroxide, carbonate or bicarbonate you need to use.

Then scale it up.

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  • $\begingroup$ Alright I did not specify, the aqua regia is about 2% because of the volume being at 60L. $\endgroup$
    – Matt
    Commented Jun 15, 2023 at 17:28
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    $\begingroup$ Aqua regia cannot be $2$%. If you mix of $3 \ce{HCl}$ + $1 \ce{HNO3}$, both being concentrated, it produces aqua regia. It can dissolve gold. If the mixture is diluted more than $2$ or $3$ times, it is not aqua regia any more. It is a mixture of diluted nitric + hydrochloric acids. And this mixture does not dissolve gold. $\endgroup$
    – Maurice
    Commented Jun 15, 2023 at 19:04

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