Is argon preferred over nitrogen for maintaining an inert atmosphere?

Question

I recently started a project in a new lab group over the Summer that does medicinal chemistry. Unlike in the undergraduate labs, they use argon instead of nitrogen for their inert atmosphere work. When I asked around, the lab members said that it was 'just because they'd always had argon', though a few people said that they thought argon was better, without explanation.

As far as I am aware, both gases are unreactive and can be dried, therefore it makes no sense to me why argon would be preferred given how cheap and abundant nitrogen is. There are some air-sensitive steps in my route but generally nothing too unstable

Is there any real difference between the two that would make one better, or is it, just as my lab said, that it's just what they have and they've never bothered to change it?

Answer 1

Speaking purely from an organic point of view, there isn't really a huge difference, labs tend to stick to one or the other, largely depending upon the availability (some universities use the boil off from the liquid nitrogen cylinders to furnish labs with nitrogen gas, whilst others have no such facilities meaning labs must buy cylinders of either nitrogen or argon).

For serious organic groups (organometallics, total synthesis) doing a lot of air-sensitive chemistry, there can be some advantage to argon due to its higher density [argon (1.714 g/L) nitrogen (1.251 g/L)], meaning it tends to blanket the reaction, and if you need to quickly open the flask to put a reagent in then the argon doesn't escape so quickly.

For inorganic chemists, there are situations where nitrogen will react with certain chemicals, and in these cases argon must be used.

Answer 2

My experience is largely colored by my days as a grad student in an organic chemistry lab that did some organometallics.

I believe for the applications we had (palladium/nickel cross-coupling), argon and nitrogen were functionally equivalent.

For more air/moisture sensitive applications in inorganic chemistry, argon may be preferred because it is heavier than air and thus better forms a protective blanket around your sensitive samples.

However, there are two other downsides to argon that come to mind. The first is price. I was actually tasked by our PI to be our argon monitor to make sure people weren't wasting argon. It was significantly more expensive than nitrogen, and the PI didn't care if you were just blowing nitrogen around.

The second one which people sometimes forget is that argon has a higher boiling point than nitrogen. If you're using argon in your manifold and cooling your trap with liquid nitrogen, you may condense argon in your trap. If then you close off your manifold without an outlet and the trap loses cooling, the manifold will be under intense pressure from the argon boil-off. This presents a serious explosion hazard if you're not paying attention.

Answer 3

I don't think either is generally preferred over the other, it's project and situation-dependent.

For most organic reactions, it won't matter as nitrogen is sufficiently non-reactive to be acceptable and the lower price makes it the go-to gas for most labs. However there is one consideration that makes argon attractive over nitrogen, and that is impurity levels. Although industrial grades for both gases are fairly pure, The specs for argon tend to have a lower impurity level of oxygen than for nitrogen, even though industrial grade nitrogen is typically purer than industrial grade argon.

For metallurgical work, argon is often preferred (actually vacuum is preferred) because nitrogen can react with metals at high temperatures to form nitrides ($\ce{Ti3N4, Si3N4, AlN, BN,}\text{etc...}$) whereas argon will not react. Additionally, nitrogen can more easily adsorb to surfaces which is the place where catalysts are active, thus it may be desired to reduce the amount of adsorbed material on a substrate (like Raney nickel) by using argon over nitrogen.

...unlike in the undergraduate labs, they use argon instead of nitrogen for their inert atmosphere work.

Given the Praxir argon has $\pu{1 ppm}$ less oxygen than the Praxair Nitrogen, it may be unconvincing that either gas matters. Considering the cost of the reagents involved and the students' time, it may not be that wasted of an investment to pay a little more for nitrogen to get more yield every few reactions. Compare that to undergraduate labs, where there tends to be less serious research, the time scales of projects and are much shorter, the reactions are less sensitive and, are typically for pedagogical purposes, which brings me to my next point:

When I asked around, the lab members said that it was 'just because they'd always had argon'

Consider the implications of switching gases:

1. Everyone in the middle of research would be introducing a change in their experiment

or

2. New experiments would have to wait a year or two for current ones to complete

or

3. A second gas system would have to be temporarily used to facilitate the long-term switchover.

None of these options are tenable unless the lab is up for remodeling, moving or new ownership and projects, thus the gas first used is the gas they continue to use.