Going Green – Solvents

First published on Cohen user blog December 2015. Updated 7th May 2017 by Andy Connelly.

Introduction

Many laboratories are looking for ways of decreasing their environmental impact and improving safety for our lab users. Something I have been interested in recently as a laboratory manager is the use of non-aqueous solvents. The most common solvents used in my labs are shown in Table 1.

As you can see we do not use a wide range of non-aqueous solvents on a day to day basis and applications are very simple. We do have some extractions using n-hexane, dichloromethane, etc. However, the use of these is carefully controlled and separate assessments are performed before these are used. In this blog post I will look at the four non-aqueous solvents in Table 1.

table1
Table 1: The main non-aqueous solvents we have historically used in Cohen Laboratories and their roles.

DISCLAIMER: I am not an expert in solvent chemistry. The content of this blog is what I have discovered through my efforts to understand the subject. I have done my best to make the information here in as accurate as possible. If you spot any errors or admissions, or have any comments, please let me know. HANDLING OF NON-AQUEOUS SOLVENTS OR ANY OTHER CHEMICAL MENTIONED IN THIS POST SHOULD BE CARRIED OUT UNDER LOCAL H&S POLICIES AND WITH APPROPRIATE RISK AND COSHH ASSESSMENTS.

General properties

Selected properties of these common non-aqueous solvents are shown in Table 2. The most important properties for our use are the boiling point and the relative polarity. Much of our use of non-aqueous solvents relies on them being quick to evaporate (low boiling point) and good at dissolving grease and removing pen (low relative polarity). Acetone fits these properties very well, this and the relatively cheap cost, make it a very attractive solvent. Some further general properties are described below:

  • Methanol is the simplest alcohol, and is a light, volatile, colourless, flammable liquid with a distinctive odour very similar to that of ethanol (drinking alcohol). Unlike ethanol, methanol is highly toxic and unfit for consumption. Methanol has the weakest bactericidal action of the alcohols [1].
  • Ethanol (drinking alcohol) is a neurotoxic psychoactive drug. It is also volatile, flammable, colourless liquid with a slight chemical odour. Hydrogen bonding causes pure ethanol to be hygroscopic to the extent that it readily absorbs water from the air.
  • Acetone is a colourless, volatile, flammable liquid organic compound. Acetone is a good solvent for many plastics (not polypropylene), some synthetic fibres, and grease so can damage plastic equipment and is bad for your skin. Acetone can also leave a white residue.
  • Isopropanol is a colourless, flammable chemical compound with a strong odour. It is hygroscopic and will absorb moisture rapidly until it reaches an equilibrium value of 65% IPA to 35% water.
Properties of the most common non-aqueous solvents
Table 2: Properties of the most common non-aqueous solvents. 1 All of these solvents are miscible in water (i.e. form homogeneous solution with water). 2 The values for relative polarity are normalized from measurements of solvent shifts of absorption spectra and were extracted from Christian Reichardt, Solvents and Solvent Effects in Organic Chemistry, Wiley-VCH Publishers, 3rd ed., 2003 (http://murov.info/orgsolvents.htm). Relative polarity is a good measure of a solvents ability to dissolve oils and grease (lower value is better at dissolving oils) 3 A short-term exposure limit (STEL) is the acceptable average exposure over a short period of time, usually 15 minutes as long as the time-weighted average is not exceeded. 4 All are highly flammable (liquid and vapour) and cause eye irritation.

EHS scores

As part of an environmental impact review we looked at the non-aqueous solvents listed above to see what the impact of them was to the environment, health and safety using reference [2]. This allows you to give a score for various parameters known as the EHS score. We will start by looking at the health issues associated with each solvent. I have added some other common non-aqueous solvents into the data for comparison.

Health

In terms of health it is clear that ethanol is the least harmful of the solvents in question (see Figure 1). This is mostly because it is not an irritant. However, clearly it does have a level of chronic toxicity (although much less than methanol). On this scoring system acetone and isopropanol are more harmful than methanol; this is because it is not very irritating – but methanol scores much higher on chronic toxicity. In fact, isopropanol and its metabolite, acetone, act as central nervous system (CNS) depressants but are still less toxic than methanol [4].

figure1
Figure 1: EHS score (Health) for some common non-aqueous solvents used in Cohen labs. Calculated using [2]

Safety

The EHS scoring system allows you to separate the environmental, safety, and health aspects of the each solvent. In this comparison the safety aspects of the selected solvents are very similar except for chloroform (see Figure 2). It is interesting that chloroform scores low on this measure this is due to it not being a fire/explosion hazard. Acetone is the most dangerous of the solvents considered but the differences are small.

figure2
Figure 2: EHS (Safety) score for some common non-aqueous solvents used in Cohen labs. Calculated using [2]

Environment

I started this process as I was concerned about the environmental impact of the solvents (see Figure 3). Unlike with the health aspects of the solvents here acetone and methanol score very well; ethanol and isopropanol poorly. However, compared to n-hexane and chloroform they are score very low. The key issue is persistency in the environment which ethanol and isopropanol score very highly on. It is also interesting to compare aqueous and non-aqueous solvents (see below).

figure3
Figure 3: EHS (Environmental) score for some common non-aqueous solvents used in Cohen labs. Calculated using [2]

Cost

General lab solvents that the laboratory I work in buy are not particularly expensive (around £2 per litre). However, this soon adds up and so the relative costs of the solvents are important. Relative costs are shown in Table 3.

As you can see the most expensive is ethanol when duty must be paid. As we are a university laboratory we do not have to pay duty on ethanol and so it is significantly cheaper for us. Clearly the cheapest is methanol, however, this is not ideal as a general lab solvent due to the health issues (see above). Acetone is a common solvent in commercial labs as it is cheapest, however, for us ethanol is a very similar price and is without some of the health and safety issues presented by acetone.

table3
Table 3: Relative costs of solvents compared to that of acetone. These are approximate and calculated from the price we pay.

Conclusion

The combined results are shown in Figure 4. These clearly show that ethanol scores the lowest on the EHS score. Initially, when I started writing this, I assumed that ethanol would come out the best on all counts. However, it is clear that ethanol does have some negatives especially when looking at the environmental impact. Despite this, I am going to strongly encourage the use of ethanol above other solvents in our laboratories. It is better in terms of health and safety and the difference in Environmental scores is not sufficient to outweigh the clear benefits.

The next best to ethanol is methanol but the health issues with methanol make it undesirable. Acetone is the obviously alternative to ethanol as it has a lower boiling point and is slightly cheaper. However, acetone is more of a health hazard and can leave a white residue after evaporation – not generally seen with ethanol.

This is not saying that we will be only using ethanol from now on. However, I will be actively discouraging the use of acetone and isopropanol and remove methanol from the laboratories as much as possible. I have also developed some guidance on how best to use general laboratory solvents.

For all this, the results should be put into context. Figure 5 shows a comparison with some other solvents. Full data.

figure4
Figure 4: EHS score (total) for some common non-aqueous solvents used in Cohen labs. Calculated using [2]
figure5
Figure 5: EHS score (total) for some common aqueous and non-aqueous solvents used in Cohen labs. Concentrations are: hydrochloric acid 32%, nitric acid 65% and caustic soda 50%. Calculated using [2]

Usage guidance

This information is a guide only. You should consult a trained health and safety officer before carrying out any work with non-aqueous solvents:

  • Avoid the inhalation of solvent vapours – use in a fume cupboard or in a well-ventilated area
  • Keep the storage containers tightly closed and store in vented cabinets
  • Never use open flames near flammable solvents
  • Never pour solvents down the sink – dispose of as hazardous waste
  • Avoid contact of the solvent with the skin — many solvents are easily absorbed through the skin. They also tend to dry the skin and may cause sores and wounds.
  • Ensure you are wearing appropriate gloves for the solvent you are handling. Butyl or Viton gloves may be required to properly protect you. Many solvents will diffuse through nitrile gloves very quickly.

References and acknowledgements

Thank you to Fiona, Stephen and the Cohen users for their feedback and thoughts on this blog.

[1] www.cdc.gov/hicpac/Disinfection_Sterilization/6_0disinfection.html

[2] C.Capello, U Fischer and K. Hungerbuhler,,What is a green solvent? A comprehensive framework for the environmental assessment of solvents, Green Chem., 2007, 9, 927–934

[3] www.sust-chem.ethz.ch/tools/ehs

[4] Slaughter RJ, Mason RW, Beasley DM, Vale JA, Schep LJ (2014). “Isopropanol poisoning”. Clinical Toxicology 52 (5): 470–8.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s