Guides and metrics: Solvent selection guides

Methodology

The tables on this page are reproduced from from D. Prat, A. Wells, J. Hayler, Sneddon, C. R. McElroy, S. Abou-Shehada and P. J. Dunn, CHEM21 selection guide of classical- and less classical-solvents, Green Chem., 2016, 18, 288–296.. The material is copyright to the Royal Society of Chemistry (RSC) and is reproduced here with their express permission. If you wish to reproduce it elsewhere you must obtain similar permission from the RSC.

After the survey of existing solvent guides[1], the CHEM21 solvent team elaborated a methodology, based on easily available physical properties and GHS statements, permitting anyone to establish safety, health and environment criteria of any solvent, even if full data on the solvents are not yet available.[2]  A simple way to obtain a preliminary ranking of the solvent (so-called “ranking by default”) was also proposed, based on a combination of these SHE criteria. The methodology was tested on the classical solvents having a clear ranking according to the survey, with a fair predictivity of 81%.

The SHE criteria are scored from 1 to 10, the highest figure representing the highest hazard level. A colour code is associated with these scores: 1 to 3: green, 4 to 6: yellow, 7 to 10: red.

The safety score derives mainly from the flash point, with contributions of the auto-ignition temperature (AIT), the resistivity and the ability to form explosive peroxides (statement EUH019 in GHS/CLP) (Table 1).

Table 1: Safety score

Basic Safety score

1

3

4

5

7

Flash Point (°C)

> 60

23 to 60

22 to 0

-1 to -20

< -20

GHS

-

H226  GHS symbol: symbol

H225 or H224  GHS symbol: symbol

1 is added to the safety score for each of the following properties:

  • AIT < 200°C
  • Resistivity > 10 8 ohm.m
  • Ability to form peroxides (EUH019)

Any solvent with a high energy of decomposition (> 500 J/g), like nitromethane, would be scored 10.

As an example, the safety score of diethyl ether, with a FP of – 45°C, an AIT of 160°C, a resistivity of 3.10 11 ohm.m and an EUH019 statement is 7+1+1+1=10.

The health score derives mainly from the most stringent GHS H3xx statements, with a contribution of the boiling point, according to Table 2.

Table 2: Health score

Health score

2

4

6

7

9

CMR

   

H341 H351 H361 (CMR cat. 2) GHS symbol: symbol

 

H340 H350 H360 (CMR cat. 1) GHS symbol: symbol

STOT

H304 H371 H373 GHS symbol: symbol

H334 GHS symbol: symbol

H370 H372 GHS symbol: symbol

   

Acute toxicity

H302 H312 H332 H336 EUH070

GHS symbol: symbol

 

H301 H311 H331

GHS symbol: symbol

 

 

H300 H310 H330

GHS symbol: symbol

Irritation

H315 H317 H319 H335 EUH066

GHS symbol: symbol

H318 GHS symbol: symbol

 

H314 GHS symbol: symbol

 

CMR: carcinogen, mutagen or reprotoxic; STOT: single target organ toxicity

1 is added to this score if the boiling point is <85°C

If, after full REACH registration, there is no H3xx statement, the health score is 1. The score of newer solvents for which the REACH registration is not complete is 5 if the boiling point is 85°C or higher, otherwise, 6, unless a more stringent H3xx is attributed by the supplier.

The environment score takes into account both the volatility of the solvent (ability to form Volatile Organic Compounds) and the energy demand for recycling. Both are linked to the boiling point. The GHS 4xx statements are also important (Table 3). The environment score is based on the most stringent of these factors. For example, the score of 7 for heptane is due to its H410 statement, whereas the score of 7 for dichloromethane is as a result of its low boiling point.

Table 3: Environment score

Environment score

3

5

7

10

BP (°C)

70-139

50-69

140-200

<50

>200

 

GHS/CLP

No H4xx after full REACh registration

H412, H413

GHS symbol: symbol

H400, H410, H411

GHS symbol: symbol

EUH420 (ozone layer hazard)

Other

 

No, or partial REACh registration

   

Water score= 1

 

If the REACh registration is not full and no H4xx has been attributed by the supplier, the environment score is 5 by default.

These S, H & E scores are combined according to Table 4 to give an overall scoring of the solvent. The ranking is imposed by the most stringent combination.

Table 4: Ranking by default

Score combination

Ranking by default

One score ≥ 8

Hazardous

Two "red" scores

Hazardous

One score =7

Problematic

Two "yellow" scores

Problematic

Other

Recommended

The ranking by default does not make the distinction between “hazardous” and “highly hazardous”. The decision to “ban” a solvent necessitates discussion at an organisational level. It is important to remember that the ranking methodology is only a model that gives a ranking by default and this should be critically assessed by occupational hygienists and other experts of any institution using it. For example, the scoring of chloroform as only “problematic” or of pyridine as “recommended” are not acceptable. This illustrates the limits of a health scoring system only based on GHS hazard statements. As the Occupational Threshold Limits of these solvents are available and very low (ppm range), CHEM21 finally ranked them as, respectively, “highly hazardous” and “hazardous”. Other decisions are more the result of the policy of the institution. CHEM21 finally decided to rank methanol and acetone as “recommended”, and on the contrary, cyclohexanone as “problematic”, for reasons which will not be debated here.

  1. D. Prat, J. Hayler and A. Wells, A survey of solvent selection guides, Green Chem., 2014, 16, 4546-4551.
  2. D. Prat, A. Wells, J. Hayler, Sneddon, C. R. McElroy, S. Abou-Shehada and P. J. Dunn, CHEM21 selection guide of classical- and less classical-solvents, Green Chem., 2016, 18, 288–296.