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How to choose the right solvent for cleaning scientific instrumentation

Measuring instruments are typically built to measure a wide range of samples. However, the measuring cell, hoses, and connections must be cleaned after measurement to keep the device in good working condition and make sure the measurement on the next sample delivers correct results.

Choosing the most suitable solvent to clean the measuring cell(s) of a scientific instrument depends on the sample or chemical which needs to be removed from the device. The aim of a cleaning procedure is to leave the measuring device ready to use for the next measurement or experiment.

If the measuring device has a sample changer, the cleaning program can be predefined to automatically use specific solvent combinations and drying times.

Before using a solvent for the cleaning task you need to consider:

The type of sample which needs to be removed from the measuring cell
Which type of solvent could be used
Which order to use the solvents in

Which type of sample needs to be removed from the measuring cell?

Samples have different properties and these properties determine the solubility of the sample in a solvent. The solubility of a substance fundamentally depends on:

physical and chemical properties of the solute
physical and chemical properties of the solvent
temperature
pressure
pH of the solvent-solute solution^[1]
…

Deciding which solvent is required to rinse out the sample follows the maxim: “Similia similibus solvuntur” (like dissolves like).

Types of solvent for cleaning scientific instrumentation

Solvents are chemicals. They can be used in their pure form or as mixtures consisting of different solvents. A solvent or cleaning agent can also simply be water mixed with some detergent.

Solvents can be classified into several categories according to their chemical structure and physical behavior^[2].

To understand their behavior as cleaning agents, it is important to understand that solvents are either polar or non-polar.

What is a polar solvent?

A polar solvent is a solvent with a high polarity. The polarity of a substance affects its solubility. To determine a substance’s polarity you can measure the dielectric constant. The higher the dielectric constant, the higher the polarity of the substance. Water is a polar solvent, which is indicated by its high dielectric constant of 88 (at 0 °C)^[3]. Polar solvents are most suitable for dissolving polar samples.

Common polar solvents:

Acetone
Water
n-butanol
Ethanol
Isopropanol

What is a non-polar solvent?

Solvents with a dielectric constant of less than 15 are generally considered to be non-polar^[4][5]. Non-polar solvents are most suitable for dissolving non-polar samples.

Common non-polar solvents:

Hexane
Toluene
Chloroform
Octanol
Dichloromethane
Xylene

What is a solvent mixture?

Solvent mixtures can have several attributes. They can be polar and non-polar as well at the same time. This ambivalence often helps to dissolve tricky samples easily.^[6]

A good example is the ternary mixture used for cleaning in-service oils. Dissolving in-service oils is tricky because the sample itself is a combination of soot, oil, and impurities such as water. Therefore, the solvent requires something to dissolve the oil, something to take out the water, and something so clean the soot. Further, the solvent needs a boiling point higher than 100 °C and lower than 200 °C.

To clean the soot > diesel
To clean the water > toluene (because toluene and water together make a perfect azeotrope)
To clean the oil > also toluene
To achieve the boiling point > n-butanol.
The perfect ratio is: toluene: diesel: n-butanol = 1:1:1

Read more about the application of this ternary mixture.

Using the right solvents in the right order

Often it is necessary to use more than one solvent, e.g. the first solvent dissolves the sample in the measuring cell and allows it to be removed easily. The second solvent rinses the first solvent out of the cell and facilitates quick drying.

Properties of the first solvent

The first solvent should dissolve the samples completely. This means that the solubility of the sample in the first solvent has to be very high. For example, samples that contain mainly alcohol can be dissolved in water or in more alcohol such as ethanol. Samples that contain fats, such as creams, can easily be dissolved in the solvent naphtha or white spirit. The easiest way to identify the type of solvent that is suitable for the sample to be diluted in is to perform a test as follows:

Take a small amount of sample and put it into a glass vial
Pour the solvent onto it
Observe whether the solvent dissolves the sample within an acceptable period of time.

If the sample aggregates in the solvent, then the solvent is unsuitable.

If the sample starts to dissolve and homogenize in the solvent, then this was a good choice for the sample.

Properties of the second solvent

For the second solvent, it is recommended to choose a solvent that helps to reduce the drying time of the measuring device. Usually this type of solvent has a very low boiling point such as acetone or ethanol. It is also advised to choose a solvent that is not harmful and widely available.

In rare cases it is recommended to use a third solvent as well. In this case, the third solvent should also enhance the cleaning function.

The following table gives an overview about combinations of solvents and samples.

Table 1: Overview of samples and suitable solvents

Samples	Suitable solvent for cleaning	Solvent for drying	Explanation
Alcohols, such as liqueurs and wines, or beer	Soluble in water and/or in ethanol	Ethanol or acetone	The sample has polar properties and is soluble in a typically polar solvent.
Honey and sugary solutions, maple syrup	Soluble in warm water	Ethanol, due to the fact that acetone is typically not allowed in food industry	Sugar dissolves in water better than in alcohol, and if heat is applied as well, then the process can be enhanced.
Waxes of biological or artificial origin	Soluble in non-polar organic solvent such as toluene, xylene, or in a warmed oil (the oil can be a low-viscosity base oil)	Cleaners naphta with a low boiling point or isopropyl alcohol	Waxes are insoluble in water and usually solid; therefore heat is needed to melt them and to properly dilute them in a solvent.
Polymers (for plastics)	Soluble in acids or organic solvent. Highly recommended as the second solvent.	Ethanol only! Ethanol quenches the organic solvent.	Polymer solutions typically aggregate in water; therefore it is necessary to follow the manufacturer’s recommendations according to these samples.
Fragrances	Soluble in acetone and ethanol	Acetone	The sample and solvent have very similar polarity. This behavior is utilized to produce eau de colognes.
Oily flavors	Soluble in acetone and white spirit	Acetone	Lipophilic samples need non-polar solvents; otherwise they would form suspensions or two-phase systems.
In-service engine oils	Ternary mixture of toluene, diesel, and n-butanol (1:1:1)	Cleaner’s naphtha or ethanol	The sample and the sample matrix need special cleaning conditions. Soot is cleaned out with diesel, toluene takes the oil, and the alcohol is responsible for the optimal boiling point
Crude oils	Toluene and/or xylene, either pure or as a mixture	Some cleaner’s naphtha	Toluene and xylene are aromatic solvents with a high boiling point and optimal to clean sticky crude oil.
Organic resin	DMSO	Ethanol	DMSO dissolves many organic compounds easily but it has a boiling point higher than 150 °C. Therefore, ethanol as the second solvent will help to reduce the drying time
Milk or dairy products	Water and some enzymes. Fat-dissolving enzyme solution such as tetrazyme.	Ethanol	Milk or dairy products are water-fat emulsions and they have a natural origin. Therefore, an enzymatic solution is necessary to remove the lipids from the system and ethanol to dry it and carry out a disinfection.
Liquid soap	Water and/or acetone. Water as a pure solvent or as a mixture with the acetone.	Acetone	Water dissolves liquid soap but also leads to foam. Therefore, the acetone is needed to break the foam and to improve the water’s washing ability.

References

En.wikipedia.org. (2017). Solubility. [online] Available at: en.wikipedia.org/wiki/Solubility [Accessed 12 Jul. 2017].
Becker, H., Berger, W., Domschke, G., Beckert, R., Fanghänel, E. and Habicher, W. (2004). Organikum. 22nd ed. Weinheim: Wiley-VCH, p.36.
Malmberg, C. G.; Maryott, A. A. (January 1956). "Dielectric Constant of Water from 0° to 100°C" (PDF). Journal of Research of the National Bureau of Standards. 56 (1): 1. doi:10.6028/jres.056.001. Retrieved 27 June 2014.
Lowry, T. and Richardson, K. (1987). Answer book to accompany Mechanism and theory in organic chemistry. New York: Harper & Row, p.77.
Anon, (2017). [online] Available at: www.idc-online.com/technical_references/pdfs/chemical_engineering/Classification_of_solvents.pdf [Accessed 12 Jul. 2017].
rushim.ru/books/spravochniki/azeotropic-dataII.pdf [Accessed January 14, 2019]

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