Heated and stirred: The story of an unsung lab hero

This month, we learn more about the humble hot plate stirrer from Kris Fairfield

The humble hot plate stirrer is a vital piece of laboratory equipment used in the vast majority of laboratories on a daily basis. This often overlooked but tireless workhorse is typically used in a broad range of chemical, biotechnical and medical applications and performs an invaluable service to scientists and researchers across the globe. Delivering lower temperatures than the open flame of a Bunsen burner, but without the associated dangers, the hot plate stirrer heats and combines different solutions in a homogenous manner, ensuring even distribution of any quantities of its constituent parts throughout the liquid.

There are many reasons why the hot plate stirrer is a highly important piece of apparatus in a broad range of laboratories. The lack of an open flame for example, makes it an ideal heating solution where sources of ignition are unsafe, and the use of magnetic fields to provide the stirring motion completely removes the requirement of lubrication. This is, in itself, an advantage in laboratory situations as lubricants could potentially contaminate the reaction vessel and product. 

The designs of the various stirring bars, brings additional advantages in terms of contamination control, as it is easier to clean than other mixing devices. In addition, the plastic, glass and ceramic coatings of the stirring bars renders them chemically inert, which also assists in ensuring the properties of the substances being mixed are not detrimentally affected by the mixing equipment. 

Among its many uses, the hot plate stirrer is crucial in performing and monitoring chemical reactions and titrations, making up buffers and media, as well as many other applications that require solutions to be heated and mixed at controlled speeds and temperatures to meet specific experimental parameters.

The hot plate stirrer has in fact been serving scientists’ needs for more than 90 years. Its first appearance in a US patent issued in 1917 was as a magnetic mixer consisting of a hot plate base housing stationary electromagnets1. A reaction vessel placed on the base contained a bar magnet immersed in liquid, which rotated under the influence of the electromagnets. 

A later incarnation of the magnetic stirrer in 1944 featured a rotating magnet placed in the mixer’s base, rather than electromagnets2. When the plastic-coated bar magnet was independently invented in the late 1940s, its inventor named it the ‘flea’ due to its tendency to erratically jump around in a solution when the rotating magnet in the stirrer’s base was accelerated rapidly3.

Today’s hot plate stirrers have advanced considerably through the years, offering laboratory researchers a selection of advanced features and capabilities that can help save time, increase laboratory productivity and improve the accuracy of experimental results.

The temperature of the hot plate is a crucial element in the mixing efficiency and can now be set very precisely and monitored effectively to meet specific experimental requirements. In addition, temperature uniformity across the hotplate is of great importance in sample heating.

Many models now employ microprocessor-based controls for accurate, consistent and repeatable temperature settings, which ensure that the temperature inside the liquid being mixed is tightly maintained. Solutions can also be heated to specified temperature much more rapidly, thanks to the integral computer technology, cutting down waiting time for scientists and so speeding up experiments. While the top temperature that a heated fluid can reach depends on its quantity and consistency, the maximum offered by some stirrers on the market is 550°C. Some devices also allow the user to precisely specify temperature increments of 1°C, or even 0.1°C for lower temperatures e.g. below 100°C.

Today’s hot plate stirrers give advanced mixing control that monitors and maintains constant speed with repeatable settings. While stirring speeds vary depending on the viscosity and size of the test solution, some devices allow you to regulate stirring speeds for all types of solutions – aqueous, viscous, or semi-solid – typically providing consistent and reliable smooth to intense mixing even at low speeds and high viscosity.
From a safety point of view, protection from accidental scalding is provided by automatic hot surface alerts and power cut outs controlled by temperature sensors are activated if the unit overheats, or when it is not in contact with a liquid. To prevent splashes from bath media, rotation speed can also ramp up slowly until a unit has reached the pre-programmed speed.
Usability has also improved too, with the use of reflective hot plate surfaces that increase sample visibility, making the observation of colour changes easier, while the chemical resistance of glass ceramic surfaces means that accidental spills won’t lead to alkali and acid damage.
But what are the most important criteria to bear in mind when choosing a hot plate stirrer? In terms of analysing its efficiency and usability, the following heating and stirring capabilities are crucial:
• Fast as possible time to reach temperature
• Ability to produce reliable temperature stability and uniform heat throughout the sample
• Efficient device insulation
• Precise stirrer control.
From a practical viewpoint, the time taken to reach the set temperature is an important factor in chemical reactions, as time is precious in the lab; scientists do not want to wait any longer than necessary for the solution to reach boiling point. Equally, if a solution is heating up over a long period of time, a vast proportion of the solution could be lost through vapour before reaching the desired temperature, which is not desirable in a carefully measured reaction.

Measuring the time to reach temperature is easily carried out by timing how long a unit takes to boil a container of water for example. Some units on the market have shown times of less than 8.5 minutes to boil 500mL in a 1L flask, with others taking more than 10 minutes4.

Uniformity and stability are two complementary factors, which are sought-after properties of hot plate stirrers. In reactions where temperature is of crucial importance, you need to know that once you set the unit to the desired temperature, the unit will remain at that temperature until you adjust it.  Looking at uniformity, a liquid on top of the unit should ideally be heated to the same temperature and at the same rate across its entirety, to avoid the danger of ‘hot spots’, and to ensure reactions proceed with the same kinetics throughout the liquid.

By running a unit for at least 30 minutes at a specified temperature, deviations can be monitored to ascertain stability and heat uniformity across the hot plate. Ideally, deviations should be as small as possible, and a number of units have been shown to achieve a temperature stability of ± 0.25°C with only a small degree of variation across the hot plate4.

The more effective the control over the stirrer bar, the greater the range of viscosities that can be stirred. For instance, a hot plate stirrer that only has a weak control over the bar will be limited to solutions with low viscosities. This is because with greater viscosities, as the speed of the vortex increases, the substance starts to control the motion of the bar and not the other way around as is intended. As a result, when further liquid is added to the mix, the mixing is not uniform.
The extent to which the stirring bar is controlled by the magnetic field is an important factor in hot plate stirring technology. If a stirring bar stops shortly after the magnetic field has been switched off then this indicates that the field is controlling the bar, and so is driving the vortex in the liquid rather than, in effect, being driven by it. 
To give an idea of the degree of control exerted by the stirrer over the stirring bar it is interesting to look at how quickly the bar stops spinning in water after turning the stirrer unit off. To do this, use a flask of water and turn the magnetic stirrer up to its maximum rpm and then switch it off – the time taken for the bar to become stationary (even with the water still vortexing) gives an idea of the magnetic force exerted on the stirrer bar by the stirring magnet. Some units provide stopping times of just 2.4 seconds4.

The hot plate stirrer is a fundamental piece of everyday laboratory equipment. Its temperature and stirrer control properties make it a preferable option to the Bunsen burner for many scientists of varying disciplines.  But a hot plate stirrer that can fulfil essential criteria – such as reaching the desired temperature quickly, offering temperature stability and uniformity, maintaining optimum stirrer control, and effective insulation – is more than just another good piece of laboratory equipment.

A high performing and energy efficient hot plate stirrer brings accompanying benefits such as time and cost savings, smoother laboratory efficiency and improved research productivity. In addition, if the lab researcher is confident that the hot plate stirrer they are using gives them consistent and reliable service, they also have greater confidence in the accuracy and quality of their experimental results.