An analytical balance is a precision instrument requiring much more care and attention than industrial scales and balances used outside academic and industrial research and QC labs. Analytical balances also represent a substantial investment with prices depending on weighing range, readout, reproducibility and linearity. For these and other reasons lab personnel should understand how and how not to use these instruments, thereby maintaining equipment accuracy and extending equipment life. Here are some basic steps to follow. The balance operations manual, however, is the authority in all cases.
Setting Up an Analytical Balance
As Maria sings in “The Sound of Music” let’s start at the very beginning. Setup instructions for Kern analytical balances offered by Tovatech stress the importance of location, which ideally will be permanent.
Before unpacking the precision scale select a firm level surface remote from extreme heat, temperature fluctuations, direct sunlight, drafts, high humidity, vapors, dust and vibration. … Read the rest
The narrator in this video has a pet peeve. A mistake that he has seen repeated time and again by not just undergraduate students. Postgraduate and doctoral candidates too make this mistake when working with an analytical balance. In the first part of the video he explains the general functioning of the analytical balance and the steps you need to take to get accurate and consistent results. In relative terms what is important is consistency. Should the analytical balance reading be wrong, if the error is consistent – due to a faulty calibration or some such issue then the results are usable to some degree. However if the readings are inconsistent and vary widely for the same type of weighing object or process then the results are both inconsistent and unusable. As for that pet peeve? Watch the video to find out!… Read the rest
Here is another video that explains the use of an analytical balance. In this video the instructor explains several key elements that are critical to making accurate measurements with the analytical balance. This includes making sure the balance pan is free of all debris and spills and measuring the object at room temperature. Objects that are above or below room temperature result in pressure changes leading to inaccurate results. While the video shows how to calculate the mass of the substance by noting the difference in mass with and without the substance on the measurement vessel, you can also obtain the mass of the substance directly by taring the empty measurement vessel.… Read the rest
If you’ve ever wondered about the difference between a precision scale and an analytical balance then this experiment will be instructive. The document explains the difference between mass and weight and the functioning of the top loading precision scale and the analytical balance. It then goes on to discuss precision, accuracy, significant figures, and error analysis (linked elsewhere), and finally, it explains the experiment that will be used to demonstrate the workings of both these balances. As with instructions on using an analytical balance in other places on our blog, this document too highlights the critical aspect of working with these sensitive measuring devices.
Here’s another instruction sheet on the proper use of an analytical balance. As with other similar how-to’s that we’ve covered this one is also similar. Avoid vibrations and drafts, do not touch the objects being weighed, allow hot objects to cool before weighing, and other similar tips. A couple though need to be called out for special attention. Avoid smudging the objects being weighted as even fingerprints will have an impact on the mass. At 0.1 mg precision fingerprints and even air can change the result. The second tip is similar. Measuring the mass of hygroscopic materials. Do this fast as the material will be absorbing atmospheric water vapor even as the weighing progresses.
Density and Volume are two critical concepts for anyone working the lab. Usually this is taken for granted when you move on to more complex experiments. For those who have some doubts about these two aspects this primer document explains the basic methods used to determine density and volume of various objects and then provides instructions for experiments that can be used to experimentally understand these concepts. The essential procedure for determining density is simple. Calculate the mass of the object (usually using an analytical balance) and then divide it by its volume. This will give you density. There are of course various use cases where the mass and volume are not directly measurable and you will need to use indirect means. The document touches on these aspects as well.
Determining the molar mass of condensable vapor may seem impossible if you don’t know how. This document explains the procedure in simple terms. The process involves adding excess of the liquid that corresponds to the vapor to a weighed bulb. This is then heated. Once heated all the liquid in the bulb gets volatilized and the excess vapor is expelled. Next the bulb is cooled and the weight of the bulb with the condensed vapor is measured. The condensed vapor’s mass is equal to the the vapor’s mass found in the flask at elevated temperature and the existing pressure. This can then be used to calculate the moles present and the liquid’s molar mass from the “ideal gas law”. Equipment used includes: ring stand, hot plate, analytical balance, beaker, flask, tap water, aluminum foil and 5 ml of an organic fluid that is volatile.
Here is another short but precise set of instructions for using the analytical balance. The key points to note are: the location where you set up the analytical balance should be free of ambient air currents and it should be free of vibrations as both these factors will have a negative impact on the readings. Not mentioned is the possibility of using a vibration dampening table – typically a counter top made of some heavy material like marble and rubber studs on the legs to absorb the vibration. This would be useful in places where it is not possible to avoid vibrations entirely. The other instructions are also relevant: be careful not to disturb the balance after taring (or zeroing) the balance, use weighing dishes, tongs, or gloves as needed, cool hot objects, weigh hygroscopic materials rapidly due to their tendency to absorb water, and always use the same procedure … Read the rest
Balances are crucial to nearly everything done in the chemistry lab. Thus it becomes all the more important that you know how to work with them. This primer on using different types of balances is a good place to start. Targeted at beginners in analytical chemistry it explains the use of the analytical balance, the proper method of weighing on an analytical balance, and the precautions to take when using the unit. Accompanied by detailed pictures on handling various lab equipment, it gives a visual cue to the detailed explanations on how to use the balances safely and properly for accurate results.
Here is an introductory lesson that you can use in the lab to learn how to weigh samples with an analytical balance. There are two exercises described in the document. The first one analyzes the percentage of water in a hydrate while the second one does a statistical analysis to determine the mass of a penny. Both exercises are good for practice that will expose the user to a wide range of functions in the analytical balance. The document describes both the objective of the experiment as well as the procedure to be followed. The complete experiment is spread over a three week period.