Weighing

Here’s a selection of YouTube videos that demonstrate weighing techniques.

Before you use any balance, it’s good practice to check the spirit level to ensure the balance is sitting on a level surface. This is necessary to obtain accurate readings. If the balance is not level, you may adjust its position using the screw legs beneath the balance. If you’re unsure about how to do this, please seek assistance from a more experienced member of staff.

For analytical balances, always close all doors to the enclosure and allow the reading to stabilise.

Weighing papers/boats

Most of these videos demonstrate weighing reagents on a piece of weighing paper or weighing boat. I prefer to weigh directly into the container that I’ll then use the reagent in. This is so that I do not lose any reagent in the process of transferring it into the weighing paper or weighing boat to the final container. However, please ensure that whatever container that you intend to place on the balance is not too heavy for the balance, with or without the reagent that you intend to weigh.

Dispensing materials over the balance

Most of the videos demonstrate dispensing materials on to the weighing paper/boat over the balance. I personally prefer to do so next to the balance, so as not to make a mess of the equipment. I usually overlay the bench with some paper towel, so that if I do make a mess, it’s easy to just pack up the paper towel and dispose of it. The only time when I dispense a material over the balance is when I need to weigh an exact amount (e.g. 100 ± 1 mg). Most of the time I don’t. I also always close the container before weighing, especially when using an analytical balance where it’s awkward to manoeuvre the container within the enclosure. Of course, this means having to tare the balance with the container’s cap/lid on.

How exact do you need to be?

You usually don’t need to weigh an exact amount of a substance to make a solution at a specific concentration. For example, if you need to prepare a 10 mg/mL solution, and you need to make 5 mL of it, you don’t need to weigh exactly 50 mg (i.e. 10 mg/mL × 5 mL) of the substance in order to make exactly 5 mL of the solution. Instead, weigh slightly over 50 mg (i.e. the minimum volume desired), and adjust the volume of solvent to maintain the concentration. In the above example, you could weigh 52 mg but adjust the final volume to 5.2 ml to achieve the desired concentration (52 mg ÷ 5.2 mL = 10 mg/mL).