Which is the right balance for weighing my sample?

In day to day analysis your weighing requirements will depend on quantity of sample available, sensitivity of weighing balance and the sensitivity of the analytical technique. The three types of digital display balances in common use in most of the laboratories are

Top pan balance

Top Pan balance Range: g – Kg

Electronic Analytical Balance

Analytical Balance Range : 0.1mg – 220g

Micro Balance

Micro Balance Range: 10^-^6 – 5g

Top Pan Balance

A Top Pan Balance is used mainly for rough weighing for preparation of reagents and stock solutions where slight weight variations do not have significant influence on the result of analysis.

Electronic Analytical Balance

Majority of times you will find an electronic analytical balance suitable for weighing your samples and reference standards. The readability or least count of such balance is 0.1mg or 0.0001gm.

Micro Balance

A Micro Balance provides the ultimate in terms of accuracy for weighing of very small amounts of samples. The readability of a micro balances 1 µg or 10^ -^6 g or 10^-^3mg and for more sensitive weighing an ultra micro balance can be used which has readability of 0.1 µg or 10^-^4 mg or 10^-^7g. It would be indeed very rare that you will have requirement of weighing below these values. Use of micro balance requires a cabinet that is dust free, wind draft free, anti-vibration and anti-accoustic so that no external disturbances affect your weighings.

Suitability of a balance

The suitability of a balance is governed by the amount of sample that is required to be weighed as the same balance cannot be used to weigh samples ranging from grams to micrograms. As per US Pharmacopoeia Section 41 the measurement uncertainty (random plus system systematic errors) does not exceed 0.1% of the reading. This means that if you need to weigh a mass of 10 mg the combined percent uncertainty should not exceed the 0.01 mg. As the readability of electronic analytical balance is 0.1 mg it is advisable that masses below 10 mg are not weighed using such balance for acceptability of results.

Having decided on the suitability of the balance it is important to follow correct weighing practices which will be covered in a subsequent article.

Hope the guidelines provided will help you choose the balance for the required application.

Please to offer your comments and suggestions.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

Comments

  1. Wind and vibrations affect readings significantly on analytical balances and are of negligible significance in top pan balance weighings.For analytical range weighings it is advisable to use vibration proof tables or work benches and take readings after closing the covers.

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