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  • Analytical Balance – Working Range

    Analytical balances are the key instrument of quantitative analysis. It is used to accurately weigh samples and standards. It is one of the primary activities of the analysis process. Its industrial application includes pharmaceuticals, plastic, food, chemical manufacturing, calibration laboratories etc.

    Analytical balance is used for accurate weighing of samples and standards; it is one of the primary activities of the analysis.

    The balances are able to provide accurate measurements, due to the extremely sensitive nature of these instruments there are several factors that can cause them to give erroneous readings.

    United States Pharmacopeia (USP) has set strict requirements for balances used for weighing analytes for quantitative measures. A recent update effective August 1, 2019, highlights the need for accuracy and stresses the concept of minimum weight as the basis for accurate weighing processes.

    Analytical weighing balance calibration

    For an analytical balance to give an accurate reading, the instrument must be calibrated. Calibration is important because it defines the accuracy and quality of the measurements that are recorded by the balance.

    Analytical balances are highly sensitive instruments that are commonly used in scientific research, pharmaceutical laboratories, quality control, and other precision measurement applications. Calibration is necessary because various factors can affect the performance of the balance over time, such as temperature changes, mechanical stress, or electronic drift.

    Reasons that can affect the accuracy of analytical balances

    Due to the extremely sensitive nature of these instruments, there are several factors that can cause them to give erroneous readings.

    • Temperature
    • Vibrations
    • Chemical Reactions
    • Air Currents
    • Calibration
    • User Mistake
    • Messy Workspace
    • Magnets
    • Slope
    • Improper Grounding
    • Plastic or Glass Weigh Containers
    • Handling of the samples

    Working Range

    Working Range is a balance is the weight range within which it will give accurate results.

    The operating range minimum and maximum means: The minimum value is the lowest value at which the balance will give an accurate reading as per the repeatability test. The maximum value is the maximum capacity of the balance.

    USP General Chapter 41 (Balances) sets three distinct requirements to be applied to materials that must be accurately weighed.

    • Accuracy
    • Repeatability
    • Calibration

    Analytical balance uncertainty

    Analytical balance uncertainty refers to the measure of the potential error or deviation in the readings obtained from an analytical balance. It provides an estimate of the range within which the true value of a measured quantity lies.

    Accuracy standards

    As per Chapter 41, the accuracy of the balance must also be tested. For the accuracy test, the weight must have a mass that is between 5% and 100% of the balance’s capacity, and the measurement uncertainty of the weight must be ≤ 1/3 of 0.10% (≤ 0.033%). The weight must be calibrated to prove the uncertainty.

    Repeatability test

    Includes at least 10 measurements using one test weight. It’s recommended to use a half-load weight (one that is approximately one-half of the balance’s weighing capacity) for this test.

    Based on the repeatability acceptance criterion, (2 x s) / (smallest net weight) ≤ 0.10 %, all masses equal or larger than (2 x s) / (0.10 %) conform with this requirement. The smallest mass that satisfies this criterion is called minimum weight:

    mmin = (2 x s) / (0.10 %) = 2,000 x s

    Note that the smallest standard deviation (s) that may be used for the assessment against the 0.10 % repeatability requirement and for the calculation of minimum weight is 0.41 d, where d is the digital increment (readability) of the balance. This lower limit is based on the rounding error of a digital indication.

    Calibration

    “The weighing shall be performed using a balance that is calibrated over the operating range and meets the requirements defined for repeatability and accuracy.” Both acceptance criteria are expressed as relative limit values, 0.10 %. Calibration is the third requirement.

    USP General Chapter 1251 elaborates more fully on routine balance tests by applying a risk-based approach to performance verification activities. Furthermore, it elaborates on the variability of the balance’s performance over time, which leads to the concept of the safety factor. 

    USP General Chapter 41 and General Chapter 1251 ensure that the operator can trust the accuracy of the balance over time.

    References:

    • US Pharmacopeia– 43
    • USP General Chapter <41> Balances,
    • General Chapter <1251> Weighing on an Analytical Balance,
    • Reichmuth, A., Weighing Small Samples on Laboratory Balances, 13th International Metrology Congress, Lille (F), 2007
    • JCGM 200 (VIM), “International Vocabulary of Metrology – Basic and General Concepts and Associated Terms”, 3rd ed., JCGM, 2010,

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