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Every measurement on ANY balance is subject to uncertainty. Understanding this uncertainty is the key to ensuring accurate weighing results and avoiding errors in downstream processing.
It is not the readability that determines the accuracy of a weighing instrument, but rather its repeatability. Its minimum net sample weight capability is also of importance.
To find the appropriate balance for your needs you need to know the smallest amount you want to weigh and how accurately you need to weigh it (i.e. at what tolerance).
METTLER TOLEDO's global weighing standard, GWP® helps you choose the right balance to meet your application requirements. Ask your local representative for a free balance recommendation.
To ensure accurate titration samples, the correct balance must be used. (See question 1.) However, to achieve the highest level of accuracy in sample preparation, we recommend using weight for liquid samples as well as solid. Using density to convert the weight value to volume is more accurate than reading the volume from the scale on volumetric glassware.
Electrostatic charging occurs primarily through stirring or rubbing during the handling and transportation of containers and materials. If the relative humidity is less than 40%, the risk of electrostatic charging is higher. Electrostatically charged samples and containers exert unseen forces on the balance weighing cell and hence cause errors in the weighing result. If your balance takes a long time to settle, electrostatic charging could be the problem. To avoid such problems, we recommend to:
METTLER TOLEDO Excellence analytical balances incorporate the unique StaticDetect™ technology which provides you with a warning if detected electrostatic charges exceed your pre-defined limits.
Error can be introduced into the titration process through the measurement of the volumes of liquid used to prepare the solutions (e.g. liquid samples, titrant solutions). There are errors associated with volumetric glassware. However, these could be adjusted for via careful calibration. In addition, the subjective reading of the volume off a scale can be prone to parallax error. By weighing the liquid and adjusting for density, the volume calculation becomes much more accurate:
Volume = Mass / Density
According to ASTM D664, for the lowest TAN values (0.05 - < 1.0), the sample size should be 20 g +/- 2.0 g and requires a weighing accuracy of 0.10 g. For the highest TAN values (100 - < 260), the sample size should be 0.1 g +/- 0.01 g with a weighing accuracy of 0.0005 g.
To accommodate these needs, METTLER TOLEDO would recommend an XPE204 or XPE205 analytical balance. To be sure you select the right balance for your needs, METTLER TOLEDO's global weighing standard, GWP® helps you choose the right balance to meet your application and quality requirements. This service is free.
Following ASTM D4739, an approximate mass of sample can be calculated based on the expected base number as following: “An approximate mass of sample (g) is 7 divided by expected base number.”
METTLER TOLEDO Excellence balances have a built-in application for titration which helps to simply and speed-up the weighing process. The application works in conjunction with an RFID reader (optional) to read and write data on an RFID tag. The tag is placed on the base of your titration beaker and automatically transfers the sample data securely to selected METTLER TOLEDO titrators.
By connecting your balance to METTLER TOLEDO's LabX laboratory software, you benefit further from full step-by-step SOP user guidance and customized processes with all data saved securely in a centralized database.
Weight values are often written down by hand or typed into a spreadsheet or other software. It's important that all weight values are recorded correctly as they will be used in several subsequent calculations. However, manual data transcription is laborious and a major source of errors. A small error in a weight value can become magnified in downstream processing and lead to costly and time-consuming rework. To simplify data handling:
For the highest level of data security and ease of data management, LabX laboratory software is recommended. LabX can be used to record all results, balance information, and user details, as well as control the weighing process for all balances in a laboratory. LabX supports you with 21 CFR part 11 compliance.
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Typical titrations require the titrant and analyte to be in a liquid form (solution). The solid samples are usually dissolved in water, but other solvents, such as glacial acetic acid or ethanol, may be used for specialized applications e.g. in petrochemistry.
In preparing for titration, the concentration of the titrant and the weight of the solid analyte must be accurately determined. It is important that all information and measurements are carefully logged, particularly when titrating multiple samples, to avoid any mix-ups between samples and incorrect calculations.
Preparation of the Sample
The table below provides information on the best techniques for preparing titration samples according to the characteristics of your sample.
Preparing titration samples is a key part of many standard titration test methods such as those from United States Pharmacopoeia (USP), International Organization for Standardization (ISO), and American Society for testing and materials (ASTM). Most standards specify the required level of weighing accuracy.
For example, the ASTM D664 Standard Test Method for Acid Number (AN) of petroleum products includes recommended sample weights plus the required weighing accuracy:
There are many more ASTM methods which use titration as a method for determination of different parameters, e.g. base number, bromine number. These methods specify sample size as a critical element of the standard. E.g. ASTM D974, D2896, D1159 and more.
Sample and Data Handling
Titration is a common laboratory technique which is relatively straightforward to carry out. However, when working with several samples, which may be from the same analyte as well as from different analytes, sample and data handling can be challenging. It's vitally important to label samples and ensure all measurements are correctly logged against the correct sample.
Particular attention should be paid to:
Accuracy and Time
Obtaining accurate titration results is highly dependent on preparing titration samples accurately. With several standards specifying precise sample sizes and an associated weighing accuracy, it is critical that the balance used meets the needs of the titration application.
For small samples, the minimum net sample weight of the balance must be considered – weighing a sample below this weight cannot be trusted to be of the required level of accuracy. In a busy laboratory, in which many samples are tested every day, a robust balance that stabilizes quickly will ensure operators can keep working on tasks and are not kept waiting for the figures on the display to settle. Manual transcription of sample data, weight values and titration results is also time consuming and prone to error.
Keeping titration and weighing equipment thoroughly clean is important to prevent cross contamination. If samples are dirty and/or viscous, weighing and cleaning can become challenging.
Connect your METTLER TOLEDO Excellence analytical balance to LabX laboratory software for the highest level of automated data handling and process security. The titration application is started directly from the balance touchscreen and users simply follow the onscreen instructions. LabX automatically takes care of all data and calculations, and all information is saved securely in a centralized database.
Excellence Balances connected to LabX have a dedicated entry point for titration tasks that require weighing. Simply select the task and LabX provides step-by-step instructions on the balance to guide you through the sample weighing process. For improved productivity, you can even start your titration whilst weighing more samples.
Either start your analysis at the instrument or from the PC and have constant access to both your current sample analysis and the latest results. No matter where you are, the LabX Mail facility can send messages or results to you.
Fully automate the selection of the correct method and transcription of samples IDs with a barcode or SmartSample™ RFID tag. Thus, eliminate sample order errors by reading the sample information as you test it, as well as ensuring the right method is selected for each product.
The Regulation option of LabX server provides all the necessary tools to help you meet the FDA regulation (21 CFR Part 11) for data management and storage. All relevant actions taken at the instrument or PC are recorded in the audit trail of LabX for full traceability and flexibility, no matter where you work.