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The best procedures for density determination of solids are the buoyancy and displacement methods, both of which are based on Archimedes' Principle. Prerequisite for these methods is the use of a liquid with known density that does not react with the sample material, but wets it thoroughly. A wetting agent can be added to the liquid.
No, you can quickly convert your laboratory balance to measure density with a Density Kit. The Density Kit can be mounted on your balance in a few simple steps.
The integrated balance application provides step-by-step instructions. Excellence level balances have workflows for 5 different density determination methods. Advanced and Standard level balances have workflows for 2.
The weighing basket on the Density Kit can be inverted so that lighter samples are held under the liquid and cannot float to the surface. If you experience that the buoyancy force is greater than the basket apparatus, place an additional weight on the top weighing pan of the Density Kit and restart the density determination procedure. Alternatively, use a different reference liquid with a lower density.
Add a few drops of wetting agent to the reference liquid. Leave it standing overnight to release any dissolved gas. Use a soft brush to brush the bubbles off the sample and the Density Kit.
The accuracy of density measurement is influenced by the tolerance of the method (air bubbles etc.) and the temperature measurement as well as the accuracy of the weight measurements. Every measurement on ANY balance is subject to uncertainty. Understanding this uncertainty is the key to ensuring accurate weighing results. It is not the readability that determines the accuracy of a weighing instrument, but rather its repeatability and the minimum net sample weight.
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. Determine if your existing balance meets your quality requirements.
ISO 1183-1 compliance requires a balance with a readability of 0.1 mg or less and specifies that the sample should preferably have a mass of at least 1 g. Weighing samples of at least 1 g on a balance with readability 0.1 mg will not generally violate the minimum net sample weight requirement of the balance. The required balance accuracy however, should be considered in conjunction with your required process tolerance. Our free GWP® Recommendation service can help you select the right balance for your specific needs.
There are several steps in the density measurement process, and sometimes you need to wait a while for the balance to settle so it can be easy to get lost off, especially when you are busy with multiple tasks. The integrated balance application provides step-by-step instructions. You confirm each instruction by pressing the OK button, so you always know where you are.
Connect a barcode reader to your balance to enable meta-data, such as sample ID, lot number and order number etc. to be read-in directly without any errors. With the METTLER TOLEDO P-50 line of printers, the meta-data, plus the date and time of the measurement, can be printed out alongside the results.
When you have a series of density determinations to make, the statistics option on METTLER TOLEDO balances enables you to quickly identify trends in your data assisting you to make decisions on a course of action where appropriate.
XPE, XSE, MS-TS, ML-T and ME-T balances have a built-in density database for the most commonly used reference liquids. The density value is adjusted according to the temperature you input.
The density application on XPE, XSE and MS-TS balances does all the calculations for you. You only need to input the temperature and select the reference liquid used. The balance records the weight values and calculates the density automatically.
The density application on MS-TS, ML-T and ME-T balances allows you to create a report of your density determination series which you can print out or save on a USB stick. XPE and XSE balances in combination with LabX software offer a higher degree of report customization with graphs and charts, and the report can be sent straight to your LIMS or EPR.
Jump to one of the following section to explore and learn more:
The solid is weighed in air (A) and then again (B) in the auxiliary liquid with a known density. The density of the solid ρ can be calculated as follows:
ρ = Density of the sample
A = Weight of the sample in air
B = Weight of the sample in the auxiliary liquid
ρ0 = Density of the auxiliary liquid
ρL = Density of air
The temperature of the liquid must be taken into account as this can cause density changes of the order of magnitude 0.001 to 0.1 per °C, the effect
of which can be seen in the third decimal place of the result.
If you know the mass and volume of a sample (solids or liquids), its density can be calculated from:
The Difficulty with Volume
It's quite simple to weigh a sample accurately, but determining the volume of a sample accurately can be challenging.
The buoyancy method avoids the problem of determining volume because it involves weighing the sample twice in two different mediums (air and a liquid). The volume can therefore be assumed to be constant in both situations.
In the simplest application of the displacement method, the volume of a solid sample is determined by observing the increase in the level of the liquid in which the sample is submerged.
Conversely, when an object of known volume is submerged in a liquid with unknown density, the difference in the weight values (in air and in the liquid) can be used to determine the density of the liquid.
A pycnometer is a specially designed glass flask, usually with a defined volume. It is most often used to determine the density of liquids. The pycnometer is first weighed empty and then full of the liquid under investigation. The difference (i.e. the mass of the sample) divided by the volume of the pycnometer is the density of the sample.
The pycnometer method can also be used for determining the density of powder samples or granulates.
Digital Density Meter
A hollow glass tube vibrates at a certain frequency. The frequency changes when the tube is filled with different substances: the higher the mass of the sample, the lower the frequency. Digital density meters work by measuring the frequency and converting it into density.
Please see the table below for a comparison of these four different methods.
Many standards and norms exist for density determination. Some of the most commonly used are:
ISO 1183-1 specifies the use of a 4-decimal place analytical balance.
The Confusion with Bulk Density
Bulk density is a measure of how many particles, parts or pieces are contained within a measured volume. Bulk density is not a property of the material itself. Bulk density includes the spaces between the particles or items as well as any voids within the items themselves. Bulk density can vary depending on how the material is handled; shaking a container, for example, allows parts to settle, increasing the overall bulk density.
The combination of a METTLER TOLEDO Excellence level balance and LabX software offers a higher level of data management and process security. Excellence analytical and precision balances can be set up with a Density Kit for density determination. LabX ensures that your density SOP is followed precisely. LabX records all weight values, performs all calculations and saves all results securely in a central database. All the data related to your density application can be transferred directly to your internal data management system.