Knauer osmometer for quality control of aqueous samples

Knauer's K-7400S Semi-Micro Osmometer

Knauer has launched an osmometer for quality control of aqueous samples.

The K-7400S Semi-Micro Osmometer can analyse samples of 50 µl to 150 µl.

The device measures the freezing point depression of aqueous samples, which correlates to the total concentration of all dissolved particles, and displays a single value.

Measurement time depends on the sample volume but is around two minutes.

Analysis of sport drinks

Dr Till Myrach said if a company wants to promote drinks as “isotonic”, osmolality should be measured, since it is related to tonicity.

The European Commission defines that an isotonic drink has to have an osmolality of 300 mOsmol/kg +/-10%.

“The benefit of an isotonic drink is that its osmotic value equals the one of human blood and therefore nutrients and minerals in the drink can quickly be resorbed and digested.

“Another example for osmolality measurements is the quality control of milk. The water content of milk has to be determined frequently before it is used for dairy production.  

“Typically, the freezing point of raw milk is between -0.525 °C and -0.535 °C. A higher freezing point, specifically above -0.515 °C, indicates that the water content is higher than normal and as a result the milk not used.”

Osmolality gives a general measure about the number of solutes in a liquid so it is not necessary to determine the concentration of each single solute but just refer to one value.

The instrument has a peltier cooler and an integrated microprocessor controlling the automated measurement.

Knauer said vapor pressure osmometry can be used as an alternative and it is possible to determine the concentration of each single solute and then calculate the osmolality but the analytical effort would be disproportionate in comparison to freezing point osmometry.

Osmolality range and resolution

It has an osmolality range of 0 - 2000 mOsmol/kg and resolution of 1 mOsmol/kg.

Pure water has an osmolality of 0 mOsmol/kg, said Dr Myrach.

“Every substance that is dissolved in water increases the osmolality of the solution and depresses the freezing point. Human blood has an osmolality of about 290-300 mOsmol/kg. Urine can vary depending on the condition of the patient 420 – 1200 mOsmol/kg.

“Vitamins Juice is approx. around 800 mOsmol/kg, beer has ~1700 mOsmol/kg and soft drinks are between 100 – 500 mOsmol/kg.”

Knauer optionally provides a printer and control software EuroOsmo 7400 for documentation.

The software automatically plots the temperature curve for each measurement and calibration and allows saving of the measured values.

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