NOVELTY - The method comprises contacting an electrolyte with a first layer containing a material (M) for reacting the surface with hydrofluoric acid derived from the reaction between the lithium hexafluorophosphate and water, measuring the variation in weight of the layer by contacting the layer with an electrode of a quartz crystal microbalance, where the variation in weight of the layer is due to the reaction between the material (M) of the layer and the hydrofluoric acid to form a second surface layer containing a fluorine compound, and calculating the weight content of hydrofluoric acid.    USE - The method is useful for detecting and dosing hydrofluoric acid content in an electrolyte containing lithium hexafluorophosphate (claimed) in lithium ion batteries, which are useful in a portable electronic devices such as mobile phones and laptops and automotive field such as electric cars.    ADVANTAGE - The method ensures accurate detection and dosage of the presence of hydrofluoric acid content in the electrolyte during storage thus avoiding operational problems encountered in the lithium-ion batteries.    DETAILED DESCRIPTION - The method comprises contacting an electrolyte with a first layer containing a material (M) for reacting the surface with hydrofluoric acid derived from the reaction between the lithium hexafluorophosphate and water, measuring the variation in weight of the layer by contacting the layer with an electrode of a quartz crystal microbalance, where the variation in weight of the layer is due to the reaction between the material (M) of the layer and the hydrofluoric acid to form a second surface layer containing a fluorine compound (MFn), where n is 1-4, calculating the weight content of hydrofluoric acid from the variation in weight of the layer, and determining the quantity of water of the electrolyte. The material of the first layer consists of IIIA, IIIB and IB elements of the periodic table. The first layer is disposed on the electrode of the quartz crystal microbalance directly contacted with the electrolyte by immersing the electrode in the electrolyte to measure the variation in weight of the layer. The first layer is weighed on the electrode of the quartz crystal microbalance in a first time, and is immersed in the electrolyte in a second time and after a time required for the reaction between the hydrofluoric acid and the material of the layer.