Determination of Total Chlorine in Different Types of Hydraulic Fluids by Coulometric Titration After High-temperature Combustion Application Note · multi EA 5100

Introduction

Hydraulic fluids are used in different technical equipment and machines such as stearing systems, brakes, versatile transmissions, aircraft flight control systems, wind engines, excavators, etc. Depending on the purpose of use and the desired properties (e.g., biodegradable, anti-wearing, fire-resistant), a wide variety of different types of hydraulic fluids is available. They are either organic, based on mineral or vegetable oil, or synthetic, e.g., based on silicone or phosphate esters. A major disadvantage of hydraulic fluids such as mineral oil-based ones is that they are flammable. This can cause serious consequences in case of an incident, especially when used in aircrafts, where immediate fire-fighting measures during a flight are nearly impossible. That is why fire-proof products are used for airplanes. The best known representative of this class is SkydrolTM, a synthetic, phosphate ester-based formulation.

To avoid dangerous situations caused by corrosion or too early degradation of the hydraulic system and fluid, quality, composition and wearing degree of the fluid have to be regularly monitored. Besides the main parameters such as acidity, presence of water and particles, the concentration of the elements calcium, sodium, potassium, sulfur, and chlorine also has to be checked.

Chlorine is not part of the composition of such high-performance hydraulic fluids. Nevertheless it can be included due to contamination effects, e.g. when cleaning the hydraulic systems with chlorinated solvents. In case the system is not completely dried before refilling with the hydraulic fluid, contaminations are easily possible. The maximum permissible value of 200 ppm must not be exceeded.

Chlorine determination in greases and other oil-based formulations (e.g., crude oil, transformer oil, engine oil) is not a big challenge due to the predictable digestion behavior of these matrices. But when it comes to high-performance hydraulic fluids like SkydrolTM, the case is completely different. Due to the matrix itself – mainly organic phosphate esters – it is not only highly fire-resistant, but also leads to the formation of large amounts of particles such as P4O10, during the combustion process. Even worse, these particles react with the formed combustion water by immediate hydrolysis to i.e. H3PO4. Measuring chlorine sensitively and reliably under such conditions without damaging the analysis system is extremely challenging.

The multi EA 5100 with its powerful Auto-Protection system effectively protects the analysis system by trapping all particles, aerosols and the liquid acid, while the formed hydrochloric acid can pass through easily thanks to the integrated heating. The analyzer’s high sensitive coulometer cell is able to determine chlorine quantities as low as 10 ng up to 10 µg. This way a wide concentration range can be covered with one and the same analysis method. A sample dilution is not necessary.