Refractometers from Anton Paar are providing an accurate solution to monitoring one of industry’s most widely-used chemicals.
Sodium hydroxide (NaOH, also known as lye or caustic soda) is a staple of the chemical industry, with applications ranging from the food and beverage industry to petroleum, and nearly everything in between.
The production of downstream products such as solvents, plastics, fabrics, adhesives, herbicides, dyes, inks and pharmaceuticals will often also require NaOH.
Along with sodium sulfide, sodium hydroxide is a critical component of the ’white liquor’ used to remove lignin from wood pulp in the production of the paper. It also plays an important role during the bleaching of the pulp as well as the manufacture of ink and dye.
These products, and many others, require the determination of NaOH concentration for product consistency. Titration has traditionally been used to determine this value, but this process is time-consuming, labour-intensive, and imprecise.
Because of the tendency of NaOH to absorb water and carbon dioxide from the air, the length of the measuring process with have a proportionate affect on the NaOH solution and reduce the accuracy of the concentration measurement.
Refractometers are used to measure the refractive index of a sample, a value that will remain constant between formulations with identical concentrations of ingredients.
Although inaccurate temperature control and low inter-operator reliability pose problems for the use of Abbe-type refractometers in quality control, modern automatic Abbemat refractometers from Anton Paar eliminate these issues with accurate internal temperature control systems and automatic operation.
Modern devices are capable of providing the rapid, serial quality checks that are required on the assembly line.
They operate by means of an LED light source, which sends a beam of single-wavelength light through a measuring prism in contact with a sample at a controlled temperature.
Depending on the difference of the refractive indices between the NaOH solution and prism, the light is partly refracted and reflected, or totally reflected.
The critical angle of total reflection is determined by measuring the reflected light intensity on a CCD array as a function of the incident angle.
Specially designed accessories for NaOH measurements are also available according to the application.
For simple measurement of low-concentrated solutions, a few drops can be placed directly in the optional Hastelloy B3 sample well.
If automation or the measurement of many samples is required, special micro flow cells, made of perfluoroalkoxy alkanes (PFA), are ideal for the measurement because of their high material resistance.
Using this setup, serial quality control checks can be performed without ever having to come into contact with the sample during measurement, which also would be recommended for high-concentration measurements.
