Bellingham
& Stanley LtdBellingham
& Stanley Ltd
At first sight, it might appear that the application of a sample to the surface of the prism would present few problems the measurement of refractive index. In the majority of cases where the highest measuring accuracy is not required this is the case. However, problems do arise from time to time from causes which are by no means obvious.
Refractometers measure the critical angle and this is determined by what takes place
at the interface between the prism and the sample. consequently, measurements can
only refer to the refractive index of the material in immediate intimate contact with the prism
surface. This very thin contacting layer of material may not have exactly the same
characteristics fluid in the bulk of the liquid in the container from which the sample was
drawn.
This may be for several reasons:
In many cases, the definition of the border line can be improved by agitating or stirring the sample on the prism face. This may make the sample more homogeneous or bring the sample into better contact with the prism surface. This effect can, of course, be easily studied on a visual instrument where the border line can be seen. The same phenomenon occurs on automatic instruments and here the operator may be made aware of what is taking place by measurement readings of an erratic nature (RFM80, 81, 90 & 91 - old series B&S automatic digitals) or by signals /Quality value displayed on screen, generated from within the instrument indicating an unacceptable border line (RFM300 and RFM500 series).
The prism surface must be clean. Suitable solvents must be used to remove all grease or other contaminants.
On instruments with plastic illumination plates (this includes most hand held refractometers,) care must be taken to avoid the use of solvents which might chemically attack the plate material. Plate materials may be acrylic or polycarbonate. Cleaning agents which are usually safe in this respect include, methylated spirits, white spirit, alcohol, soaps, detergents, acetone, genklene, arclone.
Some solvents must be avoided. These include amyl acetate, and toluene. (The latter solvent is sometimes recommended as a suitable medium for refractometry, but can attack many plastics.)
It is important not to leave material on the prisms after measurements have been taken. Some samples will `glue' the prisms together if left on the prisms for prolonged periods and damage may occur in prising the prisms apart. Again, some sample materials will attack the prism surfaces after prolonged contact. This applies particularly to glass prisms and certain types of glass are prone to attack by weak acids such as those found in fruit juices. (Glasses easily attacked in this way are usually avoided by the instrument manufacturer, but for certain instrument range alternative glasses are not always available).
Prisms are frequently mounted in the prism box by some type of epoxy cement immune to attack by most sample materials or cleaning agents. Some prisms are retained in position by silicone gaskets. In either case, the manufacturer should be consulted if samples or cleaning agents are to be used which are likely to interact with these materials. If so, then for an additional cost, in the case of the RFM300 and RFM500 series, a change from the silicone gasket to a Kalrez gasket can be made - this will allow use of the instruments with these chemical samples.
Care must be taken to prevent changes in sample concentration due to evaporation.
A thin film of sample material, when exposed to the atmosphere quickly changes its concentration, especially where volatile liquids are involved. Samples placed on the prism must never be left uncovered and prism boxes, /pressers must be closed as soon as the sample has been applied. This evaporation loss can even occur during the time of the transfer of the sample from its container onto the prism, and for accurate measurements it is recommended that, where the viscosity of the sample permits, the sample be applied by pipette rather than by a rod or spatula.
Care must be taken with highly viscous samples where a "pill size" lump of sample forms. The surface of the lump will become enriched by exposure to the air and it will be this surface layer which will contact the prism first. It is essential that this layer be ruptured by applying pressure to the lump by closing the prism unit, or by other means, thus forcing representative sample material into close contact with the prism surface.
Enrichment of the sample by evaporation cannot be over emphasised. For example:- in reading a 15% sugar solution, errors of between 0.5% and 1 .0% have been noted caused simply by transferring the sample to the prism by means of a stirring rod.
One of the most common causes of incorrect measurement is lack of proper temperature control. The sample must be at the required temperature and adequate time allowed for the sample to attain a stable temperature. This is most important especially if bulky samples with high thermal capacity (such as preserves, jams, etc.) are being tested using the reflection mode.
It must be remembered that there is a thermal time lag between the prism surface and the temperature sensor. The instrument maker seeks to minimise this lag by careful design, but same thermal lag always remains and must be taken into consideration by the operator.
The sample should cover the working area of the prism surface. This applies particularly when the transmission mode is being used. If the coverage is inadequate, several problems can arise. There may be poor border line illumination over parts of the instrument range.