osmotic pressure medium containing sugars, such as sucrose, glucose, also known that concentrated solutions used in osmosis are real solutions which might shows water activity values of glycerol solutions along with refractive index values of . In the above equation y is the activity coefficient of the solute, m is the. the first systematic study of the refractive indices of sugars other than sucrose. He observed little i difference in the indices of the different sugars in solutions of. For a solution of pure sucrose in water, the number of degrees Brix is equal to the number of degrees Brix of a pure sucrose solution with the same refractive index. by using the relation between density and solid content of the concentrate. as vapour and (c) reversed osmosis, a process whereby water from a solution.
Just as was shown for refractive index, recalibration of a refractometer with an offset error can be discussed in terms of specific gravity and salinity. Figure 11 shows what happens when adjusting the calibration screw so that the specific gravity of a 35ppt seawater standard with a known specific gravity of 1. In this figure, the miscalibrated red line moves exactly onto the green line, and the refractometer is then good to go at all specific gravity values.
Similarly, Figure 12 shows what happens when adjusting the calibration screw so that the salinity of a 35 ppt seawater standard really reads 35 ppt.
The error can be corrected using a seawater standard. By turning the calibration screw until a seawater standard reads 1. In this case, accurate calibration can also be performed using freshwater. By turning the calibration screw until a seawater standard reads 35 ppt, the red line moves onto the green line and the refractometer is properly calibrated.
This analysis makes it clear that offset miscalibration is readily corrected by turning the refractometer's adjustment screw, and that it can be corrected using either pure freshwater or 35 ppt seawater. Slope Miscalibration A second way that refractometers can give incorrect values is when they are imperfectly made or are made for an application different from seawater.
Refractive index of sucrose solutions.
One such error results in what I call a slope miscalibration Figure Essentially, the refractometer reads a refractive index that is either lower or higher than the real refractive index, and this difference changes with the difference from some point of calibration here chosen as the bottom left hand corner, matching pure freshwater.
In this case, the error becomes larger and larger as the reading moves away from the point of calibration. Such an error can arise, for example, if the scale is not made to exactly the right dimensions. In that case, no amount of moving the scale up or down can make it accurate at all values of refractive index.
The relationship between the real actual refractive index and the measured refractive index for an incorrectly calibrated refractometer red and a perfectly calibrated refractometer green. The error in reading refractive index values as far away as that of seawater can be significant, as shown.
Refractometers and Salinity Measurement by Randy Holmes-Farley - cypenv.info
Can such a refractometer be used? Yes, but only if it is calibrated using a solution known to have a refractive index close to that of the samples to be tested. Calibrating using a liquid matching seawater, for example, can lead to a slope correction as shown in Figure In this type of calibration, the refractometer is accurate at that refractive index, but not necessarily at other values.
The refractometer of Figure 13 red has a slope error, with values far from the calibration point reading incorrectly. This type of error can only be corrected by calibrating with a solution with refractive index near to the expected measurement point.
For example, to measure the salinity of seawater at 35 ppt, calibrate a refractometer using a standard with the same refractive index, and the slope miscalibration error disappears when measuring seawater samples near that salinity Figure In those cases, the measured and true salinity or specific gravity relate to one another in exactly the same way that measured and true specific gravity relate to each other in Figures 13 and Figure 15, for example, shows the relationship between the measured and actual specific gravity for a refractometer with a slope miscalibration.
Figure 16 is an expansion of the region of specific gravity of interest to reef aquarists.
Refractive index of sucrose solutions.
The relationship between the real actual specific gravity and the measured specific gravity for an incorrectly calibrated seawater refractometer red and a perfectly calibrated seawater refractometer green. This red refractometer has a slope error, with values far from the calibration point freshwater with a specific gravity of 1. The amount of error in measuring seawater is indicated.
This figure is an expansion of Figure 15 in the region of most interest to reef aquarists. Similarly, Figure 17 shows the relationship between the measured and actual salinity for a refractometer with an offset miscalibration.
Figure 18 is an expansion of the region of salinity of interest to reef aquarists. It is clear that seawater 35 ppt reads much lower in this case, at about 30 ppt. The relationship between the real actual salinity and the measured salinity in ppt for an incorrectly calibrated seawater refractometer red and a perfectly calibrated seawater refractometer green.
This red refractometer has a slope error, with values far from the calibration point freshwater with a salinity of 0 ppt reading higher than the actual value.
This figure is an expansion of Figure 17 in the region of most interest to reef aquarists. Just as was shown for refractive index, recalibration of a refractometer with a slope error can be discussed in terms of specific gravity and salinity.
Figure 19 shows what happens when adjusting the calibration screw so that the specific gravity of a 35 ppt seawater standard with a known specific gravity of 1. Figure 20 is an expansion of the region of salinity of interest to reef aquarists. In this figure, the miscalibrated red line moves onto the green line, and the refractometer is then good to go at specific gravity values near 1.
The refractometer of Figure 15 and 16 red has a slope error, with values far from the calibration point reading incorrectly. In this figure it has been recalibrated with seawater and so is adequately accurate over the range of specific gravity from 1. This figure is an expansion of Figure 19 in the region of most interest to reef aquarists. Similarly, Figure 21 shows what happens when adjusting the calibration screw so that the salinity of a 35ppt seawater standard really reads 35 ppt.
The refractometer of Figure 17 and 18 red has a slope error, with values far from the calibration point reading incorrectly. In this figure it has been recalibrated with seawater, and so is adequately accurate over the range of salinity of ppt despite the slope error.
This figure is an expansion of Figure 21 in the region of most interest to reef aquarists. This type of slope correction turns out to be important for reef aquarists, as slope miscalibration errors seem to abound in inexpensive refractometers.
Many aquarists have found that when calibrated using pure freshwater, their refractometers do not accurately read 35 ppt seawater standards. Many read 1 ppt, which is likely acceptable to most aquarists, but some read much further from the actual value.
These inaccuracies may be partly because many of these may actually be salt refractometers and not actual seawater refractometers see next section. Correction of slope miscalibration errors should be carried out using a fluid that approximately matches the refractive index of the water being tested, so for reef aquarium water, calibration with 35 ppt seawater solves this problem, while calibration with pure freshwater does not.
Scale Misunderstanding and Salt Refractometers Refractometers can lead to incorrect readings in additional ways and, again, these issues abound for reef aquarists. One is that many refractometers are intended to measure sodium chloride solutions, not seawater. These are often called salt or brine refractometers. Unfortunately, many refractometers used by aquarists fall into this category. In fact, very few refractometers used by hobbyists are true seawater refractometers.
Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large. A 35 ppt sodium chloride solution 3. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an appropriate salinity in the first place.
Figure 23 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in salinity. This figure shows the measured salinity reading for seawater being about 1. The relationship between the real actual salinity and the measured salinity in ppt for a perfectly calibrated seawater refractometer green and a perfectly calibrated salt refractometer red.
This salt refractometer effectively has a significant slope error, with values far from the calibration point freshwater with a salinity of 0 ppt reading roughly 1.
Salt refractometers reading in salinity can be recalibrated using seawater to eliminate nearly all of this error just as the refractometer in Figures 17 and 18 was recalibrated in seawater to give Figures 21 and It turns out that this is a slope miscalibration in the sense that a perfectly made sodium chloride refractometer necessarily has a different relationship between refractive index and salinity than does seawater.
This type of problem with a refractometer IS NOT at all corrected by calibrating it with pure freshwater. If you have this type of refractometer, and it was perfectly made and calibrated in freshwater, it will ALWAYS read seawater to be higher in salinity than it actually is misreporting an actual Even more confusing, but perhaps a bit less of a problem in terms of the error's magnitude, salt refractometers sometimes read in specific gravity.
However, some net energy will be radiated in other directions or even at other frequencies see scattering. Depending on the relative phase of the original driving wave and the waves radiated by the charge motion, there are several possibilities: This is the normal refraction of transparent materials like glass or water, and corresponds to a refractive index which is real and greater than 1. This is called "anomalous refraction", and is observed close to absorption lines typically in infrared spectrawith X-rays in ordinary materials, and with radio waves in Earth's ionosphere.
It corresponds to a permittivity less than 1, which causes the refractive index to be also less than unity and the phase velocity of light greater than the speed of light in vacuum c note that the signal velocity is still less than c, as discussed above. If the response is sufficiently strong and out-of-phase, the result is a negative value of permittivity and imaginary index of refraction, as observed in metals or plasma.
This is light absorption in opaque materials and corresponds to an imaginary refractive index. If the electrons emit a light wave which is in phase with the light wave shaking them, it will amplify the light wave. This is rare, but occurs in lasers due to stimulated emission.
It corresponds to an imaginary index of refraction, with the opposite sign to that of absorption. Dispersion[ edit ] Light of different colors has slightly different refractive indices in water and therefore shows up at different positions in the rainbow. In a prism, dispersion causes different colors to refract at different angles, splitting white light into a rainbow of colors.
The variation of refractive index with wavelength for various glasses. The shaded zone indicates the range of visible light. Dispersion optics The refractive index of materials varies with the wavelength and frequency of light. Dispersion also causes the focal length of lenses to be wavelength dependent. This is a type of chromatic aberrationwhich often needs to be corrected for in imaging systems. In regions of the spectrum where the material does not absorb light, the refractive index tends to decrease with increasing wavelength, and thus increase with frequency.
This is called "normal dispersion", in contrast to "anomalous dispersion", where the refractive index increases with wavelength. For optics in the visual range, the amount of dispersion of a lens material is often quantified by the Abbe number: