Reef Chemistry Question of the Day #5

Randy Holmes-Farley

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Reef Chemistry Question of the Day #5

When using a refractometer to measure the salinity of seawater, what is the basic principle involved in the measurement?

A. The change in the ability of electrical charge to move through the water due to the presence of ions.
B. The change in the speed of light moving through the water due to interaction with the ions.
C. The absorption of some wavelengths of light due to the presence of ions.
D. The interaction of ions with the glass in the prism.

Good luck!









I'll make a little white space here so the first response is not so distracting. :D
 

hart24601

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Hmmmm. I think it's like a prism, so I will have to go with change in the speed of light. I know speed of light is a constant in a vacuum, but when hits all the "stuff" in seawater it slows more than when in pure water? B
 

cope413

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E) light is sentient and communicates with a small chip in the device that then relays the information in the form of a hard to discern blue bar on tightly grouped black lines.
 

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Reef Chemistry Question of the Day #5

When using a refractometer to measure the salinity of seawater, what is the basic principle involved in the measurement?

A. The change in the ability of electrical charge to move through the water due to the presence of ions.
B. The change in the speed of light moving through the water due to interaction with the ions.
C. The absorption of some wavelengths of light due to the presence of ions.
D. The interaction of ions with the glass in the prism.

Good luck!









I'll make a little white space here so the first response is not so distracting. :D

I thought it was the change in the index of refraction with denser media.

so I guess it is B

Which if i remember my HS physics is how a prism works as well.
 

melypr1985

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B is my guess too
 
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Randy Holmes-Farley

Randy Holmes-Farley

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The answer is B, speed of light.

(A) is how conductivity probes measure salinity.
(C) could be used to measure light absorbing things in the water, but most ions in seawater do not absorb visible light.
(D) describes how a pH meter works, but it is not a prism.

Here's a blurb on how refractometers work from one of my articles:

What is the Index of Refraction?
The index of refraction (or refractive index) is the ratio of the speed of light traveling through a vacuum to the speed of light in the material being tested. Most aquarists do not realize that when using a refractometer, they are measuring the speed of light through their aquarium's water, so having such knowledge might be a good way to impress friends with your technical abilities!

Light travels through most materials more slowly than it does through a vacuum, so their refractive index is higher than 1.00000. The detailed mathematics and physics behind refractive index are actually quite complicated, because it is often a complex number with real and imaginary parts, but a simple version is adequate for all purposes that a reef aquarist would encounter. Some materials slow light traveling through them more than others, and slower light travel leads to a higher refractive index. Table 1 shows some typical refractive index values for comparative purposes.

In solutions of two compounds, such as ethyl alcohol in water, sugar in water or salt in water, the refractive index changes in step with how much of each component is present. Scientists have long known this to be true, and refractometers have a long history of use in brewing, sugar refining, analyzing blood and urine protein and many other industries where a quick measure of refractive index can lead to a good assessment of what is present.

Refractive index generally cannot reveal the identity of compounds in water, but when an aquarist knows roughly what material is there he can determine how much of it is there (within the refractive index's detection capability). Changes in refractive index are not suitable for determining trace levels of ions (such as the purity of freshwater coming out of an RO/DI (reverse osmosis/deionization) purification system), but it can do a good job when significant amounts of a known material are present.

For example, refractive index cannot determine whether a salt in water is potassium sulfate, sodium chloride, magnesium nitrate or calcium bromide, but if you know which of these you have by some other means (such as the name on a chemical's bottle), then you can determine how much is present in solution by measuring the refractive index, and then looking it up in a table that relates the refractive index to the concentration of that material.

Refractive Index and Salinity
Aquarists can use the effects that added salts have on the refractive index of a water solution to determine the salinity of reef aquarium water. As the salinity of seawater rises, the amount of salt added rises, so the refractive index rises. Figure 1 plots seawater's refractive index vs. its salinity. Figure 2 shows a similar plot of seawater's refractive index vs. specific gravity. These data are also summarized in Table 1. These sets of data demonstrate how aquarists can use refractive index to measure salinity and specific gravity, assuming they have a refractometer that can read in the appropriate refractive index range.
 

cope413

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"Light travels through most materials more slowly than it does through a vacuum"

Perhaps I'm being too literal, but is there any material that doesn't slow it down?
 
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Randy Holmes-Farley

Randy Holmes-Farley

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"Light travels through most materials more slowly than it does through a vacuum"

Perhaps I'm being too literal, but is there any material that doesn't slow it down?

Yes, actually there is, in unusual situations.

Refractive index - Wikipedia, the free encyclopedia

Refractive index below 1[edit]
A widespread misconception is that since, according to the theory of relativity, nothing can travel faster than the speed of light in vacuum, the refractive index cannot be lower than 1. This is erroneous since the refractive index measures the phase velocity of light, which does not carry information.[15] The phase velocity is the speed at which the crests of the wave move and can be faster than the speed of light in vacuum, and thereby give a refractive index below 1. This can occur close to resonance frequencies, for absorbing media, in plasmas, and for x-rays. In the x-ray regime the refractive indices are lower than but very close to 1 (exceptions close to some resonance frequencies).[16] As an example, water has a refractive index of 0.99999974 = 1 − 2.6×10−7 for x-ray radiation at a photon energy of 30 keV (0.04 nm wavelength).[16]


Negative refractive index

Recent research has also demonstrated the existence of materials with a negative refractive index, which can occur if permittivity and permeability have simultaneous negative values.[17] This can be achieved with periodically constructed metamaterials. The resulting negative refraction (i.e., a reversal of Snell's law) offers the possibility of the superlens and other exotic phenomena.[18]
 
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