Reef Chemistry Question of the Day #65 Refractometer and Ion Imbalances

Randy Holmes-Farley

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Reef Chemistry Question of the Day #65

One of the fundamental questions with any salinity measuring method is how far off from normal seawater the distribution of ions can be without incorrectly messing up the measurement.

For example, many people maintain magnesium at higher than natural levels in their aquaria. Having elevated magnesium while maintaining a total salinity of 35 ppt means some other cation (e.g., sodium) must be depleted to make up the difference. That "replacement" of magnesium for some of the sodium will have an effect on all of the ways we measure salinity, but by how much? Enough to worry about?

So the question is, if we have seawater where the salinity is carefully controlled at exactly 35.00 ppt, how much will a refractometer be off in its measurement of salinity if the magnesium is actually 1500 ppm rather than the "normal" value of about 1280 ppm by replacing some of the sodium ions? Assume the refractometer is a true seawater refractometer, is accurate, and is being used at the temperature it is designed for.

A. 35.0 ppt
B. 35.1 ppt
C. 35.4 ppt
D. 36.1 ppt
E. Cannot be determined from the information given

Good luck!













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DFW

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Reef Chemistry Question of the Day #65

One of the fundamental questions with any salinity measuring method is how far off from normal seawater the distribution of ions can be without incorrectly messing up the measurement.

For example, many people maintain magnesium at higher than natural levels in their aquaria. Having elevated magnesium while maintaining a total salinity of 35 ppt means some other cation (e.g., sodium) must be depleted to make up the difference. That "replacement" of magnesium for some of the sodium will have an effect on all of the ways we measure salinity, but by how much? Enough to worry about?

So the question is, if we have seawater where the salinity is carefully controlled at exactly 35.00 ppt, how much will a refractometer be off in its measurement of salinity if the magnesium is actually 1500 ppm rather than the "normal" value of about 1280 ppm by replacing some of the sodium ions? Assume the refractometer is a true seawater refractometer, is accurate, and is being used at the temperature it is designed for.

A. 35.0 ppt
B. 35.1 ppt
C. 35.4 ppt
D. 36.1 ppt
E. Cannot be determined from the information given

Good luck!













.


None of the above because it cannot be off the full measure of normal seawater, or more just by replacing some sodium with magnesium. It might be off 0.1, or 0.4, or even 1.1 ppt.:xd:
 

Mike J.

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Just found these questions of the day. I do like to be challenged. I'll attempt to answer this one. The refractometer reading would still be the same, 35.0 ppt. Magnesium is a cation or positive ion along with calcium, potassium, and sodium. There will be no change in the refractive index. Mag is normally 3.68 percent of seawater and raising it 220 ppm is equivalent to changing the percentage .06325 %.
 

TakedaMaru

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The same 35 approx. IMO.






IMHO, no matter with molecular masses and the refractive index of the aqueous solutions of salts ,
when we solve so stupid exercises, that in one step "replaces Na+ to Mg2+ "  

Due low concentration (really low!) of dissolved ions , we can prognose minor density/etc
change after replacing some ions(molecules) of Na to Mg ions.
Especially- with refractometer for SOHO use. Especially- salinity.
(Even without replacing 220 ppm ( 1 ppm = 0,001 ‰ = 0,0001 % ! so,
even 220ppm of Mg * 0.001 will give 0.22 PROMILLE of salinity(without replacing!).

So if somebody wants to more accurate calculate, welcome to the refractive indexes
ow water solutions of salts.
here is very similar googlebook

And you need more input data: full composition of that "seawater with replaced ions" ))



 
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Randy Holmes-Farley

Randy Holmes-Farley

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Here's a hint:

Bear in mind that if all I did was add magnesium chloride to boost the magnesium by 220 ppm, the salinity would have risen to 35.86 ppt as measured by the weight of the magnesium and the chloride added (and not based on a refractometer reading, but the weight of salts in the water). That isn't exactly what I did in the question, however, since I claimed to have maintained salinity at 35 ppt. :)
 
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treemanone2003

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I admit I have no ability to answer today's question but I really look forward to learning something. I've only found the forum as of a couple days ago and was wondering, are the weekend questions easier or harder ?
 
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Randy Holmes-Farley

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It's random. Some of the easy ones are coming from some quizzes I wrote in 2006 and 2007. Many of the harder ones I'm thinking up when I have time.

For some of these, I don't really expect folks to do the calculations, but just to think about the issue, and then if interested, see how the answer is determined.

Happy Reefing!
 

treemanone2003

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It's random. Some of the easy ones are coming from some quizzes I wrote in 2006 and 2007. Many of the harder ones I'm thinking up when I have time.

For some of these, I don't really expect folks to do the calculations, but just to think about the issue, and then if interested, see how the answer is determined.

Happy Reefing!

I'm good with random. I can't imagine some of the ones you could be coming up with, but do look forward to seeing them. Sometimes it's better (for me) to not know the answer or be totally wrong as it's the enabler to get it right next time.

Thanks for taking the time to share it !!
 
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Randy Holmes-Farley

Randy Holmes-Farley

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And the answer is...C. 35.4 ppt

The best way to try to answer this is to first figure out how the seawater composition would have to change for the magnesium to rise by 220 ppm, but for the salinity to not change.

We CANNOT simply swap a certain amount of magnesium ions for a certain amount of sodium ions in the seawater. Seawater has to have the same total amount of negative charge as positive charge in it. Magnesium carries two positive charges while sodium only has one. The only thing you could do with a simple swap and still maintain charge neutrality in the seawater is to replace two sodium ions with a single magnesium ion. But two sodium ions weigh far more than one magnesium ion, so such a swap cannot maintain the salinity at 35 ppt. It would go down.

As I mentioned earlier, boosting the magnesium from 1280 ppm to 1500 ppm with magnesium chloride raises the salinity from 35 ppt to 35.86 ppt.

So we have to do something other than a simple addition or a simple swap.

Let's start with the magnesium chloride addition to get to the right 1500 ppm magnesium. But the salinity is now too high. If we remove everything else in the seawater back down to 35 ppt, the calculation is fairly tedious, but since by far most of what is in seawater is sodium chloride, we can estimate the answer just by removing sodium chloride from the sample back down to a total salinity of 35 ppt. In fact, we have to remove 0.86 ppt (0.86 g/L) of sodium chloride.

Since we can look up the impact on the index of refraction of adding the magnesium chloride and removing the sodium chloride, we can determine how much the refractive index will change. From the change in refractive index, we can determine how much the salinity would appear to have changed in the sample despite remaining at 35 ppt.

FWIW, if we had done the calculation using some sulfate in with the chloride in the magnesium addition and with the sodium removal, the correction is pretty small as there is so much more chloride than sulfate.

This article describes the issues:

Refractometers and Salinity Measurement by Randy Holmes-Farley - Reefkeeping.com

from it:

Refractive Index and Ion Imbalances in Seawater


It turns out that an aqueous solution's refractive index is relatively insensitive to small changes in the solution's ionic makeup. For example, the usual changes in seawater's major ions that are encountered in a reef aquarium do not greatly alter the measured salinity. However, large differences in the big four ions (chloride, sulfate, sodium and magnesium) will alter the relationship between refractive index and salinity or specific gravity.


From refractive index tables found in chemical reference books, we can find that a 10 weight percent solution of sodium chloride has the same refractive index as a seven weight percent solution of magnesium chloride, a nine weight percent solution of magnesium sulfate and a 12 weight percent solution of sodium sulfate. These results indicate that some effects could relate to shifts between these ions in a reef aquarium, but that these effects are small. We can use these values to roughly predict how far off salinity measurements might be with some typical changes in the major ions. If we start with 35 ppt seawater, which normally has the following components,


Chloride 19,350 ppm
Sodium 10,780 ppm
Sulfate 2,700 ppm
Magnesium 1,280 ppm


and substitute more or less magnesium chloride in place of sodium chloride, while maintaining overall salinity at 35 ppt, we get the results shown in Table 3. The effect can be readily understood in that sodium chloride has a smaller effect on refractive index than does the same weight of magnesium chloride. So if magnesium is low, the refractive index will be low, and reported salinity will be a bit low. But overall these issues result in a very small error in salinity (in terms of the precision that reef aquarists are typically concerned with, say, ± 1 ppt), so the conclusion is that refractive index is a suitable way to measure salinity regardless of ordinary chemical imbalances.


[SIZE=-1]Table 3. The error in salinity measurement via refractive index when magnesium is present at unusually high or low concentrations. The darker blue row represents natural seawater.[/SIZE]
[SIZE=-1]Magnesium (ppm)[/SIZE]
[SIZE=-1]Salinity (ppt)[/SIZE]
[SIZE=-1]Refractive Index[/SIZE]
[SIZE=-1]Predicted Salinity (ppt)[/SIZE]
[SIZE=-1]Relative Error in Salinity (%)[/SIZE]
[SIZE=-1]800[/SIZE]
[SIZE=-1]35[/SIZE]
[SIZE=-1]1.33925[/SIZE]
[SIZE=-1]34.2[/SIZE]
[SIZE=-1]2.2[/SIZE]
[SIZE=-1]900[/SIZE]
[SIZE=-1]35[/SIZE]
[SIZE=-1]1.33928[/SIZE]
[SIZE=-1]34.3[/SIZE]
[SIZE=-1]2.0[/SIZE]
[SIZE=-1]1000[/SIZE]
[SIZE=-1]35[/SIZE]
[SIZE=-1]1.33931[/SIZE]
[SIZE=-1]34.5[/SIZE]
[SIZE=-1]1.4[/SIZE]
[SIZE=-1]1100[/SIZE]
[SIZE=-1]35[/SIZE]
[SIZE=-1]1.33934[/SIZE]
[SIZE=-1]34.7[/SIZE]
[SIZE=-1]0.9[/SIZE]
[SIZE=-1]1200[/SIZE]
[SIZE=-1]35[/SIZE]
[SIZE=-1]1.33938[/SIZE]
[SIZE=-1]34.9[/SIZE]
[SIZE=-1]0.3[/SIZE]
[SIZE=-1]1280[/SIZE]
[SIZE=-1]35[/SIZE]
[SIZE=-1]1.33940[/SIZE]
[SIZE=-1]35.0[/SIZE]
[SIZE=-1]0[/SIZE]
[SIZE=-1]1300[/SIZE]
[SIZE=-1]35[/SIZE]
[SIZE=-1]1.33941[/SIZE]
[SIZE=-1]35.1[/SIZE]
[SIZE=-1]0.3[/SIZE]
[SIZE=-1]1400[/SIZE]
[SIZE=-1]35[/SIZE]
[SIZE=-1]1.33944[/SIZE]
[SIZE=-1]35.2[/SIZE]
[SIZE=-1]0.6[/SIZE]
[SIZE=-1]1500[/SIZE]
[SIZE=-1]35[/SIZE]
[SIZE=-1]1.33947[/SIZE]
[SIZE=-1]35.4[/SIZE]
[SIZE=-1]1.1[/SIZE]
 
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Cory

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"Seawater has to have the same total amount of negative charge as positive charge in it"

Randy, are you saying that seawater always maintains a balance of charges? This is interesting to me!
 
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