Salinity And ICP Na Concentration

[Dan_P]

Would you consider a 4% increase in salinity with a corresponding 14% increase in Na determined by ICP to be a odd correlation?

 

[taricha]

"4%" salinity increase in what units? ppt?
14% increase in Na in mg/L
Assuming we're talking about both in dissolved solids per amount of water, It seems like it'd be hard to get Cl or some other element to vary inversely to the Na to make those measurements consistent.
Maybe all the Cl- ions fell to the bottom of the bag

 

[Randy Holmes-Farley Verified Community Expert]

Would you consider a 4% increase in salinity with a corresponding 14% increase in Na determined by ICP to be a odd correlation?

I'm not sure what you are asking. It certainly could be that the sodium determination has error bars of several percent or more without a salinity change.

 

[Dan_P]

I'm not sure what you are asking. It certainly could be that the sodium determination has error bars of several percent or more without a salinity change.

Since posting this question, I read your article on using a refractometer to determine salinity (ppt). In the article, you demonstrate how the actual salinity can be constant while varying the relative amounts of sodium and magnesium, which changes the refractive index and the actual refractometer reading. Thus relatively large ion concentration changes produce small refractive index changes (I think that’s what you said).

In my case, i found a very tight correlation between salinity and the ICP Na concentration, only the salinity increased 4% while the Na concentration increased 14%. I originally thought a large change in Na concentration would correspond to an equally big salinity change, like when you add too much salt. So, I went fishing for an explanation. Large uncertainty in ICP results could be part of the explanation.

 

[Dan_P]

"4%" salinity increase in what units? ppt?
14% increase in Na in mg/L
Assuming we're talking about both in dissolved solids per amount of water, It seems like it'd be hard to get Cl or some other element to vary inversely to the Na to make those measurements consistent.
Maybe all the Cl- ions fell to the bottom of the bag

Yup, 4% change to ppt and a 14% change to mg/L. And unfortunately, one vendor does not report Cl-, so, I can’t even do a simple charge balance.

Now that you ask, 14% change in Na might actually be a smaller change in ppt, though sodium Cl/SO4 does make up a large proportion of the ppt sum. OK, I need to do more figurin’.

Between you and @Randy Holmes-Farley, I will get this straight yet.

 

[Randy Holmes-Farley Verified Community Expert]

Salinity and sodium will move exactly in tandem regardless of the units used for either one if it is happening by evaporation or dilution by fresh water. If it happens for other reasons, like adding magnesium chloride, obviously they do not move together.

 

[Dan_P]

Salinity and sodium will move exactly in tandem regardless of the units used for either one if it is happening by evaporation or dilution by fresh water. If it happens for other reasons, like adding magnesium chloride, obviously they do not move together.

OK got. Thanks much.

My observation was partly rookie mistake, forgetting that salinity and density result from multiple species, sodium making up 30% of the total. A 14% change in just sodium could be only a 4% in the total salinity. Math aside, your other suggestion about large variation may also be in play here. It seems that one may not be able to use ICP concentrations to make a good estimate of the salinity of the water.