Please explain this salinity

[danielsalt]

Hey ok so this is an issue that Im not sure how much of an issue it truly is. Maybe more importantly I would like to know how to explain it best for somebody.

Basicly a person sends in ICP where you can clearly see that salinity is really high. For example Sodium is over 12k, CA 500+ MG 1500+ SR 470+.
Obviously high salinity, right?
Well if the person claims calibrated Hannah pen and Apex probe tells him salinity is 35ppt and a calculator says 38+ppt. How does one explain that?

Furthermore if he was to replace 10% water with RO (divided over day/s), would his equipment tell him 35ppt or too low?

I hope I make sense as this is a surprisingly common issue to try and explain to people.

 

[Montiman]

One of the difficult things about salinity is there are so many ways of measuring it and almost none truly read salinity. In the lab there are four ways.

1. Density using a hydrometer
2. Refractive index using a refractometer
3. Conductivity using an EC meter
4. Titration test for the Cl ion concentration.

When researchers measures salinity of NSW they actually perform a titration test for Cl concentration. This is however relying on the Principal of constant proportions in Sea Water which states that the ocean as a whole has consistent proportions of dissolved element levels and then Cl concentration can be used to estimate the concentration of other ions. The problem with our tanks is that there are not constant proportions.

In short consider why we care about salinity. From what I can gather we care about it primarily because of its effect on water density and osmotic equilibrium. If density is the most important factor it is better to rely on a hydrometer or refractometer than ICP tests because the hydrometer and refractometer are actually measuring what is most important to us. Rely on the ICP test to solve other problems for example if CL is high and S is low some additive with additional CL is likely being dosed out of proportion to the other component or their could be a precipitation issue . In your example there are other ions that can effect what an EC meter or refractometer will read such as K and S

 

[danielsalt]

Fantastic reply thankyou @Montiman
I messed a number up, 470+ was obviously K not SR.
S was 1050mg/L.

 

[Montiman]

I truly believe that a good hydrometer is one of the most overlooked salinity measuring tools. It never needs calibration and it actually measures density which is likely the most important effect of salinity on fish and perhaps some invertebrates as well. I keep a cheap in tank floating hydrometer to confirm that I'm not relying too heavily on my refractometer. I hear way too many stories about someone miscalibrating their refractometer or EC meter and experiencing losses due to incorrect salinity. The hydrometer may be harder to read and less accurate but it is more robust and likely more consistent.