DIY Refractometer Cal Solution

[slythy]

@Randy Holmes-Farley

So I saw a post that to make 35ppm salt solution I can add 3.65g of salt to 96.35ml of water. I have a 100ml graduated cylinder and a scale. I measured 3.65 grams of Kroger Branded Ionized salt and dumped it into the cylinder, then filled it to the 100ml line.

I have multiple brands of Calibration fluid and the Aquatic experts one at 35ppm has my apex salinity probe (recently cal'ed) reading ~33.5. I made your solution and I seem to get close to 37ppm when calibrated to the BRS/AE ones. The reason I am questioning the cal fluids I bought is because when I make salt in my 55g container, if I use 1/2 cup per gallon it seems to overshoot and my alk,cal and mag all come out reading low when i bring it down to my calibrated 35ppm.

I am going to make 1 Liter of solution doing this method 10x and filling a jar.

Is this solution I made with that salt a good recipe? I have a gram scale that reads out to 2 decimals but figure its probably not crazy accurate, but if I do it 10x it should average its way out to be close or closer than using measuring spoons.

Thoughts?

Thanks!

 

[infinite0180]

http://reefkeeping.com/issues/2004-06/rhf/index.php

This article explains it all. Your using the correct recipe. I switched to making my own for my conductivity meter and have never looked back. This is how we do things in the science world. I dont trust those premade solutions. I just bake some salt in my oven every so often and keep it in a tightly sealed bag. When im ready to calibrate i weight out some fresh solution and im done in a few min. My meter is usually spot on ever time. I never worry about salinity anymore…

 

[JimWelsh]

http://reefkeeping.com/issues/2004-06/rhf/index.php

This article explains it all. Your using the correct recipe. I switched to making my own for my conductivity meter and have never looked back. This is how we do things in the science world. I dont trust those premade solutions. I just bake some salt in my oven every so often and keep it in a tightly sealed bag. When im ready to calibrate i weight out some fresh solution and im done in a few min. My meter is usually spot on ever time. I never worry about salinity anymore…

No.

The correct recipe is 3.65 g of NaCl + 96.35 g of water. At 20C, that creates 100g of a solution that has a specific gravity of 1.0261, for a density of 1.0243, which means it has a volume of 97.63 mL. He created a solution that has a total volume of 100 mL. Water at 20C does not weigh 1 gram/mL. It is 0.9982 g/mL. So, the solution he created had an additional 2.36 grams of mass, at a volume of 100 mL. That solution has a density of 1.0236, for a specific gravity of 1.0255. A NaCl solution with that SG will have the same refractive index as seawater at right about S=34.0.

The correct way to use Randy's recipe is to weigh both the water and the salt.

EDIT: Using 3.74 g of NaCl to create a total volume of 100 mL would work, however.

 

[infinite0180]

No.

The correct recipe is 3.65 g of NaCl + 96.35 g of water. At 20C, that creates 100g of a solution that has a specific gravity of 1.0261, for a density of 1.0243, which means it has a volume of 97.63 mL. He created a solution that has a total volume of 100 mL. Water at 20C does not weigh 1 gram/mL. It is 0.9982 g/mL. So, the solution he created had an additional 2.36 grams of mass, at a volume of 100 mL. That solution has a density of 1.0236, for a specific gravity of 1.0255. A NaCl solution with that SG will have the same refractive index as seawater at right about S=34.0.

The correct way to use Randy's recipe is to weigh both the water and the salt.

EDIT: Using 3.74 g of NaCl to create a total volume of 100 mL would work, however.

Oh good catch. I read over his post pretty quickly and missed that detail. Yes i use a scale to measure it out like you explained. I always use a scale when possible. Density and volume make things complicated! I have a small digital scale and calibration weight i got off amazon for the purposes of making saltwater calibration solution and the occasional batch of trisodium phophate solution. Much easier and accurate to just weigh it out. Thanks for the correction Jim.

edit - heres the cheap scale i picked up for reefing!

 

[slythy]

No.

The correct recipe is 3.65 g of NaCl + 96.35 g of water. At 20C, that creates 100g of a solution that has a specific gravity

interesting, i guess incorrectly assumed that if i were to do it by mass that adding the 3.65g of salt would take up the space so topping up to 100ml would stop me from having to do anything. That was a poor assumption.

my fear now is Im actually running around S=32 ish

is this way going to be more accurate than the Calibration fluid I purchased? Im scared that I will be running s=38 or so but I think Im low based on the salt not being mixed close to spec on multiple salts…

Oh good catch. I read over his post pretty quickly and missed that detail. Yes i use a scale to measure it out like…..

I use a scale very similar to that. So I guess I will measure out 963.5 ml and add 36.5g of that ionized table salt. And see where i get.

 

[infinite0180]

interesting, i guess incorrectly assumed that if i were to do it by mass that adding the 3.65g of salt would take up the space so topping up to 100ml would stop me from having to do anything. That was a poor assumption.

my fear now is Im actually running around S=32 ish

is this way going to be more accurate than the Calibration fluid I purchased? Im scared that I will be running s=38 or so but I think Im low based on the salt not being mixed close to spec on multiple salts…



I use a scale very similar to that. So I guess I will measure out 963.5 ml and add 36.5g of that ionized table salt. And see where i get.

Why do you need to make so much? If you have a scale just tare a small cup on the scale, weigh in the salt, then add water to 100g. I like to warm the water up a tad first in the microwave so the salt rapidly dissolves. Mix and your done!

 

[slythy]

Why do you need to make so much? If you have a scale just tare a small cup on the scale, weigh in the salt, then add water to 100g. I like to warm the water up a tad first in the microwave so the salt rapidly dissolves. Mix and your done!

i was going to store it and seal it but feel like making a large solution removes some small error.

 

[gbroadbridge]

i was going to store it and seal it but feel like making a large solution removes some small error.

That is so. You don't know the absolute accuracy of your scale unless you have used a standard to check it.
A 0.2 gram error is an order of magnitude less important if you measure 30g rather than 3g.
You also need to consider the linearity of the scale if you are going to measure 30g and 970g on the same device.

 

[slythy]

That is so. You don't know the absolute accuracy of your scale unless you have used a standard to check it.
A 0.2 gram error is an order of magnitude less important if you measure 30g rather than 3g.
You also need to consider the linearity of the scale if you are going to measure 30g and 970g on the same device.

correct, i was going to do 10x 100ml so hopefully error averages out but i probably dont need to do that either.