DIY Salinity calibration solutions

[pickupman66]

Randy,

In light of another post on calibrating a conductivity meter, I decided to give it all a shot on my own system.

I read through your article on DIY salinity calibration (http://www.reefkeeping.com/issues/2004-06/rhf/) and have a few questions for my simple brain.

1. Is the Brand MORTON required or will most packages of pure iodized salt work (within reason)? Or could we use a teaspoon of our standard reef salt mix?

2. Will the same solution made for Conductivity work for my Refractometer? If not, Since i have graduated cylinders, can you tell me how much water volume in ml is required for a single TSP of Salt (as measured in your article)? (I failed to read it in the article)

I made a solution following the Conductivity standard and my Digital Aquatics SL2v2 seemed to calibrate very well to it. (once calibrated my tank was slightly low at 50mS/cm) so I need to bump it up just a bit to be spot on.

 

[Randy Holmes-Farley]

Randy,

In light of another post on calibrating a conductivity meter, I decided to give it all a shot on my own system.

I read through your article on DIY salinity calibration (http://www.reefkeeping.com/issues/2004-06/rhf/) and have a few questions for my simple brain.

1. Is the Brand MORTON required or will most packages of pure iodized salt work (within reason)? Or could we use a teaspoon of our standard reef salt mix?

2. Will the same solution made for Conductivity work for my Refractometer? If not, Since i have graduated cylinders, can you tell me how much water volume in ml is required for a single TSP of Salt (as measured in your article)? (I failed to read it in the article)

I made a solution following the Conductivity standard and my Digital Aquatics SL2v2 seemed to calibrate very well to it. (once calibrated my tank was slightly low at 50mS/cm) so I need to bump it up just a bit to be spot on.

Don't use a salt mix. Use table salt (iodized or not doesn't matter).

The recipes are different for refractometers, conductivity devices, and hydrometers.

Weighing is better than volume measurement of the salt, if you can do it.

For a refractometer:

Use 3.65 weight percent sodium chloride solution to match 35 ppt (sg = 1.0264). It can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater.

For a rougher measurement in the absence of an accurate water volume or weight measurement:

1. Measure ¼ cup of Morton's Iodized Salt (about 73.1 g)
2. Add 1 teaspoon of salt (making about 79.3 g total salt)
3. Measure the full volume of a plastic 2-L Coke or Diet Coke bottle filled with purified freshwater (about 2104.4 g)
4. Dissolve the total salt (79.3 g) in the total water volume (2104 g) to make an approximately 3.65 weight percent solution of NaCl. The volume of this solution will be slightly larger than the Coke bottle, so dissolve it in another container.

If you have a 100 mL graduated cylinder, use exactly 3.65 grams of salt and 96.35 mL of fresh water. It is tricky to volume measure such a small amount of salt, but that is about 0.59 teaspoons of Mortons.

 

[pickupman66]

Thank you. I dont have a scale so I am forced to use the teaspoon method. Can you scale the 0.59 teaspoon up for me? for instance on the Conductivity, you had 1 tsp (6.2 grams) for 182 ML of water. or help me with the formula to use a 6.2 gram measurement of salt (1tsp)

Edit
Ok, so I did Math. I am not a math guy. based on your info above (3.65 grams per 96.35ml), 1 gram of salt per 26.397ml of water. 26.397 x 6.2 =163.66 ml.

so I need 1tsp per 163.66 ml of water? I know I can get to 163.6.

Correct?

 

[Randy Holmes-Farley]

Thank you. I dont have a scale so I am forced to use the teaspoon method. Can you scale the 0.59 teaspoon up for me? for instance on the Conductivity, you had 1 tsp (6.2 grams) for 182 ML of water. or help me with the formula to use a 6.2 gram measurement of salt (1tsp)

Edit
Ok, so I did Math. I am not a math guy. based on your info above (3.65 grams per 96.35ml), 1 gram of salt per 26.397ml of water. 26.397 x 6.2 =163.66 ml.

so I need 1tsp per 163.66 ml of water? I know I can get to 163.6.

Correct?

Yes, that looks right. You are a math guy!

 

[tjgr34]

3. Measure the full volume of a plastic 2-L Coke or Diet Coke bottle filled with purified freshwater (about 2104.4 g)

Will a Pepsi bottle work or does it have to be a Coke product?

 

[Randy Holmes-Farley]

Will a Pepsi bottle work or does it have to be a Coke product?

At this point in time, both may be a bit different then when I measured them in 2004, but probably not greatly so and they are probably equally likely to be accurate.

 

[pickupman66]

Yes, that looks right. You are a math guy!

Perfect. I'll calibrate it tonight. When I used the conductivity solution I got a reading of ~32ppm.

 

[pickupman66]

bumping this so I can find it tonight when I go to calibrate things. LOL.

 

[purp]

Zerging this up from the dead, but has anyone done a DIY KCl based conductivity cal solution?

 

[Randy Holmes-Farley]

Zerging this up from the dead, but has anyone done a DIY KCl based conductivity cal solution?

They are easy to make, if you want to use KCl and have it pure. I might bake it first to dry it, but S=35 on the practical salinity scale (35 ppt for our purposes) is actually defined as matching a KCl solution.

IAPSO Standard Seawater and the Practical Salinity Scale | OSIL

IAPSO Standard Seawater is the only internationally recognised calibration standard for the measurement of Practical Salinity as approved by all the major oceanographic bodies (ICES, IOC Unesco, SCOR,etc) and is endorsed by the International Association for Physical Sciences of the Ocean (IAPSO).

osil.com

"The current definition for Practical Salinity states: a seawater of Practical Salinity 35 has a conductivity ratio of unity at 15 degrees Centigrade (and 1 atmosphere pressure) with a potassium chloride (KCl) solution containing a mass of 32.4356 grams of KCl per kilogram of solution. "

 

[purp]

What's funny is that its so much harder to really accurately measure 1L of water than it is to measure 32.4356g of KCl.

 

[Randy Holmes-Farley]

What's funny is that its so much harder to really accurately measure 1L of water than it is to measure 32.4356g of KCl.

True, especially at home.

 

[purp]

I do it by mass to .1g and say a little prayer for RODI.

 

[JimWelsh]

What's funny is that its so much harder to really accurately measure 1L of water than it is to measure 32.4356g of KCl.

If only somebody could have invented a flask designed to measure a given volume accurately, but alas, no such thing exists, I guess.

 

[purp]

So spicy Jim! Good glassware is expensive (and has to be cleaned). I have a scale that has given me pretty good accuracy/precision (0.1g up to 3 kg). I mix in batches of 1L, so I'll take 0.1 mL accuracy vs closer to 1 mL accuracy on a high quality, cleaned volumetric flask. It ain't NIST, but it hasn't steered me wrong yet.

If you got better ideas , I'd love to hear em!

 

[JimWelsh]

The uncertainty of a Class A 1000mL volumetric flask is 0.3 mL, not 1 mL. Now, that uncertainty is with regards to the accuracy of the placement of the mark, not the uncertainty when setting the meniscus. Calibrating your volumetric flask can bring the uncertainty down to much lower than that -- usually to within a drop, or around 0.05 mL.

Unless you are accurately measuring the water's temperature and adjusting for water's density at that temperature, then you likely have greater error than the 0.1 mL you cite. If you are ignoring water's density, and assuming it to be 1.000 g / mL, then you are certainly off by much more than that.

I agree that when properly done, gravimetric measurement of water can be very accurate.

I can usually find Class A 1000 mL volumetric flasks all day long on Amazon for under $30.

 

[purp]

Well I may have to pick one up then.

Either way you slice it. Measuring 1L of water is still more of a pain than measuring out KCL. Well,you do generally have to bake out the moisture. Its all a pain, i take it back.

 

[Randy Holmes-Farley]

If you calibrate the glassware with your own balance, why ever use volumetric glassware instead of the balance itself. lol

 

[JimWelsh]

If you calibrate the glassware with your own balance, why ever use volumetric glassware instead of the balance itself. lol

Seriously, Randy? I admit that this thread is a poor example, because the specification was for a kilogram of solution rather than a liter of solution, but in most cases, solutions are defined as X grams of solute per liter of final solution, and the exact amount of water to make up that liter is difficult to calculate accurately, since the density of the resulting solution is usually greater than the density of water, etc., so you can't just weigh out the "correct" amount of water, hence the utility of volumetric flasks, and their ubiquity in laboratories worldwide (even those that also have good balances, which is basically all of them).

Sorry, but I'm just arguing for more serious hobbyists to spend a few bucks on a decent VF or two. It would make so many things so much easier for them. I'm still using the first 1000 mL VF I bought years ago, and that was some of the best money I ever spent, IMHO.

 

[purp]

Jim, I feel like you need a hug. If it weren't for the global pandemic I'd give you one.

Can you link me to something you'd consider to be a reasonably priced 1L VF?