Salt concentration...

NedL · Apr 7, 2014

Hello chemists in our midst,

Is there an easy, crude way to estimate the concentration of salt, say to within a percent or so? I have about 500ml of water with an unknown quantity of ( mostly sea, but maybe some bittern ) salt. Searching around a bit but end up at aquarium websites that say you need a fancy conductivity meter.... I have an ohmmeter, thermometer, scale, graduated cylinder... I'd be perfectly happy if I could even distinguish between say 1% and 5% solution. The two ideas I've thought of so are are:

1) weigh a small cup, add a small measured sample, boil in microwave until no liquid is left, then weigh again.
2) stick my ohmmeter probes in, then do the same with a cup of water adding salt until I get the same reading...

Oh, if it matters, I've already added 1.5g citric acid.

Or, alternatively, if I make a salt print, what are the signs of too high and too low a concentration? I've read that if it is too high the print will be weak and pinker, but anything more specific?? The last time I used sea salt for a salt print, I got staining and fog, but I didn't play with the concentrations much....

ME Super · Apr 7, 2014

IDK how accurate your ohm meter is, so I don't know if this little bit of info will help you or not, but conductance = 1/resistance. So a resistance of 0.5 ohms has a conductance of 2 mhos (mho is ohm spelled backwards). Now that that is out of the way, you can figure out the conductivity of your salt solution from your ohm meter by converting resistance to conductivity. Past the conversion, I am unable to help further at this time.

NedL · Apr 7, 2014

I've got a digital ohmmeter that reads down to 0.1 ohms ( for the <200 ohm scale ) and an analog meter that reads to within about 50 ohms or so. What I don't have any clue about is how sensitive the measurement would be to surface area of electrodes or types of salt, etc. If it can get me in the ballpark that would be fine! I know there are some kinds of measurements that don't work well with digital ohmmeters....

I have a vague memory of holding the leads of an ohmmeter in a glass of water and adding salt and watching the reading change... but I was probably a kid and it was probably 40 years ago!

Herzeleid · Apr 8, 2014

E. Young's thesis on salt prints might give you the idea of what happens at different salt concentrations.
http://researchbank.rmit.edu.au/view/rmit:7850

chrisaisenbrey · Apr 8, 2014

I guess the classical way would be to measure the density of the salt water by a hydrometer.
Perhaps you find some Alcohol meter or some Saccharometer which are relatively common in rural environment

anikin · Apr 8, 2014

Measure out 100 grams of the solution. Then measure its volume. Subtract the measured volume from 100mL. The value in mL is a rough approximation of your concentration.

ME Super · Apr 8, 2014

The standard "Pure water weighs 1g/mL" standard only applies when water is at its densest. This is at 4oC. Warmer or colder, and water's density is < 1g/mL. Do the above at 4oC and this might actually work.

NedL · Apr 8, 2014

Herzeleid said:
E. Young's thesis on salt prints might give you the idea of what happens at different salt concentrations.
http://researchbank.rmit.edu.au/view/rmit:7850

Thanks that's a good idea. I read that some months ago and I'll look through it again!

anikin said:
Measure out 100 grams of the solution. Then measure its volume. Subtract the measured volume from 100mL. The value in mL is a rough approximation of your concentration.

This is very clever. Just the kind of thing I was hoping for.

ME Super said:
The standard "Pure water weighs 1g/mL" standard only applies when water is at its densest. This is at 4oC. Warmer or colder, and water's density is < 1g/mL. Do the above at 4oC and this might actually work.

I will try anikin's idea with this enhancement! Thank you all very much

Regardless of how this works, I'm hoping to try a salt print with this mystery liquid tomorrow... one paper is already salted using it undiluted....
By combination of this method and reading Young's thesis I can hopefully not waste too much silver getting to a nice print! Thanks again everyone!

I'll drop back by this this thread and let you know how it went!

mrred · Apr 8, 2014

Or to factor out a multitude of errors, I would weigh equal volumes of your saline solution and a solution with everything except the salt. The difference would be how much salt you have.

chrisaisenbrey · Apr 9, 2014

Here is the DIY manual for a hydrometer. It is simply calibrated with salt of known concentration.

http://www.wikihow.com/Build-a-Hydrometer

Jim Noel · Apr 9, 2014

By all means not only read Young's paper, but follow it. SAlt is too cheap to worry about the concentration of what you have, start over and match the salting solution and silver nitrate concentrations as indicated.

NedL · Apr 9, 2014

Jim Noel said:
By all means not only read Young's paper, but follow it. SAlt is too cheap to worry about the concentration of what you have, start over and match the salting solution and silver nitrate concentrations as indicated.

Hi Jim thanks! I wasn't very clear about why I'm doing this. I certainly do mix my own salt solutions carefully with my normal salt printing! I did read her paper when I first started a few months ago.

This is a special one time project that is using salt water that I collected, with unknown salinity. The salinity was supposed to be somewhere between about 2.5% and hypersalinity -- so I was expecting it to need dilution. But now I have some reasons to doubt that, hence my question!

NedL · Apr 9, 2014

chrisaisenbrey said:
Here is the DIY manual for a hydrometer. It is simply calibrated with salt of known concentration.

http://www.wikihow.com/Build-a-Hydrometer

Brilliant! I can do this!

The thing that is so perfect about this idea is that I can make a measurement relative to a known 2% salt solution... all I really need to know is if my mystery saltwater is much less or much more than 2%. This will even give me a rough estimate of how much!

Thanks! I'll report back with results!

NedL · Apr 9, 2014

Hi Everyone,

In the spirit of the article that crisasenbrey pointed me to, and in keeping with the spirit of what I'm doing and my knowledge of chemistry, I made the hydrometer using a straw and playdoh. My bottles of mysterious salt water and distilled water were in the same closet, so they should be very close to the same tempertature.

Here is 100ml of distilled water:

here after adding 2 grams of salt:

here with a total of 4 grams of salt:

and here is my mystery saltwater:

I'd say it looks like my mysterious saline solution is a little bit less than 2% salt. It certainly isn't 6 or 8% salt and it is not supersaturated.

This fits my suspicions-- it's why I asked the question here in the first place! It also means the solution should be close to right for a salt print without dilution. The proof will be in the print, but I just put the contact frame outside 10 minutes ago, and the area around the negative turned purple right away, just like normal for a salt print. The color of purple reminds me of the few times I've used sea salt, rather than the more lavender color from plain NaCl.

If this all works out I will make a post in the MSA thread where you will be able to read the whole story and find out what this was really all about.

Thanks everyone!

Edit 5 hours later: the print came out great, thanks again!

eli griggs · Jun 20, 2024

chrisaisenbrey said:
I guess the classical way would be to measure the density of the salt water by a hydrometer.
Perhaps you find some Alcohol meter or some Saccharometer which are relatively common in rural environment

Hydrometers are common at beer and wine makers stores where you can buy the materials and tools involved.

grahamp · Jun 21, 2024

Way back, when I used to do geology for a living, we used a pocket salinity refractometer to measure salinity of water in the field. A drop of the liquid is placed on the surface of half a split prism pair, the pair is closed, leaving a thin film. Looking through the eyepiece shows the concentration based on the diffraction. We cross checked it against solutions made in the lab.

The nice thing is that it is not subject to temperature variation. The same could not be said of the operator on a snowy, icy, November day...

Salt concentration...

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