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Q & A: Salt Concentration in Water

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Q:
How do I find the concentration of salt in water?
- Science Class (age 14)
Wisconsin
A:
Salt is a general term. It is the product created when an acid and a base are combined. Salts themselves can be acidic or basic, depending on the properties of their so-called "conjugate acid/base"

I think what you mean is table salt, otherwise known as Sodium Chloride: NaCl

Well, there are two ways at least that I can think of.

1. Take a sample of the water and measure its volume. Then boil away the water and record the mass of whatever salt is left behind. You can then, using proportions, figure out how much salt was in the entire container based on how much salt was in your sample. Then you can divide the mass of all salt by the volume of the water, to get the concentration (measured in the units of mass over the units of volume, like g/mL , g/L, etc)

2. Take a water sample and record its volume and add a compound that will produce an insoluble chloride salt, like Silver. So if you used AgN03 (Silver Nitrate .. be careful with it!). Fill a buret with Silver Nitrate and take a sample of the salt solution. Add Silver Nitrate until the solution stops becoming cloudy. It's very important that you determine exactly how much silver nitrate you need, so that you can calculate the amount of salt.

This way is a bit harder, because you have to use some chemistry. When you find that adding Silver Nitrate no longer causes cloudiness, you can record the mL of Silver Nitrate you used.

The reaction is:

Ag+ + Cl- ---> AgCl(s) (s) means solid.

So the formula to find the mass of it is:

x mL AgNO3 used
---------------- = y
molar mass of AgNO3

To find the molar mass, you look on a periodic table and add the atomic weights of Silver, Nitrogen, and Oxygen (times 3 for oxygen because there are 3 of them in the molecule)

Let whatever that is to be y.

You take this number and multiply it by the molar mass of Sodium chloride, which is the atomic weight of sodium added to the atomic weight of chlorine.

So:

y * (Atomic weight of NaCl) = z g NaCl in sample

This number z is the amount of salt in your sample. You can then use proportions to figure out how much salt is in the entire solution based on how much was in the sample of water you had.

The calculations I have shown you are actually a part of chemistry called stoichiometry. The idea is to convert grams of a compound to moles (a common unit in chemistry. 1 mol is 6.022*10^23 atoms/molecules ... whatever you have)

Good luck!

Jason


p.s. If you're pretty sure that the main solute in your water is NaCl, there's another quick way to figure out how much is present.It's based on the idea that in fairly salty water almost all the electrical conductance comes from the dissolved salt. It won't work if the salt level is low. You can make two electrodes out of copper wire, maybe glued to a little slab of wood or something to hold them a fixed distance apart. Then you can dip those in your solution and measure the resistance with an ohmmeter. You may do best to switch the leads back and forth between the wires and measure the average value, to get rid of some problems that arise in dc measurements. If you have access to an ac ohmmeter, it would work better, but they aren't so easy to find. How then do you convert the conductance (the inverse of the resistance) to an estimate of the salt concentration? Simple- you make a series of measured salt solutions, and compare with them.

Mike W.

(republished on 07/13/06)

Follow-Up #1: experimental help

Q:
I TILL NEED HELP IN FIGURING HOW TO DO IT IT DID NOTCOME OUT RIGHT
- azar
PACOIMA, CA, U.S.
A:
Ok, we could walk you through this, but not without some idea what happened. Did you try a series of different salt solutions, to see if the resistance went down as the salt concentration went up? Did you get fairly consistent resistance readings? If not, what went on?

Mike W.

(published on 04/25/07)

Follow-Up #2: measuring viscosity

Q:
i want to use a specific concentration of salt in water to find its viscosity variance in stokes region.so how can i estimate that
- moni (age 18)
india
A:
I'm not sure how accurately you need to measure the viscosity. Here's a way to get it about right.  The flow of liquid through a pipette due to gravity will, for a given level of fluid, be proportional to the density and inversely proportional to the viscosity. The time it takes the liquid to drain from one level to another is then proportional to the viscosity divided by the density. It's not hard to measure the density of your salt solution. You can compare the drainage time of it to that of water.  The ratio of the drain times, corrected for the different densities, will give you the viscosity ratio. Water at room temperature has a viscosity of about 1.0 centipoise. You can check that more carefully on a table.

Mike W.

(published on 02/04/12)

Follow-Up #3: measuring salt concentration by resistance

Q:
I need to be able to find the percent concentration of NaCl in a solution with water. I need to be able to test from 1% to 10%. I was interested in the method of testing resistance and I was wondering if I could find the percent concentration with this method. Thanks!
- adarsh (age 15)
lawrenceville, georgia, usa
A:
You certainly could. You could make a bunch of test solutions with known concentrations, measure their conductances, and determine the salt concentration in an unknown solution by comparison. There are some things to be careful about, however.

A lot depends on how accurate you need those measurements. If you only need to know the salt concentration to say 20%, the crude methods described above should work. If you need good accuracy, there are straightforward ways to improve it.

The conductivity is temperature sensitive. You'll need to either keep everything at a fixed temperature, or, maybe more practical, do your initial calibration curve at several temperatures and measure the temperature on your test solution.

To get reliable results, you'll need to use an ac, not dc, conductance meter. You can buy these, or make one yourself. The electrodes that go in the solution also have to be rigidly mounted and made of some metal that doesn't corrode so that your calibration won't drift. It's best to use on pair of electrodes to carry the current and a second one to measure the voltage. You can buy conductance cells with good platinum electrodes. If that's too expensive, you could probably make something that worked well enough using wires mounted in an epoxy holder. Platinum wires would be best, but maybe nickel or even copper would work well enough for your purposes.

I just looked online, and it seems that there are meters for $400 that would do everything you're likely to need. There are even conductivity meters under $50, but they may not include thermometers and may require that you dilute your solutions to get them within the range the meter covers.

I'd be happy to go into more detail if you're interested.

Mike W.

(published on 02/13/12)

Follow-up on this answer.