How to determine electrolyte concentration in solutions?

Question

The original question is here

We've been given the sports drink vs orange juice challenge. To find out which has more electrolytes. I could test for conductivity but that alone wouldn't tell me which has more actual electrolytes in it. So I'm wondering if there's a way I could separate each ion from each other and measure them that way?

I'm posting it here because I'm very curious as to the answer.

Several people have suggested evaporation.

You can try evaporating the liquids and seeing what's left behind. The electrolytes will be left behind as solids.

However, the issue with evaporation is that it leaves behind all solids - electrolyte or not.

Some people have suggested conductivity measurements.

Wouldn't conductivity simply be proportional to the concentration of substances that promote conductivity?

However, this objection was raised:

Well my understanding was that different ions produce different voltages, therefore, there could be a lot of one type of electrolyte producing a large voltage but there could be more electrolytes in the other solution producing a smaller voltage. I'm not sure though as I am in first year and haven't had this completely covered yet. My professor said that conductivity might not give enough information so I may need a second approach as well, I'm not sure what he means exactly.

Is this a valid point? I guess we could empirically test out this assertion that "different ions produce different voltages" by comparing the conductivities of two otherwise identical NaCl and, say, MgBr2 solutions. Wouldn't the MgBr2 have a conductivity measurement that is roughly 1.5 times that of the NaCl solution? Or is it true that the ion's charge affects the conductivity?

Answer 1

Electrolytes turn water into a conductive solution. Conductivity is affected by how free the electrons are to move. Conductivity (which is the reciprocal of resistivity) is affected by by concentration, temperature, and how much charge the ions carry. Conductivity alone will give an equation with both ions and concentrations; however, this equation will have multiple solutions. For some fun understanding this phenomena look at this this link.

Conductivity is different than voltage. Voltage in this situation is likely referring to a half-cell reaction with a standard hydrogen electrode in a galvanic cell. Using this approach has the same issue with a multiple solution equation.

Electrolytes are typically ions. Ion concentration is measured with ion chromatography. Ion chromatography involves separating the ions from the solution based on various properties to perform a measurement. It is explained here, here, and here.

When electrolytes of drinks are measured it is important to include if the electrolytes are bio-available. Bio-available electrolytes depend on the activity performed. An activity such as sports uses a different set of electrolytes than sitting.

Testing bio-available electrolytes requires more work than analytic chemistry. Analytic results both the drink and bodily fluids of a population doing a certain activity are need to draw conclusions about the electrolyte composition of a drink.

Reading a patent for a sports drink ions include $\ce{Na+}$, $\ce{K+}$, $\ce{Mg^2+}$, $\ce{Cl-}$, $\ce{SO4^2-}$, $\ce{PO4^2-}$. The patent also includes citrate, sucrose, dextrose, ascorbic acid and pyridoxine as these are known to affect the uptake of electrolytes.

If you have access to a university library the values of these compounds for orange juice are available.

Answer 2

OVER THE TOP, 2nd ANSWER

There are many ways to analyze things so with respect to the original challenge, to find out which has more electrolytes (orange juice or gatorade), since there is not a strict definition of what a electrolyte is, you could try to define electrolyte: Electrolytes are minerals, acids, bases, or salts that carry an electric charge... but that's debatable and still leaves room for interpretation. So rather than try to be specific, it will be easier to simply list a few analytical methods that should provide a totally comprehensive analysis.

A practical preparation and test for many elemental-ions and minerals (sodium, calcium, zinc, iron etc.) and subsequent analyses will be to sample the ash content.

Measuring the pH of the solution may help confirm further analyses, but should not be technically necessary.

Atomic absorption spectroscopy (AAS) would be the most straightforward technique is used for determining the concentrations of a elemental ions in a sample. I would use AAS to analyze both the original and reconstituted (from ash) sample for veracity.

However, AAS will not account for mixed organic molecules such as citric acid or ascorbic acid. High performance liquid chromatography (HPLC), in conjunction with IR spectroscopic analysis of the analytes would be the most straighforward method for separating organic molecules/electrolytes. Combustion analysis or Mass Spectrometry could possibly be employed for obtaining more information about the carbon, sulfur, and nitrogen, hydrogen, and oxygen content of the sample.

And to put a nail in this coffin, gas chromatography (which is often coupled with NMR these days) may also be used to separate and analyze a sample of mixed materials. And the Nuclear Magnetic Resonance (NMR) data from each analyte can be obtained (which is very useful information for both approximating concentration and identifying the analytes).

So, given HPLC + IR Spectroscopy, Gas chromatography + NMR spectra, Ash content, Mass Spec analysis, AAS, and the pH... there's really nothing more that could possibly be necessary to determine the contents of an orange or a bottle of gatorade, and then you can call an electrolyte whatever you please.

Answer 3

There are many ways to do this and essentially you are after the mineral and ash content. So, with that in mind, phosphorus boils at 280.5 °C and here are the common methods for mineral anlaysis (like for food or gatorade).

But for about the same price of a crucible (not to mention the oven) you can have someone else analyze the ash content. This is not a service recommendation, I'm just saying it's another, more affordable option.

Whatever analysis you perform, a practical consideration is that analysis will be easier and more accurate after reducing the sample to ash.