In electrolytes, traveling ions are the charge carriers, playing the role of electrons in a metal conductor. Ionic solutions, which are electrolytes, vary in resistivity depending on small changes in concentration. Ohmmeter probes dipped in a glass of distilled water produce a high-ohm reading, while adding a pinch of salt greatly increases the conductivity.
Electrolytes are increasingly in the news because of their role in next-generation power systems such as fuel cells and batteries for electric vehicles. So it can be helpful to understand what makes a substance an electrolyte and how its electrical qualities are measured.
Any substance that becomes an electrically conducting solution when dissolved in a polar solvent, such as water, is considered an electrolyte. The dissolved substance separates into cations (plus-charged ions) and anions (minus-charged ions), which disperse uniformly through the liquid. Dissolving ordinary table salt in water, for example, results in sodium and chloride ions: NaCl(s) → Na+(aq) + Cl−(aq).
Electrically, the electrolyte solution is neutral. Applying a voltage to it forces the cations of the solution toward the electrode that has an abundance of electrons and the anions toward the electrode that has a deficit of electrons. This movement of anions and cations constitutes an electrical current.
Possible electrolytes include most soluble salts, acids, and bases. Some gases, such as hydrogen chloride, at high temperatures or low pressure, can also function as electrolytes.
It is worth exploring the details of how electrolytes behave while conducting current. When a voltage is applied, lone electrons normally can’t pass through the electrolyte. Rather, a chemical reaction at the cathode provides electrons to the electrolyte. Another reaction takes place at the anode, consuming electrons from the electrolyte. Consequently, a negative charge cloud develops in the electrolyte around the cathode, and a positive charge develops around the anode. The ions in the electrolyte neutralize these charges, enabling the electrons to keep flowing and the reactions to continue.
Typically in systems such as batteries, the electrode reactions can involve the metals of the electrodes as well as the ions of the electrolyte. In batteries, two materials with different electron affinities are used as electrodes; electrons flow from one electrode to the other outside the battery, while inside the battery electrolyte ions close the circuit. In some fuel cells, a solid electrolyte or proton conductor connects the plates electrically while separating the hydrogen and oxygen fuel gases.
Though many electrolytes take the form of a liquid, not all do. Gel electrolytes, for example, closely resemble liquid electrolytes in the sense they are liquids in a flexible lattice framework. Various additives are often applied to boost their conductivity.
Solid electrolytes have several structural advantages over liquids and gels, and there is a lot of research in this area. Dry polymer electrolytes differ from liquid and gel electrolytes in that a salt is dissolved directly into the solid medium. The medium is a relatively high-dielectric-constant polymer (PEO, PMMA, PAN, polyphosphazenes, siloxanes, etc.) and the salt has a low lattice energy. Often, composites and an inert ceramic are used to make the electrolyte mechanically stronger and more conductive.
There are also solid ceramic electrolytes where ions migrate through the ceramic phase by means of vacancies or interstitials within the lattice. And there are also glassy-ceramic electrolytes.
Then there are electrolytes that exhibit both solid and liquid properties. Organic ionic plastic crystals are a type of organic salts exhibiting what are called mesophases, a state of matter between liquid and solid. Here mobile ions are orientationally or rotationally disordered while their centers reside at the ordered sites in the crystal structure. They have various forms of disorder and therefore plastic properties and good mechanical flexibility as well as improved electrode-electrolyte interfacial contact.
Of course, some electrolytes are better than others for passing current. It used to be that a “strong electrolyte” was a material that was good at passing current when in solution. That definition has been revised. Now, a strong electrolyte is a solution/solute that completely, or almost completely, ionizes or dissociates in a solution. But the ions are still good conductors of electric current in the solution. A concentrated solution of a strong electrolyte has a lower vapor pressure than that of pure water at the same temperature. Strong acids, strong bases and soluble ionic salts that are not weak acids or weak bases are strong electrolytes.
In general, the lower the concentration and the lower the charges on the ions, the “stronger” the electrolyte. Alkali metals other than lithium are usually strong electrolytes especially in dilute solutions. Alkaline earth metal compounds are weaker electrolytes, and other metals are even weaker still.
It turns out that solubility of the electrolyte enters into whether or not it is a good or bad conductor. It’s possible that a compound is a strong electrolyte but just not particularly soluble. So in solution, it would not produce lots of ions. In general, if the ions in a compound are strongly attracted to each other, the compound will be less soluble, and thus probably a weaker electrolyte in solution. Additionally, electrolytes also look stronger at lower concentrations, because if the ions split up, they are less likely to find each other again.
That brings us to how electrolytes are measured. The measurement process for traditional unsealed lead-acid car batteries is easy. The lead-acid battery holds an electrolyte solution consisting of 65% water and 35% sulfuric acid. The specific gravity or weight of this solution rises as the battery charges and drops as the battery discharges. In the charged state, the chemical energy of the battery is stored in the potential difference between the pure lead plates at the negative side and the PbO2 on the positive side, plus the aqueous sulfuric acid. The electrical energy produced by a discharging comes from energy released when the strong chemical bonds of water molecules are formed from H+ ions of the acid and O2− ions of PbO2.
The specific gravity of the electrolyte depends on the 65%/35% water-sulfuric acid ratio for the necessary chemical reaction to take place. This ratio is affected by the amount of sulfuric acid and the temperature of the solution. As the temperature drops, the electrolyte contracts and increases its specific gravity. As the temperature rises, the electrolyte expands and specific gravity drops.
A battery’s specific gravity is a measure of its state of charge. This is because, during discharge,
the specific gravity drops linearly with the loss of capacity. The specific gravity also rises as the battery
A hydrometer is used to measure the specific gravity of the electrolyte solution in each cell. It basically uses a float to gauge the density or weight of sulfuric acid compared to the density of an equal amount of water. The greater the concentration of sulfuric acid, the more dense the electrolyte. A lead-acid battery cell is fully charged with a specific gravity of 1.265 at 80°F. This figure must be adjusted for temperature by adding 0.004 for every 10°F above 80°F and subtracting 0.004 for every 10°F below 80°F. Hydrometers frequently include a built-in thermometer that indicates the correction factor based on specific temperatures that must be added or subtracted from the float reading to get an accurate specific gravity reading for each battery cell.
The hydrometer makes use of Archimedes’ principle: A mass suspended in a fluid is buoyed by a force equal to the weight of the fluid displaced by the submerged part of the suspended mass. The lower the density of the fluid, the deeper the mass of a given weight sinks; the stem is calibrated to give a numerical reading.
Of course, hydrometers are of no use in solid or gel electrolytes. The typical way of measuring electrolyte specific gravity in sealed batteries or those employing solid electrolytes is indirectly through circuits that monitor the battery state-of-charge. Here the state of the electrolyte is inferred from knowledge of the battery chemistry and make-up and from electrical behavior while powering the load.