At Seachem, we spend an increasing amount of time answering questions from hobbyists. I believe this is happening because the options for aquarium management are growing at such a dizzying pace that the art and science of it has not kept up. Here is one sampling of some of the more frequently asked and more important questions asked.

What’s the difference between iodine and iodide, chlorine and chloride, ammonia and ammonium?

All matter is made up of atoms consisting of a core of positively charged particles called protons and neutral particles called neutrons, surrounded by an orbital field of an equal number of negatively charged particles called electrons. Forces in the orbital field of electrons drive atoms to acquire orbital stability. There are two mechanisms to achieve orbital stability: 1) the gain or loss of electrons, or 2) the sharing of electrons between atoms. The first mechanism leads to the formation of ions. Since ions have more or less electrons than protons, they have either a negative or positive charge, respectively. Negative ions (called anions) associate with positive ions (called cations) to form ionic compounds. The second mechanism, sharing of electron between atoms, leads to the formation of uncharged covalent molecules. Sodium is an example of an atom which can give up an electron to achieve stability, acquiring a positive charge (Na+). Chlorine stabilizes by gaining an electron and acquires a negative charge (Cl-). Sodium ions associate with chloride ions to form sodium chloride, an ionic salt, common table salt. If, on the other hand, chlorine atoms acquire stability by sharing electrons then they form a covalent compound, Cl₂, elemental chlorine. The same is true for iodine and bromine. Ions have properties totally different from their elemental forms. Chlorine, iodine, and bromine are highly toxic, even at extremely low concentrations, whereas chloride, iodide, and bromide are essential for life and are relatively non-toxic. Chloride ions are a major dissolved component of sea water. Hydrogen consists of a single proton, no neutron, with a single orbital electron. If hydrogen gives up its electron, it acquires a positive charge and becomes a hydrogen ion (H+). Hydrogen is unique in that its ion is a proton, although hydrogen ions or protons do not exist as such, but protonate a solvent, usually water, to form a hydronium ion, H₃O⁺. If hydrogen shares an electron with another hydrogen, it forms a covalent bond or elemental hydrogen (H₂). Oxygen can share electrons with another oxygen and form elemental oxygen (O₂) or it can share electrons with hydrogens and form a covalent compound H₂O, water. Water (H₂O) is entirely innocuous when compared to hydronium (H₃O⁺), which is corrosive at even low concentrations. Hydrogens can share electrons with nitrogen and form a covalent compound NH₃, ammonia. When ammonia gas is added to water, it removes protons from hydronium water and forms ammonium ions, NH₄⁺. The extent to which this takes place is dependent on the hydronium concentration (pH): the higher the hydronium concentration (the lower the pH), the more ammonium is formed. Ammonia (NH₃) is very toxic, but ammonium (NH₄⁺) is relatively safe.

What is pH?

Water is a covalent compound and, like other covalent compounds, does not extensively dissociate. Its limited dissociation takes place according to the equilibria:

H₂O + H₂O<——> H₃O + OH-.

The extent of dissociation is expressed as a rate constant K= (H₃O)(OH-)/(H₂O)(H₂O), which is equal to 1.0 x 10-14. In such an equation, () designate concentration. At the equivalence point, the hydronium concentration is 1.0 x 10-7 moles per liter or 0.0000001 moles/liter. This is usually expressed as pH, or the negative log of the hydronium concentration, 7.0. pH is a very useful expression for expressing cumbersome concentrations. pH is the negative log of the hydronium ion concentration, or, if you're a little sloppy with concepts, the hydrogen ion or proton concentration. At pH 7.0, the concentration is 0.0000001 moles/L; at pH 3, the concentration is 0.001 moles/L; at pH 9.0, the concentration is 0.000000001 moles/L. As you can see, pH is just a convenient way of writing an inconvenient number. Each pH unit represents a 10-fold concentration difference of hydronium ions, the number of decimal places in moles/L of hydronium ions. The most reliable measurement of pH is that obtained with a functioning, calibrated electrode under proper conditions. The most unreliable measurement of pH is that obtained with a poorly functional or miscalibrated electrode or one obtained under improper conditions. A steady and reproducible digital reading is not assurance of accuracy. If you are not able to recognize the inadequacies of pH meter measurements, you are better off with pH dye methods. pH electrodes may appear to function, but have a clogged reference junction. An electrode which jumps around with the approach of static charge or ground (your hand) cannot reliably measure pH. An electrode should have rapid and consistent response to not only strong pH reference buffers, but diluted buffers as well. An electrode should be calibrated at two points on both sides of the target pH and it should behave linearly between those points. Always remove a sample of water from the aquarium to measure pH. Measurement by immersing the electrode directly in the tank can be severely compromised by otherwise undetectable stray electrical currents. Very soft water (e.g., distilled water) cannot be measured reliably unless a small amount of potassium chloride is added to improve conductivity. pH dye measurements work well, but have two limitations: measurement is dependent on visual perception, and dye response is subject to error from protein interference, salt concentrations, inadequate buffering or any combination of these. For that reason, only dyes with clear- cut color changes around the target pH should be used. Otherwise, try to use two different dyes to help visual judgment and also catch interference.

What is the difference between distilled, deionized, and R/O water?

Distilled water is obtained by condensing water from its vapor form to its liquid form. It is a relatively pure form of water, free of dissolved solids. It will, however, contain gases and vapors. Distillation does not free water of carbon dioxide, ammonia, or other vaporous contaminants.