Developing Good Lab Procedure

Proper technique results in accurate, reproducible test results. The best test kit will give bad information if not used correctly or if the aquarist's technique is sloppy. Some tips for proper testing:

1. Read the instructions carefully and make sure all equipment is handy before beginning test procedures.

2. Use a timer when a test specifies a certain waiting period.

3. Wash test tubes and other equipment carefully, rinse in distilled water, and drain dry after each use.

4. Rinse test vials and pipettes with the water to be tested immediately prior to carrying out the test.

5. For very accurate results, perform the test in triplicate and average the readings.

6. Parts per million (ppm) and milligrams per liter Cmg/L) are the same thing, for practical purposes. To be completely technical, multiply mg/L times specific gravity to obtain ppm.

7. Don't go to all the trouble to buy test kits, learn good techniques, and make regular tests unless the results are recorded in a notebook for future reference. (Pages 139 and 140 are log sheets that can be reproduced on a copier or used as a guide to help make record keeping easier.)

of this term and to better understand the concept of alkalinity, return to the discussion of neutralization reactions. The general formula for a neutralization reaction is written as follows:

126 Natural ReeT Aquariums

Note that equivalent amounts of both hydrogen and hydroxide ions are involved. Next consider that different chemical compounds will yield up different amounts of hydrogen or hydroxide ions when dissolved in water. Nitric acid, HN03, yields only one hydrogen ion per molecule, while sulfuric acid, H2S04, yields two. This can be determined by inspection of the formula. Similarly, calcium hydroxide, Ca(OH)2, yields two hydroxyl ions per molecule, and sodium hydroxide, NaOH, yields only one. The number of hydrogen or hydroxyl ions available per unit of a solution of a compound is the number of equivalents per unit of that solution.

MOLAR SOLUTIONS. Chemists long ago recognized the need for a standardized way of making solutions so that experiments could be consistently repeated. Combinations of ions are important to chemists, so it was natural that they would develop a standard solution that is defined in terms of the combining ratios of the ions in the solution. When a compound such as salt (sodium chloride, or NaCl) is added to water, it breaks up into ions (Na+ and CI"). When these ions combine with others added subsequently, they do so in precise ways, each negative charge matching exactly with a positive charge. We often need to know exactly how many individual ions there are in a solution, so we can add precisely enough of something else to achieve some desired result. This is accomplished through the use of molar and normal solutions.

To prepare a molar solution, it is first necessary to total up the molecular weight of the compound in question. For the salt, NaCl, in this example, the atomic weight of sodium is 23, and that of chlorine is 35, so the molecular weight is 58. If we weigh out 58 grams of salt, we will have a gram-molecular-weight, or mole, of this compound. In the case of water, two atoms of hydrogen, atomic weight 1, are combined with one atom of oxygen, atomic weight 16, so one mole of water is 18 grams.

Why is this important? A chemist named Avogadro proved a few centuries ago that one mole of anything will contain exactly the same number of individual particles, whether atoms, molecules, or ions. The number is 6.02 x 1023, or 602,000,000,000,000,000,000,000 particles.

Avogadro s number is not really important by itself. What is significant is that a mole of anything contains this same number of particles. When one mole of a compound is dissolved in an aqueous solution to make a total volume of one liter, we have a standard solution, called a molar solution, abbreviated as 1 M.

Note that in a molar solution of sodium chloride there will be one mole each of Na+ and CI" ions present. Similarly, in a molar solution of nitric acid, we have one mole of hydrogen ions (H+) and one mole of nitrate ions (N03~).This is a solution of acid, and it receives special consideration because it contains one mole of those very important hydrogen ions. Chemists call such a solution a normal solution (1 N). A 1 N solution delivers one equivalent of either acid or base. In other words, one teaspoon of a 1 N solution of acid will neutralize exactly one teaspoon of a 1 N solution of base. It does not matter which acid or base is the source of hydrogen or hydroxyl ions, because all that is important is the combining ratio, based on Avogadro's number. The combining ratio, or equivalency, of a normal solution is a constant.

Returning to the measurement of alkalinity, then, we are determining how many equivalents of acid must be added in order to combine completely with the bicarbonate, carbonate, borate, hydroxide, and other ions present in the water sample, without adding an excess of hydrogen ions. We do this by performing a titration, adding a standardized acid solution drop by drop and noting when the endpoint is reached by means of an indicator that changes color in response to different pH levels. As it happens, the endpoint of the total alkalinity titration is reached at a pH of 4.5, and the

Chapter Five 127

indicator used, for example phenolphthalein, changes color at this pH. Phenolphthalein changes from colorless to bright red, while bromthymol blue, an indicator often used in carbonate hardness (KH) tests, changes from blue to yellow at the endpoint. Determination of the endpoint can be done precisely using a pH meter. If one does this, recording the solution s pH as each drop of acid is added, the results appear as in the accompanying titration graph.

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Aquarium and Fish Care Tactics

Aquarium and Fish Care Tactics

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