Determination of experimental quantities

The equipment and conditions that can be used are so varied that no general statement of precision can be made. If the pH is used as a general indicator of water masses and water structure a precision of 0.03 unit is probably sufficient and should be easily obtainable. 

For measurements designed to be used in the calculation of the total carbon dioxide content of sea water a precision of 0.02 unit should be attempted and can be obtained under sea-going conditions if care is taken. 

For measurements designed to be used in the calculation of the relative amounts of the various forms of carbon dioxide (CCF , HCOj , and free COg) the highest possible precision is required in pH measurement. Ideally a precision of 0.005 unit should be attempted but under most conditions it is doubtful whether better than 0.01 can be achieved and then the greatest care in sampling and standardization is required. 

Ilie pH of a sample is measured by a glass electrode and electrometer-type pH meter after taking certain precautions in sampling and standardization. The pH is measured at laboratory temperature and pressure and the pH of the sea water in situ is derived by calculations involving a knowledge of the temperature and pressure of the sample when taken. 

Normally a Beckman Model G pH meter, or an instrument of similar specification, should be employed with wide range (—5 to 80 C, pH 0-11) glass electrodes and saturated calomel half-cell electrodes. Automatic reading instruments may be satisfactory for shipboard use, where a precision of about 0.03 unit is acceptable. A high-amplification pH meter such as the Beckman G.S. meter should be used for the most refined work, as in tracing marine photosynthesis, respiration, etc., where pH changes of 0.01 unit or less may be significant. Wide-mouth screw-capped 50-ml polyethylene bottles are needed, one for each sample. 

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We have concentrated so far on the determination of experimental quantities within DFT. The calculated electronic structure can also be studied with the goal to get chemical insight from it and answer questions like what is the bonding pattern How are bonds formed and broken Determination of localized orbitals has proved to be a useful tool in this direction. 

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