In a chemistry lab, you could theoretically create a solution that has high alkalinity and low pH. However, in a reef aquarium, so many things would have to be catastrophically wrong with the water chemistry that nothing would be alive in the aquarium long before that happened.
pH
The pH scale measure how acidic or basic a solution is. This is determined by how much ionized hydrogen is dissolved in a sample. The pH scale ranges from 1 to 14. If a solution has a pH of 7, it is neutral, neither an acid or a base. If the pH is lower than 7, it is an acid. If the pH his higher than 7, it's basic. This scale is a useful shorthand for several areas of water chemistry and an important test for saltwater aquariums.
Alkalinity
Alkalinity has a close relationship with pH. It is a measurement of how much a solution will resist dropping in pH due to acid. Functionally, it is a measurement of the concentrations of certain minerals, often calcium carbonate, that have a buffering effect on aquarium water, preventing pH from dropping. Several units of measurements are used for alkalinity, including meq/L, degrees of carbonate hardness, abbrivated dKH, and CaCo3 equivalents.
Desired Levels
While both pH and alkalinity are closely related, you should measure both water parameters in a reef aquarium. For a reef aquarium, keep the pH between 7.8 and 8.5; 8.1 to 8.3 is the ideal range. The ideal parameters for alkalinity in a reef aquarium are 2.5 and 4 meq/L, 7 to 11 dKH or 125 to 200 ppm CaCO3 equivalents. These all translate to the same range.
Reef Aquariums
For reef aquariums, prepare your water by mixing commercial salt mix with water. You need to use water purified by either distillation or reverse osmosis. Salt mix, in addition to salt, contains calcium carbonate and other buffers that get the pH and alkalinity exactly where they should be. Since many reef invertebrates absorb calcium, you may need to supplement it. The process of coral sucking calcium out of the water can lower pH and alkalinity. Meanwhile, toxic nitrogen compounds that result from overfeeding, overcrowding and rotting organic matter also reduce pH and alkalinity.