To standardize a solution of acid you simply titrate it against a solution of alkali of known concentration. Once you have calculated this you can use it in turn as a standard solution, to find the concentration of a solution of alkali.
Standard solutions are also commonly used to determine the concentration of an analyte species. By comparing the absorbance of the sample solution at a specific wavelength to a series of standard solutions at differing known concentrations of the analyte species, the concentration of the sample solution can be found via Beer's Law. Any form of spectroscopy can be used in this way so long as the analyte species has substantial absorbance in the spectra.
See also: Primary standard