His invention relates to the treatment of phosphoric acid and more specifically to the treatment of concentrated phosphoric acid. Phosphoric acid suitable for use by the semiconductor industry must be of high purity. Increasingly stringent specifications of the semiconductor industry are continuously being established because of the efforts expended in miniaturization of the components. The calcium content of the phosphoric acid is one of the critical impurities permissible in the acid and presently 85% phosphoric acid containing up to 5 ppm calcium is acceptable by the industry. As miniaturization proceeds, the industry specifications will reduce the acceptable calcium level. The present invention provides a means for reducing selectively and substantially the calcium content of relatively high purity 85% phosphoric acid; that is, phosphoric acid containing about 61% p2 o5 and having a specific gravity of about 1.68. A convenient source of a commercial grade of phosphoric acid of relatively high purity is food grade 85% phosphoric acid. The specifications for this grade of phosphoric acid do not include a limit for the calcium content which may vary up to about 0.02%. It has been found that the calcium content of the concentrated phosphoric acid may be reduced substantially by contacting the acid with a strong cation exchange material or resin in its acid form.