WATER, QUALITY:  Water Analyses - Phosphate interference in the determination of fluorides

                           UNITED STATES
                    DEPARTMENT OF THE INTERIOR
                         GEOLOGICAL SURVEY
                     Water Resources Division
                        Washington 25, D.C.

In reply refer to:                            December 2, 1958
                                              Code 4053 0001


To:       District Chiefs and Staff Officials, Ground Water Branch

From:     Acting Chief, Ground Water Branch, Washington, D.C.

Subject:  WATER, QUALITY:  Water Analyses - Phosphate interference
                           in the determination of fluorides

A recent memorandum (November 12, 1958) by the Chief of the
Quality of Water Branch, called attention to the fact that all of
the quality of water laboratories should be on guard for the
interference of phosphates in the determinations of fluorides.
Phosphates, other than the ortho forms, are not active to water
but may be added in well development or in water treatment (for
example "Calgon").

Results of a study of the Raleigh District, North Carolina,
confirmed the statement in the Quality of Water Manual that 10 ppm
ortho-phosphate can be tolerated if the Eriochrone Cyanins R
reaction is allowed to go for 24 hours.  Polyphosphate
interference becomes significant at a much lower level.

In the future all water samples collected where polyphosphates
have been added should be labeled so that the Quality of Water
laboratories can make allowance for these chemicals in determining
the fluoride content of the water.  Comercial laboratories,
however, may not make an allowance for polyphosphates, so care
should be exercised in the interpretation of commercial analyses.
Water samples from wells that have been treated with
polyphosphates, and that have been reported as having abnormally
high fluoride, probably should be resampled to determine whether
the analyses correctly measured the content of fluoride in the
water or whether the results were erroneous because of
interference by polyphosphates.

                             (s) H.M. Babcock

Distribution:  A, B, S3, FO3, SL