Your best source for complete information on the NFPA 70E Standard is via the actual standard, available for purchase at www.nfpa.org. In 2000, the NFPA 70E Standard first addressed electric arc flash hazard and required flame resistant arc flash clothing to protect workers. The current edition of the NFPA 70E standard was released in 2012, and further details personal protective equipment.
Fluoride ions exist predominantly as hydrofluoric acid in acidic environments with a pH below 5.2. Fluoride ions corrode the glass. Therefore, the pH measurement with the glass electrode for a solution which includes fluoride ions is basically impossible. Although the useful life of the electrode may be shortened, however, the measurement with immersing the electrode in the sample for a given short time is possible.
Fluoride: Uses, Effects and Controversies. Fluoride, the 13th most abundant element on the earth's crust, is a chemical ion of the element fluorine - fluoride has one extra electron that gives it a negative charge. Fluoride is found naturally in soil, water, foods, and several minerals, such as fluorapatite and fluorite. Fluoride concentration in seawater averages 1.3 ppm (parts per million), while in fresh water supplies the natural range is typically between 0.01 to 0.3 ppm.