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Simultaneous determination of fluoride species plus acid anions in etching baths by ion chromatography

MetrohmMetrohm presents a method for the straightforward ion chromatographic determination of HF, HNO3, short-chain organic acids and H2SiF6 in acidic texturing baths used in the manufacturing process of solar cells. Standard ions such as fluoride, nitrate, acetate and sulfate are determined via suppressed conductivity detection while dissolved silicate is spectrophotometrically detected in the same run after downstream post-column reaction (PCR) as molybdosilicic acid.

Solar cells are manufactured from monocrystalline and polycrystalline silicon wafers whose surface is treated in etching baths before being spiked with foreign atoms (P, B). Efficiency of this process is enhanced by replenishing the used etching solutions with concentrated acids.

The dissolution of silicon in HF-HNO3 etching baths leads to the formation of SiF62-. Under the given alkaline eluent conditions, SiF62- completely hydrolyzes into undissociated orthosilicic acid and fluoride:
NaSiF6 + 4 NaOH –> Si(OH)4 + 6 NaF.

After separation of the resulting HF-HNO3-Si(OH)4 mixture, the fluoride, nitrate, sulfate and acetate anions are determined by suppressed conductivity detection. The determination of the undissociated orthosilicic acid is achieved by downstream UV/VIS detection at 410 nm after post-column reaction with molybdate:
H4SiO4 + MoO42- + 24 H+ –> H4[Si(Mo3O10)4] + 12 H2O.

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