Potentiometric flow injection determination of total water hardness in tap water by using cupric ion buffer.

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A rapid and simple flow injection method for the determination of total water hardness using a cupric ion buffer and a flow-through type cupric ion selective electrode detector is proposed. The flow injection system was constructed with three stream channels. A sample solution of calcium or magnesium ion (200μl) was injected into a carrier (water) stream and was merged with a stream of 0.5 M sodium nitrate solution to maintain a constant ionic strength in the sample. This mixed stream was merged with the stream of cupric ion buffer solution consisting of 5×10-3 M cupric nitrate and 1×10-2 M EDTA adjusted to pH 9.4. The calcium or magnesium ion formed a complex with EDTA. In the reaction coil (0.5 mm i.d.×5.4 m) decreasing the concentration of EDTA in the buffer and as a result liberating the cupric ion from the EDTA-cupric ion complex. The change in the concentration of the free cupric ion was monitored with the cupric ion selective electrode and the concentration of the calcium or magnesium ion could be determined from the peak height of the electrode response. In order to make the sensitivity to the calcium ion equal to that to the magnesium ion, the pH of the cupric ion buffer was adjusted to pH 910 for determination of total water hardness. In this pH region, the amounts of EDTA reacted with the calcium and magnesium ions are almost the same and thus the change in the free cupric ion concentration is almost the same. A small amount of triethylenetetramine was added to the cupric ion buffer to suppress the effect of the chloride or bromide ion coexisting in the sample. The present method was successfully applied to determination of total water hardness in tap water with a sampling of 30 h-1 and a relative standard deviation of 0.7%. The results obtained by the proposed method were in good accordance with those obtained by the conventional chelate titration method.