The Preservation Method of Water Samples to Determine Sulfide in Groundwater by the Methylene Blue Spectrophotometric Method

Sulfide is one of the important characteristic indices of groundwater pollution evaluation. In order to obtain accurate results for sulfide in water samples, Zn(Ac)₂ solution and NaOH solution are usually used as fixatives to reduce the loss of sulfide and H₂S from the water by oxidization. The numerous preservation methods from the national standard methods and documents have differing adding sequences and amounts of Zn(Ac)₂ solution and NaOH solution. Using these methods, the recovery rates of sulfide were in the range of 65%-108%. For this experiment the sulfide in the water sample was measured by using the methylene blue spectrophotometric method. The effects on the sulfide recovery rate are discussed in this paper, including different addition sequences and amounts of Zn(Ac)₂ solution and NaOH solution. The experimental results showed that the recovery and accuracy of sulfide are improved when the adding sequence and amounts are 1.0 mL of Zn(Ac)₂ solution, 500 mL of water sample and 2.0 mL of NaOH solution. The recovery rates of sulfide were in the range of 94.2%-98.0% for low and high concentration samples, which were higher when compared to other results in the literatures. The recovery rates of sulfide were satisfied with high concentration samples through increasing amounts of Zn(Ac)₂ solution and NaOH solution.