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The hydrolysis of sodium tripolyphosphate was investigated in sterile aqueous buffers at pH 3, 4, 5 and 7 by Zidner et al (1984) and gave a pseudo first order reaction law. At pH 7 and 40°C the rate constant for hydrolysis of sodium tripolyphosphate to pyrophosphate is 4.81E-4 h-1
The kinetics of hydrolysis was also studied in sterile and algal culture media (clesceri and Lee, 1965). STPP hydrolysed much quicker in sterile lake water than in distilled water and about 5-10 times slower in sterile algal media than in sterile lake water. STPP half lives in sterile lake water and algal medium ranged between 83 and 608 h at 23ºC.
From these data on hydrolysis, the rate constant in natural surface water has been calculated at 25ºC by linear regression analysis of the logarithm of the concentrations versus time:
KSTPP/water/25ºC = 0.2369 d-1
DT50 STPP/water/25ºC =70.2h
An hydrolysis rate constant at 12ºC (default average value for EU surface waters) can be extrapolated using the Arrhenius law:
KSTPP/water/12ºC = 0.0837 d-1
DT50 STPP/water/12ºC =198.7h
Davis and Wilcomb (1967) studied the rate of hydrolysis of condensed phosphates STPP and TKPP (tetrapotassium pyrophosphate) in raw domestic sewage from a municipal treatment plant at 28ºC. Rapid hydrolysis were observed in the first 48 h. 80% and 86% of all the condensed forms present (condensed phosphate originally present in the sewage plus STPP or TKPP respectively) were hydrolysed in the first 48 h, while only 35.6% of the condensed forms in the control were hydrolysed in the same length of time. The degradation of sodium tripolyphosphate exhibited pseudo first order reaction kinetics with a rate constant of KSTPP/sewage/15ºC=0.093 h-1 corresponding to a half life of 7.42h at 15ºC. An additional study by Halliwell et al (2001) performed on a small, predominantly domestic waste water treatment facility measured the half life of detergent phosphates (triphosphate) in waste waters as 7.3 hours at 15°C and 3.0 h at 20°C....

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