chemistry journal of moldova

V. Gladchi et al./Chem.J.Mold. 2008, 3 (1), 70-76The ammonia ions in the section line of Dniester were present in concentrations 0.015 – 0,136 ml NH 4+/l.Starting from Cosauti towards the Dubasari hydroelectrical plant their content was steadily increasing. The maximalcontent was observed below the dam at Dubasari. The seasonal dynamics of ammonium nitrogen was manifested by the+increase of its concentration in autumn. In spring the NH 4ions were not identified in any site but Bosernita.-In average for the year, the content of NO 3in the section lines constituted 5.5 – 7.4 mg NO 3-/l, and the lowest-values were detected in autumn; the highest concentrations of NO 3were observed in spring in all the sites butNaslavcea.The nitrites, during the investigation period, were a permanent component of Dniester waters. Their absencewas registered only once in May 2006 above and below the Dubasari barrage. The average content of nitrites throughoutthe year constituted between 0.027 and 0.058 mg NO 2-/l. In summer the content of nitrites in Dniester waters wasincreasing, while in spring it was decreasing. The permanent presence of nitrites in the background of low content or-complete absence of ammonium nitrogen can occur due to unfavorable oxidation state and slow oxidation of NO 2toNO 3-.3-The phosphates were present in the river in quantities 0.11 – 3.22 mg PO 4/l. Their average concentrations in3-the sections lines varied in the range 0.48-0.73 mg 4/l. The maximal pollution with phosphates was observed in3-the summer of 2006 at Cosauti and Bosernita sites, and constituted respectively 3.22 and 1.37 mg 4/l. The seasonaldynamics have shown an increase of the phosphates content in the segment Naslavcea – Mereseuca in spring, anotherincrease being detected in summer between Cosauti and Dubasari.The average content of organic compounds according to the BOD 5in the section lines of the investigated sectorof Dniester constituted 3.1 – 3.9 mg O 2/l. In summer this indicator was rising above the yearly average at Mereseucaand Cosauti sites and constituted respectively 4.6 and 3.7 mg O 2/l, in spring – at the Dubasari water reservoir (Bosernita– Dubasari, lower reach) and constituted respectively 6.1 – 5.0 mg O 2/l. At Naslavcea the maximal amount of organiccompounds was 4.45 mg O 2/l that was detected in spring.The hydrogen peroxide and the redox state of watersThe analyses of the waters of the sector under investigation have never detected an oxidant state throughout theperiod of research (table 2). The most favorable state, according to this parameter, was registered in September 2005when hydrogen peroxide was detected in the samples collected from all the sites (the range of concentrations: 1.44·10 -6 M– 9.52·10 -6 M) except Naslavchea. At that site the waters were continuously in reducing state, the hydrogen peroxidewas missing and only the organic substances of peroxidazic type that are easily titrated by hydrogen peroxide werepresent. The reducing state of water below the Naslavchea dam could be caused by a shift from the equilibrium statein the waters of the accumulation reservoir above the dam resulting from the inflow of substantial amounts of reducingperoxidazic substances. The negative impact of the resulted disequilibria has been attenuated along the investigationsegment of the river, that was confirmed by the presence of hydrogen peroxide in the waters of the Mereseuca site,where its concentration was the lowest (1.19 . 10 -6 M) in the hole investigated part of the river. The detection of hydrogenperoxide can be related to the occurrence in the segment Naslavcea – Mereseuca (18 km) of self-purification processesthat lead to the total oxidation of peroxidazic substances and to the low excess of hydrogen peroxide.The maximal content of H 2O 2in September 2005 was registered below and above the Dubasari dam (8.48· 10 -6 Mand 9.52·10 -6 M), that can be explained by the existence of large quantities of microflora that eliminates metabolic H 2O 2in the surrounding environment.Table 2The content of hydrogen peroxide and reducing substances (H 2O 2·10 -6 M/Red·10 -6 M) in the waters of Dniester River)Month, year Naslavcea Mereseuca Cosauti Bosernita Dubasari, above Dubasari, belowthe dam the dam09.2005 -1,191,771,449,528,481,10-----10.2005 1,241,051,90-1,52-------11.2005 0,320,350,70-0,330,42------03.2006 ----------0,900,9205.2006 0,30-0,50-0,500,40-0,30----06.2006 4,805,441,26------0,500,460,6408.2006 0,500,490,480,490,490,48------72