Analytical 

 Methods 



I 



Co-Chairmen 

 M. J. Schneider 

 B. G. D'Aoust 



Methods of dissolved gas quantification were dis- 

 cussed and compared. Presently available methods 

 report data for a single discrete sample and each 

 has unique features making the selection depen- 

 dent on intended use of the data, operator skill, 

 and equipment cost. In addition, the two methods 

 being developed for continuous and semi-continu- 

 ous unattended monitoring were discussed as was 

 the need for such capabilities. Each of the methods 

 will be treated separately below. 



The Weiss saturometer or tensionometer mea- 

 sures total dissolved gas tension directly. Dissolved 

 gases are permitted to diffuse across a semi- 

 permeable membrane into a closed, gas-filled 

 space connected to a pressure sensor. The design 

 most commonly used consists of a coil of Silastic® 

 tubing inside a protective cage which is submerged 

 in the water being analyzed. The tubing is con- 

 nected to a pressure gauge providing total dis- 

 solved gas pressure including water vapor relative 

 to the surface atmospheric pressure. A positive 

 pressure, therefore, represents a supersaturated 

 condition. 



Three different formulae have been used to 

 compute the percentage of equilibrium saturation. 

 If the degree of total dissolved gas saturation (in- 

 cluding water vapor pressure) relative to moist air 

 is desired, use the formula: 



Saturation = 



barometric saturometer 



+ 



pressure pressure 



barometric 

 pressure 



x 100 (1) 



For total gases excluding water vapor relative to 

 moist air, use 



water 

 barometric saturometer _ vapor 

 pressure pressure pressure 



Saturation 



barometric 

 pressure 



x 100. 

 776 Round Table Discussion 



(2) 



And for the degree of total gas saturation excluding 

 water vapor relative to dry air, use: 



water 



barometric saturometer vapor 

 + 

 pressure pressure pressure 



Saturation 



barometric water 



pressure - vapor 

 pressure 



x 100 



(3) 



Equation (2) correlates with values normally re- 

 ported in the literature for other methods; however, 

 the appropriate formula to use depends on the 

 intended use of the data. In any case, the baro- 

 metric pressure and water temperature should be 

 reported with data obtained with the saturometer 

 to permit conversion between values. 



Advantages of the saturometer include low 

 cost and capability of use in the field. It is readily 

 portable and provides data on site. It does not, 

 however, provide data for individual gases and 

 requires running a simultaneous Winkler titration 

 to determine oxygen concentration so that sepa- 

 rate gas data may be computed. Several partici- 

 pants were critical of the instrument due to prob- 

 lems arising from the relatively long equilibration 

 time (approximately 20 min) and the need to vigo- 

 rously agitate the instrument during this equilibra- 

 tion time. It was agreed that the operator must be 

 familiar with potential problems including leaks 

 and the equilibration time and must conscientiously 

 use the saturometer. 



Dr. Brian DAoust suggested that the concept of 

 the saturometer has not been thoroughly explored 

 through radical design changes including reducing 

 the volume of the dead space in the tubing and 

 pressure sensor. As little as 2 in. of tubing may be 



Schneider: Battelle-Northwest, Ecosystems Department, Rich- 

 land, Washington; and D'Aoust: Virginia Mason Research 

 Center, Seattle, Washington. 



