181 



In the block diagram, figure 6, the error signal is the difference between 

 the electrode potential and the reference potential. The latter is set equal to 

 the end point potential of the titration. This arrangement is convenient not only 

 from the point of view of the construction of a closed-loop servo-system, but al- 

 so it is the means of reducing the current drain on the electrodes to a minimum 

 at the end-point. 



In the ACT-I, as in the Lingane apparatus, the geometric arrangement 

 of the indicator electrode with respect to the burette tip allows an anticipatory 

 feature to be designed into the instrument. With the indicator electrode close 

 to the tip, the control circuit "sees" a region further along on the titration curve 

 than exists in the bulk of the solution. By proper spacing it is possible to make 

 the instrument approach the end point by the introduction of smaller and smaller 

 volume increments of the titrant. 



Instrunnents for the measurement of chlorinity (or related functions)are 

 summarized below: 



(1) The Knudsen titration is the standard method for chlorinity determi- 

 nation. It can be carried out on shore or on shipboard, but cannot 

 be readily modified to in situ measurements. Although usually per- 

 formed with specially designed Knudsen burettes and pipettes, the 

 titration can be done, with no loss in accuracy or precision, with 

 conventional type laboratory glass apparatus. 



(2) Conventional conductimetric methods can be applied to shore, ship- 

 board, and in situ measurements. Special precision electrical 

 equipment is needed for all applications. Cell design and behavior 

 appears to be the limiting factor in the reliability of these measure- 

 ments, especially when adapted to in situ techniques. 



(3) High frequency oscillometric techniques have considerable appeal be- 

 cause of their "electrodelessness" but require the solution of high 

 frequency oscillator design problems before direct chlorinity meas- 

 urements can be made. 



(4) Automatic potentimietric titrations can be conducted at sea and ashore 

 but are not adaptable to in situ measurements. Electrochemical con- 

 siderations suggest that the sensitivity of these nnethods can be much 

 greater than that obtainable by other methods. Specialized equip- 

 ment is needed for automatic titrations but manual potentiometric 

 titrations can be carried out with commercially available apparatus. 



COLORIMETERS AND SPECTROPHOTOMETERS 



Colorimetric and spectrophotometric procedures are used for the deter- 

 mination of several of the minor constituents of sea water. To name only a few, 

 methods have been described for the determination of inorganic phosphate, ni- 

 trate, nitrite, silicate, as well as for plant pigments and for several metallic 

 ions. The last named usually require concentration of the test substances from 

 a large volume of sample before analysis, a process which will be discussed lat- 

 er. The instrumentation of colorimetric analyses can be very simple, especial- 

 ly when the test color has an absorption peak in the visible part of the spectrum 

 and when only a few samples are to be analyzed at one time. The use of Hehner 

 tubes, in which the column heights of solutions of a test sample and a standard 

 are adjusted until the depth of color in both appears identical by visual inspec- 

 tion, is an example of extreme simplicity. Until recently all colorimetric anal- 

 yses performed at sea were read with visual comparators or photometers. Wat- 

 tenberg (1937) describes a simple comparator, constructed on the Hehner tube 

 principle, which is typical of many early seagoing instruments. Mellon (1950) 

 gives a detailed discussion of commercially available instruments but makes no 



