Mohr-Knudsen titration

To derive Eq. (3-1), Knudsen and co-workers mainly used samples from the Baltic and from the Mediterranean and the Red Sea. The offset of 0.03 in Eq. (3-1) reflects that the salt composition especially in the Baltic Sea is not exactly constant (Millero and Kremling, 1976) and thus contradicts the basic assumption of constancy that led to Eq. (3-1). However, the proposed titration method also had advantages it could be performed in reasonable time onboard a ship, and for salinity measurements in open ocean areas where S is close to 35 %o, the error induced by the non-constancy is less than that from titration (0.02 %o in salinity). Therefore, the so-called Mohr-Knudsen titration method (Mohr, 1856 see Chapter 11) and the Knudsen formula (3-1) served oceanographers for more than 60 years to determine salinity from chlorinity. 

In this method, the chlorinity is first determined approximately by a modified Mohr-Knudsen titration to an accuracy of about 0.02 %o (Grasshoff and Wenck, 1972). Next, for the precise determination, a suitable amount of concentrated silver nitrate solution (standardized by comparison with batches of Standard Seawater) is weighed in a glass beaker. Then... 

Accurate burette For example, Metrohm Dosimat, 20 mL, used for the Mohr-Knudsen titration. The connections and the piston burette should be either amber glass or painted black to avoid any photochemical reaction of the silver ions (as for apparatus under 6.). 

Grasshoff and Wenck [29] have described a version of the Mohr-Knudsen silver nitrate procedure for the determination of the chlorinity of sea water. In this method, which overcomes the disadvantages of conventional burettes, use is made of a motor-driven piston burette of 20 ml capacity, which is sufficient for a chlorinity range of 0 - 45 per thousand. The accuracy is the same as for conventional titration. The apparatus is compact and portable. 

Based on the relatively constant comjxjsition of the bulk of seasalts, Knudsen and co-workers Forch et al, 1902) suggested salinity, S%o, to be determined from the titration of chlorinity with silver nitrate using Mohr s (1856) method as... 

The precipitable halide halogens in a 15-ml volume of seawater sample are determined by titration with a silver nitrate solution using a chromate end point, the Mohr titration. The silver nitrate solution is standardized against 15 ml of a seawater standard of known chlorinity ( Eau de Mer Normale ), obtainable from the Depot d Eau Normale, Laboratoire Hydrographique, Charlottenlund Slot, Copenhagen, Denmark. Corrections are made to allow for temperature and density, so as to convert titration values to chlorinity, from which salinity values are obtained. Special Knudsen-type automatic pipettes and burettes are used. 

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