Potassium dichromate, fine crystals, C. P., 

 A. C. S. (60 grams). 



Silver nitrate, crystals, C. P. (371.1 grams 

 per jar — 3 jars). 



iSodium chloride, fine crystals, C. P. 

 Sodium thiosulfate, crystals, C. P., A. C. S. 

 (small bottle). (Sec. 13-12.) 



Sulfuric acid, cone. 36N, AS and Nj pure, 

 sp. gr. 1.84, A. C. S. (480 ml.). 



Standard Sea Water, Copenhagen, Eau de 

 Mer Normale (12 vials). 

 13-22 Apparatus Required. — In addition to 

 the apparatus previously listed in sections 

 13-2 through 13-8, the following apparatus 

 is required to carry out salinity titrations by 

 the Knudsen method. 



Bottle, dropping, S. T. ground pipette, 

 Pyrex, 125 ml. capacity. 



Bottle, wide-mouth, 5-gallon capacity. 

 Bottle, Pyrex, narrow-mouth, S. T. ground 

 flat glass stopper, capacity 1 liter. 

 Bulbs, rubber, pipette, small. 

 Burette, Knudsen, automatic tvpe, Pyrex; 

 range: 16-22 double ml., 12-18 double ml., 

 17-23 double ml. 



Pipette, Knudsen, automatic type, Pyrex, 

 capacity, 15 ml. 

 Reader, meniscus. 



Wipes, disposable paper, absorbent, lint 

 free, Kimwipes type. 



13-23 The Knudsen Automatic Pipette and 

 Burette. — The titrations are carried out using 

 the special Knudsen automatic pipette and 

 burette. The pipette is calibrated to deliver 

 almost exactly 15 ml. of sample water. It is 

 used to measure the amount required from the 

 sea water sample bottle. At the top is a three- 

 way stopcock for suction filling, neutral, and 

 delivery. Although the pipette is built to 

 deliver 15 ml., each one differs very slightly 

 from the others; therefore, it is important that 

 the same pipette be used throughout a complete 

 set of titrations. 



The Knudsen burette is a very delicate and 

 somewhat complex instrument for releasing a 

 measured amount of silver nitrate solution. 

 It is fitted with a filling stopcock at the base, 

 a three-way stopcock similar to that of the 

 pipette at the top, and a delivery stopcock 

 below the graduations. It differs from the 

 ordinary burette in that it is calibrated in 

 double milliliters. A drainage value of 20.00 

 on the burette is equal to 40.00 milliliters of 

 solution. The Knudsen tables, which are 

 explained in sections 13-30 and 13-31, are 

 designed to take care of the different calibra- 

 tions for this type of burette. The double 

 milliliter graduations permit the use of a larger 



amount of less concentrated solution which 

 increases the accuracy of titrations. 



13-24 Setting Up the Titration Apparatus.— 



Before the pipette and burette can be set up, 

 they must be meticulously cleaned and the 

 stopcocks lubricated in the manner described 

 in sections 13-14 and 13-15. Figures 13-1 

 and 13-2 show schematic arrangements for the 



RUBBER TUBING 



Figure 13-1. Knudsen pipette assembly. 



Knudsen pipette and burette. The pipette and 

 burette should be clamped to two support 

 bars which are rigidly attached to the titration 

 table. Attach a filter pump, or aspirator, to 

 the salt-water tap and connect it to the pipette 

 with heavy-wall rubber tubing. A piece of 

 rubber tubing is connected to the overflow valve 

 and terminated in an Erlenmeyer flask, con- 

 nected to the supporting rod, to collect the 

 overflow of the silver nitrate solution. 



Attach the magnetic stirring apparatus to 

 the support rod below the burette so that it 

 will be centered under the delivery tap. It is 

 recommended that a doughnut-shaped piece of 

 half-inch-thick foam rubber be cemented to 

 the top of the stirrer to prevent the 100-ml. 

 beaker, used for titration, from sliding off. 



The table should be provided with a sea 

 rack to hold the bottle of standard sea water, 

 the dropping bottle of indicator solution, 2 or 

 3 100-ml. beakers, and 2 sea-water sample 

 bottles. 



H. O. 607 



121 



