size of a match head, and apply it carefully to 

 the stopcock with the index finger. Smooth 

 the grease evenly over the ground-glass surface 

 avoiding the holes by about one-sixteenth of an 

 inch. Insert the stopcock into the barrel. By 

 rotating the stopcock it will be apparent imme- 

 diately where there is an excess or lack of grease. 

 If too much grease is used, clean the entire 

 assembly again with benzine and repeat the 

 lubricating procedure. When properly lubri- 

 cated, the stopcock will operate smoothly and 

 provide a nonleaking seal. 



Do not use a stopcock lubricant that contains 

 silicon because the acid-dichromate cleaning 

 solution used to clean burettes and pipettes is 

 inaffective in removing it. Silicon stopcock 

 lubricant requires the use of a basic or alcoholic 

 cleaning solution which will not clean as thor- 

 oughly as the acid-dichromate solution and in 

 the presence of silver nitrate, as used in the 

 salinity analyses, will result in the formation of 

 a black precipitate inside the apparatus. This 

 deposit is extremely difficult, if not impossible, 

 to remove. 



13-15 Cleaning Burettes and Pipettes. — 

 Before titration apparatus can be set up, the 

 burettes and pipettes to be used must be cleaned 

 meticulously. They must be inspected fre- 

 quently during analyses and recleaned at the 

 first signs of adherence of solutions or samples 

 to the inner sides of the glass. The results of 

 an analysis can be distorted greatly by the 

 presence of a single droplet of solution or par- 

 ticle of grease adhering to the inside of the 

 pipette or burette. For example, a one drop 

 error in delivery of the Knudsen pipette can 

 cause an error of 0.16 parts per thousand of 

 salinity. 



To clean this glassware remove the stopcocks 

 and clean and lubricate them as described in 

 section 13-14. After reassembling, rinse the 

 instruments inside and out with fresh water and 

 fill them with the special acid dichromate clean- 

 ing solution described in section 13-16. This 

 solution is very concentrated and must be 

 handled with extreme care. Before filling, 

 warm the solution to about 60° C Do not let 

 the solution come in contact with the gradua- 

 tions or other markings on the burette as it will 

 remove the color from the lines and figures. 

 Leave the solution in the instruments for at 

 least 2 hours. 



Drain the acid-dichromate solution carefully 

 from the instrument. The solution is returned 

 to its container for reuse. Rinse the instrument 

 for about 5 minutes in tap water and then make 

 a final rinse with distilled water. If there is 

 any sign of water adhering to the inside of the 



H. O. 607 



instrument, then fill it again with the cleaning 

 solution and let stand for at least 2 hours. 



If the instrument is clean, remove the stop- 

 cocks and clean, lubricate, and reassemble them. 

 The pipette or burette is now ready to be set 

 up for titrations. When running the titrations, 

 keep a close watch on the condition of these 

 instruments. At the first evidence of droplets 

 adhering to the inside, they must be cleaned 

 again. 



13-16 Making and Handling the Acid- 

 Dichromate Cleaning Solution. — The special 

 acid-dichromate cleaning solution is an ex- 

 tremely powerful and dangerous liquid and 

 must be handled with great care at all times. 

 It must be made up only when there is ample 

 diluting water available for immediate use and 

 only when there are two or more persons present 

 in the laboratory. Wear rubber gloves, use 

 Pyrex glassware, and remember to always add 

 acid to water slowly by means of a stirring rod, 

 or down the side of the beaker. 



The acid-dichromate cleaning solution is a 

 saturated solution of concentrated sulfuric acid 

 (H2SO4) with potassium dichromate (KjCraO?). 

 This is made up in the following manner: 



1. Dissolve 60 grams of technical grade 

 potassium dichromate in 300 ml. of distilled 

 water at about 20° C. 



2. Place the beaker in a cold water bath and 

 slowly add 480 ml. of technical grade concen- 

 trated sulfuric acid to the saturated solution 

 of potassium dichromate. Stir the solution 

 continuously. 



3. When cool, pour the solution into a liter 

 ground-glass-stoppered bottle. 



A crystalline precipitate will form at the 

 bottom of the bottle. The precipitate indicates 

 that the solution is saturated and may be used 

 over again as long as it remains. When the 

 dark brown color of the solution begins to show 

 a greenish hue, it is an indication that too many 

 impurities are present and the solution should 

 be disposed of and a new solution made up. 



13-17 Purifying Chemicals by Recrystalli- 

 zation. — It is sometimes necessary to use certain 

 solid chemicals of a greater purity than provided 

 by commercial manufacture. In purifying a 

 solid by recrystallization, a roughly weighed 

 amount is dissolved in a sufficient volume of 

 distilled water to give a saturated or nearly 

 saturated solution at the boiling point. The hot 

 solution is filtered to remove insoluble material, 

 which is always present to some degree. Filter 

 paper is placed in a funnel which has had the 

 stem removed to prevent crystals forming 

 within and clogging the bore. The filtered 

 solution is cooled rapidly by stirring to form 



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