handling strong chemicals xoear I'ubher gloves 

 arid safety goggles. Wlien mixing acid solutions, 

 the safest method is : place container of water in 

 a cold water bath, stir water constantly with a 

 stirring rod, and pour acid slowly into the water. 

 Never pour water into acid. Keep acid away 

 from, coTTibuMiile material. 



The most serious accidents that occur in a lab- 

 oratory usually result from contact with strong 

 chemicals. It is important that safety and first 

 aid equipment be readily available. In addition 

 to a first aid kit, an overhead quick-pull, safety 

 shower should be provided. If a corrosive chem- 

 ical is spilled on any part of the body, flush the 

 contaminated area immediately with large 

 quantities of water. 



H-9 Cleaning General Laboratory Equip- 

 ment. — Keeping laboratory glassware and other 

 equipment clean is of extreme importance. Con- 

 tamination of samples will result in invalid 

 analyses if dirty glassware is used. 



Always clean and dry the workbench tops and 

 tables after completing analysis of samples or 

 after making up chemical solutions. 



H-10 Cleaning Burettes and Pipettes. — Be- 

 fore titration apparatus can be set up, the bu- 

 rettes and pipettes to be used must be cleaned 

 meticulously. They must be inspected frequently 

 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 particle of grease adher- 

 ing 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, rinse the instruments 

 inside and out with fresh water and fill them 

 with the special acid-dichromate cleaning solu- 

 tion. This solution is very concentrated. Wear 

 safety goggles when handling. Do not let the 

 solution come in contact with the graduations 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 12 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 

 instrument, fill it again with the cleaning solu- 

 tion and let stand for at least 2 hours. 



If the instrument is clean, remove the stop- 

 cocks and allow them to dry ; lubricate, and re- 

 assemble them. The pipette or burette is now 

 ready to be set up for titrations. When running 

 the titrations, keep a close watch on the condi- 

 tion of these instruments. At the first evidence 

 of droplets adhering to the inside, they must be 

 cleaned again. 



H-11 Preparing and Handling the Acid- 

 Dichrontate Cleaning Solution. — The acid- 

 dichromate solution is prepared from concen- 

 trated sulfuric acid and a commercially avail- 

 able solution called "Chromerge." Since this 

 acid-dichromate solution is very corrosive, 

 safety glasses or goggles and rubber gloves 

 must be worn during its preparation and use. 

 To prepare the solution perform the following 

 steps : 



Step 1. Into a 9 lb bottle of concentrated sul- 

 furic acid, slowly add the small bottle of 

 "Chromerge" concentrate. 



Step 2. Recap the 9 lb bottle tightly and mix 

 well. 



Step 3. Cool the slightly warmed mixture to 

 room temperature before using. 



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. The solution should be 

 disposed of and a new solution made up. 



H-3 



