analyses are only limits of tolerance they are of little use in comparing 

 different brands of chemicals which may contain varying amounts of other 

 impur it ies . 



The purest grades include Me r c k ' s " Reagent Grade", Mai inckrodt ' s 

 " Analytical Reagent ", Eimer and Amend' s "Tested Purity", Baker's 

 " Analyzed" , etc. 



Reagent grade and special chemicals are to be issued only when 

 this grade of purity is requested on the order. 



The confusion of different grades of materials is to be guarded 

 against. For example, do not issue immersion oil for clearing oil just 

 because both are different kinds of cedar oil. Certain of these confusing 

 substances are specially marked. In the case of expensive materials issue 

 no more than the amount marked on the bottle unless permission of the 

 person in charge is given for a greater amount. 



SOLUBILITIES: 



If a special solution calls for a large amount of a material consult 

 the table of solubilities, Chapter 5, or for stains. Chapter 4, to see 

 if solution is possible. The Merck Index, Chemical Rubber Handbook of 

 Chemistry and Physics, Lange's Handbook of Chemistry, International 

 Critical Tables , and the Dictionary of Chemical Solubilities are excel- 

 lent reference books concerning solubilities of compounds. If in doubt, 

 consult with the person in charge before attempting the preparation. 

 This may avoid an error and the wastage of much material. Chloretone, 

 in particular, cannot be made stronger than 0.6% in water at room 

 temperature . 



ACCURACY AND DECIMALS: 



Note that accuracy is relative. An error of 0.1 gram in 500 grams 

 is only a 0.02% error, while the same error in 1 gram is a 10% error. 

 Use the proper scales or balance for the particular job to be done. If 

 there is any question as to the accuracy required in an order, consult 

 with the investigator placing the order or with the person in charge. 

 When a decimal is to be placed on the label of any preparation conform 

 to the following rule: Write the decimal to the number of places known 

 to be correct, and only one doubtful figure. This same rule can be 

 applied to figures where the known value does not extend to decimals 

 by writing one doubtful figure and the rest zeros. 



ACCURACY AND ERRORS: 



Absolute errors x, -x are deviations from the correct values and 

 their sign is important for correct statement. They are expressed as 



2- 



