SAMPLING AND TESTING HIGHWAY MATERIALS. 21 



num, reduced in volume if need be, 1 c. c. of ammonium hydroxide shall be added, 

 the solution brought to boiling, 25 c. c. of a saturated solution of boiling ammonium 

 oxalate added, and the boiling continued until the precipitated calcium oxalate has 

 assumed a well-defined granular form. The precipitate after one hour shall be filtered 

 and washed, then with the filter shall be placed wet in a platinum crucible, and the 

 paper burned off over a small flame of a Bunsen burner; after ignition it shall be 

 acidified with hydrochloric acid, concentrated on the steam bath to about 150 c. c, 

 and made slightly alkaline with ammonium hydroxide, boiled and filtered (to remove 

 a little aluminum and iron and perhaps calcium). When cool, 10 c. c. of saturated 

 solution of sodiiun-amrnonium -hydrogen phosphate shall be added with constant 

 stirring. When the crystallin ammoniiim-magnesium orthophosphate has formed, 

 ammonia shall be added in moderate excess. The solution shall be set aside for 

 several hours in a cool place, filtered and washed with water containing 2.5 per cent of 

 NH 3 . The precipitate shall be dissolved in a small quantity of hot hydrochloric acid, 

 the solution diluted to about 100 c. c, 1 c. c. of a saturated solution of sodium-ammo- 

 nium-hydrogen phospate added, and ammonia drop by drop, with constant stirring, 

 until the precipitate is again formed as described and the ammonia is in moderate 

 excess. The precipitate shall then be allowed to stand about two hours, filtered and 

 washed as before. The paper and contents shall be placed in a weighed platinum 

 crucible, the paper slowly charred, and the resulting carbon carefully burned off. 

 The precipitate shall then be ignited to constant weight over a Meker burner, or a blast 

 not strong enough to soften or melt the pyrophosphate. The weight of magnesium 

 pyrophosphate obtained multiplied by 72.5 gives the percentage of magnesia. The 

 precipitate so obtained always contains some calcium and usually small quantities of 

 iron, aluminum, and manganese as phosphates. 



(27) A permissible variation of 0.4 will be allowed, and all results in excess of the 

 specified limit but within this permissible variation shall be reported as 5 per cent. 



DETERMINATION OF SPECIFIC GRAVITY. 



(28) The determination of specific gravity shall be made with a standardized Le 

 Chatelier apparatus which conforms to the requirements illustrated in figure 7. This 

 apparatus is standardized by the United States Bureau of Standards. Kerosene free 

 from water, or benzine not lighter than 62° Baume, shall be used in making this 

 determination. 



(29) The flask shall be filled with either of these liquids to a point on the stem 

 between zero and 1 c. c, and 64 grams of cement, of the same temperature as the 

 liquid, shall be slowly introduced, taking care that the cement does not adhere to 

 the inside of the flask above the liquid and to free the cement from air by rolling 

 the flask in an inclined position. After all the cement is introduced, the level of 

 the liquid will rise to some division of the graduated neck, the difference between 

 readings is the volume displaced by 64 grams of the cement. 



The specific gravity shall then be obtained from the formula 



c, -r, Weight of cement (g.) 



Specific gravity — 



Displaced volume (c. c.) 



(30) The flask during the operation shall be kept immersed in water in order to 

 avoid variations in the temperature of the liquid in the flask, which shall not exceed 

 0.5° C. The results of repeated tests should agree within 0.01. 



(31) The determination of specific gravity shall be made on the cement as received. 

 If it falls below 3.10, a second determination shall be made after igniting the sample 

 as described in section 20. 



