Spectrophotometric Determination of Total Nitrogen in Oils -
A Semi m ic ro Kjel d a h1 Method CECIL H. HALE, MARGIE N. HALE, AND WILLIAM H. JONES Esso Laboratories, Baton Rouge, La.
I t is frequently necessary to analyze petroleum feed stocks and products for small amounts (less than 0.1%) of combined nitrogen. The conventional macroKjeldahl method requires large samples, prolonged acid digestion, and large amounts of reagents. A semimicro-Kjeldahl procedure has been devised to use small samples and to allow the determination of very low concentrations of nitrogen. The oil samples are digested with acid in special Kjeldahl flasks which fit onto an all-glass apparatus for steam distillation of the ammonia. The ammonia is determined by spectrophotometric measurements of the color developed with Nessler’s reagent. Nitrogen present as amino, nitro, nitroso, azo, and ring compounds can be determined on an average to *lo% over a concentration range of 0.002 to 1.0vc.
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flask for the introduction of sodium hydroxide is flared COILsiderably on the end. This prevents plugging of the tip by sodium sulfate precipitated during the neutralization of the sulfuric acid. Fifty-milliliter volumetric flasks are used to collect the d i e tillate under the condenser. A Coleman Model 11, Universal spectrophotometer and, for a few analyses, a Beckman Model DU spectrophotometer were used to measure the concentrations of ammonia in the distillates.
T IS frequently necessarytoanalyze petroleum feed stocks and
products for nitrogen content; small amounts of nitrogen are known t o have deleterious effects upon some types of catalysts and also lead to the formation of deposits of solid ammonium w cyanide salts in lines. The conventional Kjeldahl method ( 2 ) is satisfactory for the analysis of oils that contain as much as 0.1% nitrogen. When the nitrogen content is less than O.l%, however, large samples of oil are necessaiy to obtain sufficient ammonia to titrate satisfactorily. With samples of oil larger than 1 gram, increased amounts of sulfuric acid are required and excessive foaming occurs during the early period of digestion. Furthermore] with some types of oils, when samples large enough to give adequate titrations are used, several days are required to complete the digestion. In such cases the blanks are often larger than the actual sample titrations and are very erratic. I t is desirable, therefore, in the analysis of oil to use small samples in order to obtain rapid digestion with minimum amounts of sulfuric acid. This requires the measurement of very small amounts of ammonia for samples low in nitrogen content. Beeghly (1) described a method for the determination of small amounts of combined nitrogen in steels without the use of large samples. His procedure involves dissolution of the steel in hydrochloric acid, steam distillation of the ammonia, and spectrophotometric measurement of color developed by ammonia with Nessler’s reagent. This procedure has been applied, with a few modifications, to the analysis of petroleum. A small sample is digested with sulfuric acid in a special Kjeldahl flask. The ammonia is liberated by steam distillation and reacts with Kessler’s reagent, and the color is measured with a spectrophotometer. APPARATUS
The samples are digested in 100-ml. Kjeldahl flasks (over-all length, 25 cm.) equipped with 28/15 spherical joints to fit on the distillation apparatus. The apparatus for steam distillation of the ammonia is similar to that described by Beeghly ( 1 ) for the determination of nitrogen in steel, with the following modifications: Spherical joints and clamps are used instead of tapered joints, except on the neck of the steam generator. With tapered joints the Kjeldahl flasks frequently froze to the apparatus and had to be broken loose; with spherical joints, the apparatus is more flexible and less likely to be broken by ordinary use. The joint that connects to the condenser is placed midway between the condenser and the Kjeldahl flask to make the apparatus more flexible. The delivery tube extending to the bottom of the Kjeldahl
REAGEYTS
Sulfuric acid, c.P., 96%. Catalyst hXixture. Mix 5 parts of anhydrous potassium sulfate: 2 parts of mercury (11) oxide, 1 part of anhydrous copper (11) sulfate, and 1part of selenium. Salicylic acid. Sodium hydroxide, 50% solution, containing 1% of sodium sulfide. Sessler’s Reagent ( 4 ) . Dissolve 50 grams of potassium iodide in a minimum volume of cold water (ap roximately 35 ml.). Slowly add a saturated solution of mercury TII) ciiloride until the first slight red precipitate persists, and then add 400 ml. of 9 AI sodium hydroxide. Dilute to 1 liter with ammoriia-free water and allow to stand until the supernatant liquid is clear. Standard ammonium chloride solution, 100 mg. of nitrogen per liter. Organic nitrogen compounds used for the preparation of synthetic samples were obtained from the Eastman Kodak Company. PROCEDURE
Preparation of Calibration Curves. Pipet 5 - , lo-, 1 5 , 20-, and 25-ml. aliquots of the standard ammonium chloride solution into 50-ml. volumetric flasks. Dilute almost to the necks of the flasks with ammonia-free water and pipet into each flask 1 ml. of Nesslcr’s reagent. Swirl the flasks during the addition of the Nessler’s reagent to prevent precipitation. Fill to the marks with ammonia-free water, mix, and allow to stand for 5 minutes. Measure the absorbancies a t 450, 475, and 500 mp, with distilled water in the reference cell. Prepare calibration curves for the three wave lengths by plotting the concentrations of nitrogen per 50 ml against the corresponding absorbancies for each wave length. Analysis of Sample. JVeigh a 0.05- t o 1.0-gram sample, from a weight buret if the material is a liquid, into a 100-ml. Kjeldahl flask equipped with a 28/15 spherical joint (to fit on the distillation apparatus). Add approximately 0.1 gram of salicylic acid and 0.1 to 0.2 gram of catalyst mixture. Take care that the sample and other materials added do not stick on the neck of the flask. Add 7 to 8 ml. of concentrated sulfuric acid and allow to stand 5 to 10 minutes. Heat the mixture under a hood until the solution is clear. This usually requires 0.5 to 2 hours. Care must be exercised a t the beginning of the digestion to prevent excessive foaming. For certain types of oils, it is sometimes necessary to add 2 to 3 ml. more of acid during the digestion when the volume of the solution becomes low. After the solution clears. continu(,
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ANALYTICAL CHEMISTRY
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the digestion for 1 hour and allow to cool. Dilute with 5 to 10 ml. of ammonia-free water, again cool, and add several drops of phenolphthalein. While the sample is digesting, the steam-generating flask of the distillation amaratus should be half filled with distilled water and heat applied: The steam-generating flask should contain 1 to 2 ml. of sulfuric acid to prevent the distillation of any ammonia in the water. Connect an empty KjeldahI flask to the unit and allow the steam to pass through it into the condenser to free the apparatus of traces of ammonia. With the steam-generating flask maintained at a temperature just below the boiling point, connect the Kjeldahl flask containing the digested sample to the unit, Fill the drop ing funnel above the Kjrldahl flask with 50% sodium hydroxi&. Cautiously neutralize the acid to phenolphthalein and add about 1 ml. of excess sodium hydroxide. Increase the temperature in the steam-generating flask sufficiently to cause a steady evolution of steam. Place a 50-ml. volumetric flask under the condenser to collect the water and ammonia. Allow the flask to fill exactly to the mark with the condensate; remove it from the condenser, and mix the solution thoroughly. The temperature of the steam-generating flask can be lowered by the addition of more water, and the apparatus is immediately ready for the substitution and distillation of a second sample.
Table I.
‘Table 11. Determination of Nitrogen in Miscellaneous Plant Samples Sample A
Type of Material Gas oil
B
Gas oil
C
Gas oil
0,011 0,012
1)
Gas oil
0.025 0.025
F:
Gas oil
0,064 0.063
I
Gas oil
0.094
G
Gas oil
0.16 0.16
€I
Gas oil
0.30 0.30
I
Catalyst
0.18
.I
Catalyst
0.21 0.56 0.56
I
