A Flame Spectrophotometric Method for Sodium, Potassium, and Calcium in Fuel Oil SIR: Traces of metals such as sodium, potassium, and calcium in fuel oils and other liquid hydrocarbons are of considerable interest to the petroleum and associated industries. These metals are usually considered as contaminants, but in some cases they are a constituent of an additive which improves the petroleum product ( 5 ) . Usually the concentration of these metals is of the order of a few parts per million. Investigation of the methods for determining trace metals in petroleum products has established the need of a simple yet reliable method for their determination. Several flame spectrophotometric methods for determining these metals have been described in the literature. These methods usually involve ashing of the sample and subsequent redissolving of the ash in water ( 7 ) . These methods require a large amount of time and are subject to error from sample contamination or volatilization ( 5 ) . Direct flame spectrophotometric methods have been applied to the determination of additive elements in lubricating oils (1, 4 ) and various metals in crude oil ( 7 ) . This research was conducted to develop a rapid and accurate method of determining sodium, potassium, and calcium in distillate fuel oils. EXPERIMENTAL
Apparatus. A Beckman DU recording flame spectrophotometer with hydrogen-oxygen flame attachment was used for these determinations. T h e instrument was equipped with a spectral energy recording attachment and an exhaust system for the burner housing. Reagents. A straight-run distillate fuel oil was used in t h e preparation of a calibration curve. T h e properties of this fuel are listed in Table I. Procedure. An aliquot of t h e fuel oil being analyzed ( 2 . 5 milliliters or more) is mixed with a n equal volume of ethanol-toluene (1 :4) solvent. A portion of the resulting solution is analyzed on the flame spectrophotometer for sodium, potassium, and calcium. The conditions used for these measurements are shown in Table 11. Two more solutions are prepared in a similar manner for each metal containing standard additions that are equal to and twice the estimated metal concentration of the original sample (3). The instrument is readjusted for each metal to obtain maximum emission with minimum background interference. Three readings of emission are recorded for each metal for the original and standard-addition samples. The emission readings are corrected for back-
Table I.
Properties of Distillate Fuel Oil Used in the Preparation of Standards Ash nil 37.1" API 630" SU viscosity at 100" F 3 6 . 8 178" F (P-hl) 15' F Sodium < O . 1 nle./l. 0 Potassium
