s74
ANALYTICAL CHEMISTRY
order to ensure maximum color development. From data showing that 1 molecule of iron is required for each molybdenum molecule, Dick and Bingley ( 2 ) suggest that iron may be a part of the colored complex, the compound Fe[h400(CNS)~lbeing formed when sufficient iron is present; in the absence of iron, 11102[?vloO(C?rTS)s]3is formed with only 60% as great color intensity as obtained with excess iron present. The reduction in color appears to be directly proportional to the decrease in the proportion of molybdenum in the [MoO(CSS),]-- which is suggested as the chromogenic portion of the molecule. I n the present study this possibility has not been investigated in detail, but it has been observed that the addition of iron does intensif? the color when the amount of molybdenum present is greater than 4 y; presumably sufficient iron is present as a contaminant in the reagents to permit full color development when less than 4 y of molybdenum are present. I n the absence of added iron, linearity between absorbancy and amount of molybdenum does not alwaj-s obtain but in the presence of excess iron linearity is obtained oi7er the entire range 0.1 to 100 y of molybdenum taken. Therefore, standard iron additions are recommended for blanks and standards as well as samples. There is no loss of molybdenum to the portion of the solvent added and discarded prior to the addition of the thiocyanate and stannous chloride. Precision and Linearity. T h e data of Table I illustrate the precision obtained over a range of known amounts of molybdenum from a standard solution. Precision obtained on plant samples at three different levels of molybdenum is shown in Table 11. The samples were ashed in porcelain dishes a t 550OC. for 16 hours. Wet ashing is also suitable, provided the precautions described by Evans et al. ( 3 ) are observed. Results obtained by the proposed method are also compared with those obtained by extraction of the colored complex with two portions of isoamyl alcohol essentially as described bv Sandell (4). The results with the proposed method for the
molybdenum-deficient tomato blades are somewhat lower than those obtained by the double extraction method. I n part, the higher result with the double extraction method may be due to more complete extraction but, in addition, traces of iron thioryanate seem to be extracted with the isoamyl alcohol, as evidenced by a slight off color of the extract. I n the proposed method, a second extraction with carbon tetrachloride-isoamyl alcohol failed to extract additional color. Also, more variable results were obtained by the reference method than by the proposed method. The results for the samples containing large amounts of molybdenum were slightly higher when obtained by the reference method but not significantly so as determined by the t test for the significance of differences between means. The principal advantages of the modified method over the usual method, using extraction of the colored complex with lighter-than-water solvents, are ease of manipulation iyith attendant saving of time, and freedom from large errors caused by variations in salt concentrations in the samples. The vapor pressure of the mixed solvent is fairly high and prerautions t o prevent loss of solvent should be observed a t all times. Provided loss of solvent by evaporation is prevented, the colored roniplex is stable and exhibits constant absorbancy for several hours, although nothing is gained by deferring measurement of aheorbancy for a long time. LITERATURE CITED
( I ) Brownlee, K. A , , “Industrial Experimentation,”
Brooklyn,
N. Y., Chemical Publishing Co.. 1949. ( 2 ) Dick, ;1. T., and Bingley, J. B., Australian J . E.epf/. Biol. .lied. Sci., 25, 193-202 (194i). ( 3 ) Evans, H. J., Purvis, E. R., and Rear, E‘. E., . ~ N A I . .kqiib:ic,, 22, (4)
1568-9 (1950). Sandell, E. B., “Colorimetric Determination of Traces of ~ I c t a l s , ” 2nd ed., S e w York, Interscience Publishers, 1950.
KECEIYED for review July i , 1033.
Accepted November 19, l 9 i 3 .
Dissolving Sodium- Potassium AIIoys LEONARD P. PEPKOWITZ Knolls Atomic Power Laboratory, General Electric Co., Schenectady, HE
use of liquid metals as heat transference agents has in-
Tcreased rapidly because of the impetus of the current reactor
programs. Oneof the important substances for such use is sodiumpotassium alloy. This alloy, which can exist within a wide range of the sodium-potassium ratio, is liquid a t room temperature and is exceedingly reactive toward oxygen, \Tater, etc. These properties present a difficult problem in sampling and in the subsequent dissolving steps before analysis. Because the sample is liquid and is usually contained in a sealed tube or bulb to prevent its catching fire, the actual mechanism of adding the sample to the appropriate solvent without a hazardous and violent reaction is usually difficult unless complicated dryboxes with inert atmospheres are used. With the obvious solvents that are used-such as the alcohols or hydrocarbons-the rate of addition is an important factor in determining whether the system will react violently and release large volumes of hydrogen with the associated foaming and spattering, resulting in the loss of 8ample even if the explosion hazard can be controlled b y eliminating oxygen from the immediate a t mosphere. This perplexing problem can be solved by the following technique (I), which has been used in this laboratory for the past 3 years without ever having a fire or explosion. No special dry boxes with oxygen-free atmospheres are required. The dissolution can take place in the usual laboratory hood.
N. Y.
The S a K sample is frozen hy placing it in a clean borosilicate, glass dish supported on dry ice. The glass container is shattered with a sharp blow. The solid S a K which is kept frozen on t h c dry ice is cut into small pieces and dropped into an dcohol-dry ice slurry. The reaction to form the alcoholate gom very smoothly a t these low temperatures and the rate can be controlled very easily by adding small pieces of dry ice to the slurry. The rate can be increased by allowing the mixture to warm slightly or slowed down by adding dry ice. The faster rate is evident by the increased rate of formation of gas bubbles in the solution. The resulting carbon dioxide blanket in combination with the low temperature prevents any reaction with the oxygen in the air and even though some water is frozen out, the reaction rate is too sloiv to cause any trouble. At high temperatures, t h e NaK will react with carbon dioxide, so that a carbon dioxide extinguishei should never be used to put out alkali metal fires; soda ash is preferred. After the sample has been dissolved, the slurry is alloxed to. come to room temperature and the sodium and potassium ethylate are dissolved by adding acid. The usual practice in this laboratory is to titrate to a phenolphthalein end point and make u p to volume. By thiq simple means a gram of S a K can be disqolvetl in 15 minutes. LITERATURE CITED
(1) Pepkowitz, L. P., Knolls Atomic Power Lab., P u b . 976 (1953).
RECEIVED f o r review August 20, 1953. Accepted October 29, 1953
The Knolls Atomic Power Laboratcry is operated by t h e General Electric Co. for t h e Atomic Energy Commission. T h e work reparted here was carried out under Contract number ‘8-31-109 Eng-52.
