696
V O L U M E 19, NO. 9
furic acid are added. The solution is brought to 80” t o 90” C. for titration with 0.050 N potassium permanganate. For samples of low calcium content, two t o four digests of 500mg. samples can be combined. Since only 500-mg. samples are used the feed, food, or other products analyzed should be finely ground and well mixed, so t h a t uniform samples can be taken.
and t h a t all the calcium in t h e ash was brought inta solution only by long standing in hydrochloric acid. Thus i t appears t h a t t h e digestion methods might be preferable t o the ashing method. The combined vanadate-phosphorus and calcium methods of digested samples have been used for a number of feed and food samples with much saving of time.
DISCUSSION OF RESULTS
LITERATURE CITED
T h e d a t a in Table I show t h e results obtained on several materials and t h e per cent difference of t h e two digestion methods as compared to the official ashing method. The results with the digestion methods obtained with 500-mg. samples are slightly higher t h a n those with the ashing method; hiorris, Nelson, and Palmer ( 4 ) found t h a t in the case of certain materials the calcium values obtained by the regular nshing method Tvere slightly low
(1) Assoc. Official Agr. Chem., Official and Tentative Methods of .&nalysis, 6th ed., p. 127 (1940). and Stamberg, 0. E., IKD. ENG. CHEW.,ANAL. ( 2 ) Bolin, D. W., ED., 16, 345 (1944). (3) Koenig, R. A., and Johnson, C. R., Ibid., 14, 155 (1942). (4) Morris, H . P., Selson, J. IT., and Palmer, L. S.,Ibid., 3, 164 (1931). PUBLISHED with the approval of the director of the Agricultural Experiment Station as Research Paper 254.
C 0 R R E S P 0 N D E N.C E Explosion in Determination of Cobalt as Potassium Cobaltinitrite SIR: I was interested to read in the January issue of ASALYTICAL CHEMISTRY [19, 72 (1947)l of the occurrence of an explosion during the determination of cobalt as potassium cobaltinitrite. Some years ago as a student I was preparing sodium cobaltinitrite by the usual method (Bilts, Hall, and Blanchard, “Laboratory Methods of Inorganic Chemistry,” New Tork, John W l e y 8: Sons). The salt failed to precipitate out, so the solution was concentrated by evaporation. During evaporation the solution turned purple, frothed considerably, and, on cooling, solidified in the form of a solid foam. Another student was carrying out flame tests and, Tyishing to obtain a sample of the substance, touched the surface with a hot platinum wire, whereupon i t exploded with some violence, covering everyone with a sooty powder. At the time this was regarded merely as an interesting phenomenon and it was not until I read of this rather similar occurrence that I recalled i t to mind. R. J. GOWSES Veterinary Research Laboratory Vom. P.O. Bukuru Xorth Nigeria
Pressure Regulators SIR: An article published in your AKALYTICAL EDITION[ b ~ ENG. . CHEM.,ANAL.ED., 18, 156 (1946)l describes a “Variable Pressure Manostat” based on the same principle as Kewman [Ibid., 12, 274 (1940)l. Unfortunately, simple as the apparatus looks, it does not fulfill the purpose of a pressure regulator and calling it a manostat is erroneous. 811 vacuum pumps work irregularly, and a manostat should serve to keep the pressure of the evacuated system. Several such devices have been described in your journal, all of which should give satisfactory results [Ibid., 13, 418, 908 (1941) ; 15, 283 (1943); 18,214 (1946)l. The principle of this device is theoretically wrong, because all pressure changes of the vacuum pump are fully transmitted t o the system. The constant level of the liquid in the apparatus does not equalize pressure changes caused by the irregular working of the pump. 48 Hopetoun Ave.
ERNEST CHALLEN
x-aucluse, N.S.W., Australia
vacuum pump, and pressure changes of the pump are therefore transmitted to the system. I n our experience, however, a good commercial pump, kept in proper working condition, will deliver a constant vacuum, and by using a New-man-type regulator in conjunction with such a pump we have been able consistently t o conduct distillations at constant pressure for several days at a time. HUGH B. DOXAHOE University of Kansas Lawrence, Kans.
ROBERTR. RVSSELL C. A. VANDERWERF
SIR: Challen’s point concerning the use of the word “manostat” is well taken. If the word “regulator” had been used, all would have been well. The success of the devices described depends on the vacuum pump’s being used. I can check VanderWerf’s statement that a constant pressure can be maintained with a good pump protected by a dry ice-cooled trap. Ohio State CniversitjColumbus, Ohio
MELVINS. XEWJIAN
Determination of Olefins, Aromatics, Paraffins, and NapMhenes in Gasoline SIR: I n the article on “Determination of Olefins, Aromatics, Paraffins, and Naphthenes in Gasoline” [ANAL.CHEX., 19, 178 (1947)l the following paragraph appears: Recently Groennings has suggested that the 0.16 factor recommended by Grosse and \Tackher for correcting the specific dispersion for olefinic unsaturation should be replaced by a value read from a curve of correction factor 21s. boiling point. Groennings in deriving his correction assumed that there was a 50-50 distribution betw-een cyclic and noncyclic olefins throughout the boiling range. The last sentence is slightly misleading, since Groennings [IKD. ENG. CHEM., AXAL.ED.,17,361 (1945) I recommends the assumptionof a 5050 distribution between cyclic and noncyclic olefins only in case there is no information concerning the type of olefin present and no basis for any better approximation than the 50-50 approximation. Groennings does not assume such a distribution in all cases. V e regret that in attempting to discuss a complex subject briefly a slightly misleading statement was made concerning Groennings’ valuable paper. S. S.KL-RTZ. JR. I. IT,MILLS
c. c. 5 1 . 4 R T I S
SIR: The Newman-type regulator is obriously designed t o maintain a constant pressure differential between the system as a whole and the
Sun Oil Co. Sorwood and Marcus Hook, Pa.
TT T. HARVEY l f . R.LIPKIN
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