February, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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The Value of Cryoscopy in the Technical Investigation of Varnishes‘ By Harry E. Hofmann VALENTINS & Co., N s w YORK,N.
CHEIBER and Nouvelz have reported the apparent molecular weights of varnish bases, resins, and stand oils determined cryoscopically in naphthalene, and have drawn certain conclusions with regard to the action of different varnish resins when heated with linseed oil, as in the manufacture of varnishes. They also draw attention to the important consideration that the values obtained are not really true molecplar weights, but that they furnish an insight into certain changes that take place on heating. The determinations made by Scheiber and Nouvel on mixtures of equal parts of the resins and linseed oil, heated at certain temperatures for certain periods of time, are either (1)greater than the calculated values (for the oil and resin heated separately under the same conditions), or (2) equal to or slightly less than the calculated values; and from this it was concluded that in the first group chemical combination had taken place between the oil and the resin, and that in the second there had been merely solution or mechanical mixing of the resin in the oil. Representative of the first class is the kauri gum-linseed oil base, which showed the greatest difference between observed and calculated values. In the second class are several other common resins, but the resin-linseed oil and the Borneo kauri-linseed oil bases gave molecular weights that were considerably less than the calculated values. No explanation was offered for this phenomenon, but it was suggested that the greater difference between the observed and calculated values of the molecular weights of the kaurilinseed oil mixtures was in accordance with the superior results obtained with kauri-linseed oil asd varnishes-thus indicating th@ there was chemical combination between the oil and the resin in this instance, which did not take place in the cases of some of the other resins, notably common rosin (colophony). It is the purpose of the present paper to present an explanation of the behavior of the rosin-linseed and Borneo kaurilinseed bases, and to record some supplementary data obtained by the author on Chinese wood oil and ester gum. Practical varnish men have long recognized that satisfactory varnishes cannot be-or, rather, were not-made from linseed oil and rosin, and Beams states that “varnishes made from mixtures of rosin and oil do not readily dry off hard, but tend to remain sticky, or ‘tacky’.’’ He further states that in order to prepare a satisfactory varnish using rosin it is necessary to “harden” the rosin previously-which usually means neutralizing a part of the free acid with CaO, ZnO, etc. Inasmuch as the molecular weight of linseed oil increases quite rapidly a t high temperatures, and that of the resin does not change considerably, and since the resultant molecular weights when certain resins, especially common rosin and Borneo kauri, and linseed oil are heated together, are less than the calculated values, it seems quite probable that these resins retard the polymerization of the oil. It is an established fact that rosin, ester gum, etc., retard the polymerization of a rapidly polymerizing oil such as Chinese wood oil, and Rhodes and Potts4 have shown that, in general,
S
1
2 8
4
Received September 5, 1924 2.angew. Chem., S6, 353 (1923). “Chemistry of Paints, Pigments, and Varnishes,” London, 1923, p. 223. Chem. Met. Eng., 99, 533 (1923).
Y.
the retarding influence of a substance heated with Chinese wood oil is proportional to its acidity. Therefore, since rosin and Borneo kauri both contain a very high percentage of free acid, it is reasonable to assume in the light of Scheiber’s and Nouvel’s work and of practical observations, that they slow down the polymerization of linseed oil. Assuming, therefore, that in the majority of rosin-linseed oil varnishes the oil is not sufficiently polymerized or “bodied,” it is easy to conclude that these mixtures must be heated for longer periods of time in order to make them dry satisfactorily. The following experiment made by the author bears out this conclusion: Equal parts, by weight, of water-white rosin and refined linseed oil were heated together at 290’ C. for over 3 hourfi (longer than is the usual practice) and then made into a varnish by the addition of turpentine and a very small quantity of cobalt lineolate. This varnish was very hard and dry in about 15 hours, indicating that the extra time of heating was quite effective. The author has made several molecular weight determinations by means of the cryoscopic method, using benzene, on Chinese wood oil, rosin, ester gum, and mixtures of these, and, although they may not be strictly comparable with those of Scheiber and Nouvel, which were made with naphthalene, they give an indication of the behavior of these materials when heated separately or together. Weights of V a r n i s h M a t e r i a l s
T a b l e I-Molecular
Raw
MATERI A L
891 921 891 883 854
Chinese wood oil
Heated 1.25 hours a t 220’ C. 1414 1398 1423 1416 1464
91 7
Mean. Rosin (W. W.)
Mean. Ester gurnKcomrnercia1) ~~
......... .. . . . .. . .
Mean.....
.....
886 886 855 886 590 571 560 556 562 587
820 823 840 825 814 800 820
1423 625 602 621 609
597 611 855 860 873 867 864
Mixtures of equal parts of Chinese wood oil and each of the resins were then heated for 1.25 and 3 hours at 220’ c., with the following results: T a b l e 11-Molecular
Weight8 of Mixtures of Chinese Wood Oil with Rosins and with Ester Gum
-
HBATSDAT 220’ c. FOR: 7 3 hours -1.25 hoursCalcd. Obsd. Calcd. Obsd. Chinese wood oil and rosin 1017 824 (0) 968 841 936 969 830 984 872 857 964 Mean 845
EQUALP A R T S OF:
Mean . . . . . . . . 1048 1265 a Chinese wood oil alone becomes insoluble in 3 hours a t this temperature.
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It will be noted from these data that, on the basis of the calculated values, the observed values of the molecular weights are 16.9 and 8.4 per cent less than the calculated values, in the cases of Chinese wood oil heated with rosin and ester gum, respectively-thus indicating that rosin is twice as good a
Vol. 17, N o . 2
retarding agent for Chinese wood oil as is ester gum. This is aIso in strict accordance with the common practical knowledge that twice as much ester gum as rosin is required in varnish-making sufficiently to retard the polymerization of Chinese wood oil.
The Analysis of Sodium Sulfide' By W. S. Calcott, F. L. English, and F. E. Downing E. I . D U
PONT DE
NEMOURS & CO.,WILMINOTON, DEL.
T
HE usual method of analysis of sodium sulfide, by de- subsequent storing and sampling of the material involved in an termining the total iodine absorption value of the Sam- extended investigation. Instead, sodium sulfide of the highple with subsequent correction for iodine-consuming est purity available was recrystallized from water as Na2S.impurities after removal of the sulfide as zinc sulfide, has been 9Hz0.immediately made up into stock solutions using freshly found untrustworthy, as the bulky gelatinous p-ecipitate of boiled distilled water, and stored in filled, tightly stoppered zinc sulfide readily absorbs v o l u m e t r i c flasks. If a the impurities present and series of analyses required The portions of sample used for the determination of handicaps the analyst in more than a day for comsodium sulfide and sodium thiosulfate are freed from the selection of a suitably pletion, the samples taken sodium sulfite by treatment with barium chloride. The sized sample. Results by each day were withdrawn sulfide is evolved as hydrogen sulfide, using ammonium this method tend to yield from a fresh flask of stock chloride which does not attack the thiosulfate, and is abhigh values f o r sodium solution, or fresh samples sorbed in ammoniacal cadmium chloride solution, the sulfide and low values for were used and a re-analysis cadmium sulfide being titrated with iodine. The thiosulthe impurities. The followmade for comparison. Solufate is titrated with iodine after removal of the sulfide in a ing system of analysis has tions free from sodium sullarger sample by treatment with ammonium chloride and fite were obtained by using therefore been developed, evolution under reduced pressure. The sodium sulfite is sodium sulfide containing in which each principal condetermined by direct precipitation as barium sulfite in a considerable percentages of stituent of the material is medium of ammonium acetate solution made alkaline directly determined. sodium polysulfide, in which with ammonia, and subsequently titrated with standard sulfite is converted to soiodine solution. Sodium carbonate is precipitated as Determination of Sodium dium thiosulfate by reaction Sulfide barium carbonate under the conditions for the precipitawith the sulfur present. tion of barium sulfite. The acid-consuming value of the The ordinary procedure T h e effects of l a r g e precipitate is determined and the sodium carbonate conof determining sulfide by amounts of the impurifies tent calculated after deducting the equivalent acid-conevolution as hydrogen sul(sodium thiosulfate, sodium suming value of the sodium sulfite as previously deterfide with hydrochloric acid hydroxide, and sodium carmined by iodine titration. is not applicable to matebonate) upon the sodium The percentages of sodium sulfide, sodium thiosulfate, rials containing sulfite or sulfide determination are sodium sulfite, and sodium carbonate found in ordinary thiosulfate. Both these shown by the series of analsodium sulfide should be accurate to *0.3 per cent. impurities are decomposed yses of a sodium polysulfide by the acid treatment, with solution given in Table I. It is apparent from these results that the impurities, even &e evolution of sulfur dioxide, which reacts with hydrogen sulfide liberating sulfur, thus introducing serious errors into when present in large amounts, cause relatively small errors the sulfide analysis. The interference of sulfite may be over- in the sodium sulfide determination. come by its precipitation and removal as barium sulfite, but the interference of thiosulfate cannot readily be overcome so Table I-Effect of Added S o d i u m Thiosulfate S o d i u m Hydroxide. a n d S o d i u m Carbonate upon Determlnatio; of S o d i u m Sulfide long as the hydrogen sulfide is liberated by means of hydroAnalysis of stock solution chloric acid. It has been found, however, that hydrogen NazS No. Per cent sulfide is readily evolved by a mixture of ammonium chloride 1 18.OB and sodium chloride solutions, which form a medium not suffi2 18.20 3 17.99 ciently acid to decompose sodium thiosulfate appreciably.2 Average 18.08 ' With these modifications, the evolution method for sodium Added sodpum thiosulfate = 30 per cent of NazS present sulfide furnishes an accurate direct method for the estimation NazS NazS Difference found present of sulfide in the presence of considerable amounts of sodium Per cent Per cent Per cent No. sulfite and sodium thiosulfate. -0.2s 17.80 1 18.08 -0.18 17.90 2 18.08 EXPERIMENTAL-The preparation of sodium sulfide ab-0.18 17.90 18.08 3 solutely free from impurities such as sodium sulfide and sodium Added sodium hvdroxide = 60 aer cent of NazS present thiosulfate was not attempted, owing to the difficulties in18.05 -0.03 1 18.08 -0.00 17.99 18.08 2 volved in avoiding oxidation during preparation and in the +0.03 18.11 18.08 3 Received August 14, 1924. essential to the system of analysis was made at this laboratory by George Barnhart. 1
* This discovery
1 2
,Added sodium carbonate = 30 per cent of NazS pvesent 17.SO -0.28 18.08 17,99 -0.09 18.08
