T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Aug., 1921
699
should be exactly 50 cc. If necessary, adjust the solution and re-standardize. One cc. of this solution is equal to exactly 0.002 g. of vanadium. The author has little confidence in the so-called “practical test” method of standardization, except as i t is used in conjunction with theoretical values. If results and theory do not agree, something, i t would seem, is decidedly wrong STANDARD PERMANGANATE SOLUTION-T~~ &andard Potas- with either the process or the standard used. We believe sium permanganate solution is 0.0392156 N and is n ~ &UP the standardization with sodium oxalate establishes with as follows: certainty the vanadium value of the permanganate. We are, however, careful to run, parallel with each series of Potassium permanganate.. ......................... 2.5 g. Sodlum or potassium hydroxide. ..................... 5.0 g. tests, a standard sample, as a check upon the work. 2000 CC. Distilled water to m a k e . . .......................... It may be said in conclusion, that a rigid comparison of To standardize this solution, weigh 0.1314 g. of Bureau the new method with that of Blair, as modified by the Amerof Standards sodium oxalate into a 400-cc. beaker, and cover ican Vanadium C O . ,with ~ Cain’s phosphomolybdate method,. with 200 cc. distilled water at 80” C. Add 10 cc. 1 : 1 SUI- and with the method of the Bureau of Standards, bringp furic acid, and titrate slowly with the permanganate solution out an excellent agreement. I n the particular case of t h e until the further addition of a single drop produces a faint high-speed steel mentioned in the introduction the new method permanent pink. Match this color in another beaker con- gives 1.53 to 1.55 per cent vanadium, and for the Governtaining the same amounts of water and acid. Deduct the ment No. 50 Standard (0.756 per cent), 0.75 to 0.77 even, amount of permanganate needed to produce the end-point with ordinary care. The blank, in these instances, was, from that used in the titration of the oxalate. The result found to be quite high, from 1.9 to 2.0 cc.
examination, are to be carried through parallel with each series of tests, and the amount of permanganate required by the blank is to be deducted from the amount required to titrate each sample. Blanks on some materials, especially those containing chromium, will be quite large, but they are accurate, and may be deducted with confidence.
The Gold Number of Commercial
gel at ins'^"'^
By Felix A. Elliott and S. E. Sheppard EASTMAN KODAKCo., ROCHSSTER, N. Y .
As piirt of an inquiry into the physical and chemical properties of gelatins i t was of interest to determine the extent to which gelatins could be clmsified by the variation of their gold numbers. Zsigmondy4 has defined the gold number of colloids as the number of milligrams of colloid necessary just to prevent the precipitation of 10 cc. of standard gold solution by 1 cc. of 10 per cent sodium chloride solution. L
PREPARATION OF GOLDHYDROSOL A well-prepared gold solution should not appear turbid in reflected or transmitted light and should appear deep red in a thickness of 6 to 7 cm. It will withstand heating to boiling without precipitation. A turbidity noticed in direct reflected light is caused by particles larger than the average and indicates an inhomogeneity of the solution. Under Ibe ultramicroscope a well-prepared solution exhibits a lively Brownian movement. The presence of an unresolvable hazy background indicates a considerable number of amicroscopic particles generally tolerated, but large yellow flashes in any great number across the field condemn the solution, as these are caused by the larger, undesirable gold particles. Many methods are to be found in the literature on the preparation of gold solutions, but Zsigmondy’s is very simple and satisfactory for this work. Very pure water is obtained by distilling water twice in a block tin stdl with a block tin Coil condenser. MTater with a conductivity of 1.2 X was easily obtained and was satisfactory when used. The next precaution is to make all glassware to be used in the preparation chemically as well as physically clean. Zsigmondy insists on Jena resistance glass, but four Of the leading American makes were used when properly cleaned, with excellent results. Three solutions are necessary: (1) 6 g. AuCL.HC1.Received April 8, 1921. Published as Communication No. 118 from the Research Laboratory of the Eastman Kodak Company. 8 This work was done in October 1917 and discontinued on account of the wux. 4 Ann., a01 (1898),29. 1
3H20 dissolved and made up to one liter with conductivity water; (2) one liter of 0.18 N K2C03 solution; and (3) a 0 . 3 per cent solution of formaldehyde. According to Zsigmondy, 120 cc. of water prepared as above is heated and 2.5 cc. of gold chloride solution are added, then 3.5 cc. KzCO3 solution. This is stirred to insure uniformity and heated to 100” C. It is removed from the heater and 3 to 5 cc. of the formaldehyde solution are added with lively stirring. A much smaller amount of formaldehyde solution was found sufficient. This had been observed by L. W. Eberlin in this laboratory in 1914, but it was not known whether this variation was due to uncertainty as to the strength of the formaldehyde. The more recent work has shown that the amount necessary is a function of the rate of addition (roughly inversely proportional). If the formaldehyde is added (0.5 per cent solution) a drop a t a time and well stirred after each addition, this procedure followed untiI the solution begins to show a faint red tinge, and the additions now made only after a further color change is no longer produced by the previous drop, a deep red and extremely clear solution will be obtained. This solution was made up in lots of 2 liters to insure maximum uniformity throughout many tests, and about 2 CC. of 0.3 per cent formaldehyde were used. PREPARATION OF GELATIN SOLUTIONS So-called hard, medium, and softgelatins were tested. ’rhe solutions were all made up with conductivity water a t 500 c., each being heated 4 hrs. to complete equilibrium. After slow cooling to 200 these so~utionswere diluted to 0.001 per cent. A series in four steps was formed and the gold hydrosol added, followedbythe sodium chlorge solution. I n Table 1 are shown representative data obtained with seventeen different commercial gelatins of the three classes on the market, soft, medium, and hard. The data for the five samplesquoted aFe representative of the different classes. The two color changes under each concentration are, first, the immediate color and, second, that observed after 24 hrs.. 1
See Scott,
“
Standard Methods of Chemical Analysis,” p . 273.
THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
700 SAMPLE
No. 1 2
4
6
9
B Blue, Slightly
TABLE I MILLIGRAMS O F GELATIN USED 0.005 0.015 0.015
CHARACTER
OF GELATIN
B-P SP-P VSP-P Soft B-P v-P VSP-P Hard B-P P-P SP-P Soft B-P B-P P-P Medium BV-P s P-P VSP-P Medium P Purple, SP Slightly Purple, R Red, BV Bluish Violet, VSP Very Purple, VLP Very Light Purple, V Violet.
The data showing the actual color changes with increasing concentrations for each sample, while interesting, are omitted to save space. The gelatins fell roughly into three classes. The first class had a gold number of 0.005 to 0.010 and contained Samples 1, 3, 9, and 17. Class 2 had a gold number of 0.010 to 0.015 and contained Samples 2, 4, 5, 7, 10, 11, 12, 13, 14, and 15. Class 3, with a number of about 0.015, was made up of Samples 6, 8, and 16. TABLEI1 (1 per cent solution, 4 hrs. a t 50" C.) 0.55 0.40 0.35 0.30 0.15 0.10 Milligrams of gelatin' Immediate color LP VLP PV VPV B B Color after 24 hrs.2 LP LP LP PB LP B 1 T h e reqidual color, when different from that produced immediately after the addition of the sodium chloride solution, indicates t h a t the precipitation equilibrium is reached slowly and t h a t D fraction of the gold is protected before the precipitation is complete. 2 Intervening concentrations of gelatin were tried but these d a t a are omitted t o save space. Only the concentrations are cited a t which distinct color differenceswere observed.
It was: of course, known that there is not great precision in the differentiation between hard, medium, and soft gelatins even from the same maker, so that it was not surprising t o find no agreement a t all between numbers in Classes 1, 2, and 3, and numbers classed as hard, soft, and medium. As this classification is too coarse to be of value, a more exact differentiation was sought. Preliminary work along this line showed that there are three factors to be controlled very carefully: 1-The 2-The 3-The to stand
amount and the degree of heating. concentration a t which the gelatin solution is heated. time of aging, i. e., the time the solut on is allowed after having been made.
Inasmuch as the mere classific'ation is of little scientific interest consideration will be given only to the control and influence of these three factors. For this part of the investigation, Sample 14, a granulated gelatin, was selected, thus insuring maximum uniformity and ease of working. Three widely separated concentrations were selected, and all experiments were made with solutions a t these strengths, viz., 1, 0.01, and 0.001 per cent. This range would bring out the effect of concentration on protective action as denoted by the gold number. Solutions at these three concentrations were made up in three different ways : 1-By making the solutions up directly without subsequent dilution, as 1 g. gelatin to 100 cc. solution for a 1 per cent solution, to be heated for 4 hrs. a t 50' C. to establish equilibrium and cooled in a water bath at 20°, at which lution of 1 per cent, heating at
Nofor 4 hrs., cooling knd diluting to 0.01 and 0.001 per cent at 20'. 3-By making the original solution of 1per cent a t 50", heating for 4 hrs., and making further dilutions of 0.01 and 0.001 per cent at 50'. with a Further 2-hr. heating t o equilibrium and cooling at 20'.
The results shown in 'the condensed Table I11 clearly indicate that the gold number decreases with decreasing concentrations, that is, the protective action of the gelatin increases with decreasing concentrations. This is ment with the work of W. Mens.' The protect 1
Z. p h y s i k . Chcm., 68 (I909), 129.
Vol. 13, No. 8
TABLE111-SUMMARYOF DATA STRENGTH OF SOLUTION Per cent GOLDNUMBER Original 1.0 0.15 0.01 0.020 0.001 cu 0.015 Diluted pi 50' 0.01 0.9075 0.001 0.02 Diluted at ZOO 0.01 0.0075 0.001 0.02
is not increased by a decrease in the quantity of gelatin, but as the concentration is lowered the state of division of the gelatin present is altered. At high concentrations there is a majority of large particles with some smaller particles also; a t low concentrations, a majority of very fine particles and very few of the larger particles. According to Menz's theory the larger particles exert very little, if any, protective action (schutzwirkung). His data and those here reported are in good agreement, tending to verify this theory. He further proved (and our ultramicroscopic observations corroborate him) that the amicrons actually do increase in proportion as the concentration is decreased. The slow increase in size of particles which occurs a t room temperature when gelatin solutions '%et" to a gel and even in the dilute solutions which do not congeal is due to a lag in adjustment of equilibrium which increases with dilution. Taking this into consideration, it is clear that the effect of aging is the agglomeration of amicrons forming larger particles, thus lowering the protective action. This accounts for a gold number of about 0.020 obtained for the 0.001 per cent solutions no matter how they had been prepared. The 0.001 per cent solutions had stood 2 days after having been made and had doubtless had time to agglomerate. The temperature of dilution seems to have very little effect, a t least within the range observed, and it is improbable that from 20" to 100' any difference would be noticed, because complete cooling to a uniform temperature of 20' would tend, a t identical concentrations, to give similar distribution of small and large particles. If the gold numbers were determined a t the dilution temperatures there would undoubtedly be a difference.
SUMMARY 1--Modifications of Zsigmondy's method for the preparation of deep red, highly homogeneous, gold hydrosols are given which make the preparation both easier and more certain. 2-Seventeen different gelatins of all grades and methods of manufacture were ghown to differ but little in their protective action. 3-The classification thus made possible is too rough, and moreover does not bear any simple relation to those properties of chief interest to users of gelatins. 4-It has been shown that the gold number decreases as the concentration decreases. 5-It has been shoti;n that the gold number increases the longer the gelatin solution stands or "ages" after it has been made. 6-The method o king the solutions was investigated, and the effect of coo for different times and temperatures indicated. 7-The state of subdivision of gelatin sol definitely indicated by these gold numbers and by ultramicroscopic observations. These solutions were shown to contain varying proportions of large and small depending on the temperhture! at which they were i rium, there being a predominance of large prticles qt lower temperatures.
