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INDUSTRIAL AND ENGINEERING CHEMISTRY
lution is diluted to 300 cc., heated to 70-75" C., and titrated with 0.05 N Ce(SO&. Determination of Vanadium in Chrome-VanadiumTungsten Steels
After the steel was dissolved in hydrochloric acid, the tungsten oxidized by nitro-hydrochloric acid, and the tungstic acid filtered off, sulfuric acid was added and the solution evaporated to fumes. This material was diluted somewhat, and the tungstic acid added, after it had been converted into sodium tungstate solution either by dissolving in dilute sodium hydroxide or by fusing with sodium carbonate in an open crucible and extracting with hot water. A clear solution was obtained. The use of persulfate or permanganate was unnecessary in this case after the nitro-hydrochloric treatment of the steel. Therefore, ferrous sulfate was added, either in excess, or to an end point, depending upon the method used. Titration was then made with ceric sulfate, two end points being obtained if excess of ferrous sulfate was used, otherwise one end point. The conditions for sharp breaks were the same as for chrome-vanadium steels and the curves entirely similar to those already given. Results for a chrome-vanadium-tungsten steel are shown in Table IV.
Vol. 20, No. 9
the HZWOI, add 5 cc. HCl, 3 cc. HN03, and boil down again. Dilute to 60-70 cc. with hot water, boil a few moments to dissolve any salts, and let settle on the hot plate until the supernatant liquid is clear. Filter the HzWOa, using hot 2 per cent HC1 to transfer the precipitate and to wash it. Add 5 cc. HzSO4, sp. gr. 1.83, to the filtrate, and evaporate on a low-temperature hot plate to fumes. Dilute to 70-80 cc. and heat until all salts have dissolved. Place a 150-cc. beaker under the funnel containing the H2W04, puncture the paper, and wash through most of the H z W O ~with water. Dissolve the remainder of the materia! on the filter with hot 4 per cent NaOH. Add 5 to 10 cc. more to the original beaker to dissolve any precipitate which adheres to the glass, using only about 15 cc. in all. Filter this liquid and pour the filtrate into the main filtrate from the HzW04which has been diluted previously to 70-80 cc. A clear solution will result. Add 40 cc. H2S04, sp. gr. 1.5, cool to 5-10' C., and then add 0.1 N FeS04,either in excess or to the (VO8)- 3 (VO) end point, depending on the method being used. From this point the procedure is the same as that given for chrome-vanadium steels. +
+
Summary of Analyses of Steels
Recommended Procedure
I n Table IV, 4- to &gram samples of the chrome-vanadium steels were used, and 1- to 1.3-gram samples of the chromevanadium-tungsten steel.
A 1-gram sample is convenient for a steel containing 1 per cent or more of vanadium, and a 2-gram sample for a smaller per cent. Add 10 cc. of water and 30 cc. HCl, sp. gr. 1.18, to the sample in a 400-cc. beaker. Warm gently until the steel is completely decomposed and the tungsten separates out as a black powder. To the boiling hot solution add 8 cc. HN03, sp. gr. 1.42, at first cautiously, until the violent action is over. Boil, rotate the liquid frequently to expose a fresh surface of the tungsten to the oxidizing agent, and evaporate to 20 cc. If there are any dark particles in
Table IV-Vanadium in Alloy Steels EXCESS FeSOd, FOLLOWEDFeSOd ADDED TO BY TITRATION WITH (VOa)-+ (VO) END Ce(S0dz TO 2 END POINT, FOLLOW~D BY STEEL POINTS TITRATION WITH Ce(SOI)? Per cent Per cent Cr-V, No. 1-(0.230% V, 0.230 0.234 2.35% Cr) 0.227 0.232 0.232 0.211 B. of S.Cr-V, No.30(b) 0.219 0.212 0.216 (0.215% V, 1.03% Cr) 0.213 B. of S. Cr-V-W steel, No. 50 0.764 0.745 0.751 (0.766% V, 3.61% Cr) ++
A Precision Pipet Viscometer' S. W. Ferris THEATLANTICREFININGCOMPANY, PHILADELPHIA, PA.
ANY instruments have been designed for the purpose of determining the viscosity of liquids when the sample at hand is too small to allow the use of a standard instrument such as the Saybolt Universal viscometer. One of the most obvious means of obtaining rough comparative results is to observe the efflux time of the oils from a pipet of small volume. Dean and his co-workers2 have devised and extensively used a pipet which they describe as highly satisfactory for use with small samples, but for which they claim no unusual degree of accuracy. Further reference to their results will be made. The present instrument was also developed primarily as a small-sample accessory to the Saybolt. Its construction is shown in Figure 1. The capacity of the pipet itself is 4 to 5 cc. between the marks. I n constructing the tip, the following method has been found very satisfactory: A glass tube of about 5 mm. inside diameter is drawn in a flame to an inside diameter of about 2 mm. Heating a t the portion having an inside diameter of about 2 mm., the tube is again drawn, reducing the diameter to somewhat less than 0.5
M
1 Presented before the Division of Petroleum Chemistry at the 74th Meeting of the American Chemical Society, Detroit, Mi&., September 5 to 10,1927. 2 Dean, Hill, Smith, and Jacobs, Bur. Mines, Bull. 307, 45 (1922).
mm. The decrease in diameter should be gradual, as shown in the drawing. The tip is then broken off and polished on a fine emery stone in a plane perpendicular to the long axis of the tip. The tip proper, when so made, is not a true capillary, but the taper is very gradual. By making rough trials of efflux time on some oil of known viscosity, the size of the tip may easily be adjusted to give the approximate efflux time desired. As will be more fully discussed later, there is no attempt whatever to make the efflux times of any two pipets identical. Indeed, they may vary markedly without in any way destroying the usefulness of the instrument. To give times approximating one-half Saybolt, the tip when finished will probably have an orifice of 0.5 to 0.6 mm . It has been found inadvisable to fire-polish the finished orifice, because the walls of the tip are so very thin that unless the work is performed with great care the walls will be thickened at the orifice, so reducing the size of the orifice and very probably giving an orifice not circular in shape. It has not been definitely shown that such an orifice will fail to give the same results as ground orifices; however, it has been found that when the walls are thickened a t the orifice by fire-polishing in such a way that the diameter is considerably smaller than that of the walls just above, small
September, 1928
INDUSTRIAL AND ENGINEERI-VG CHEMISTRY
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Table I-Calibration a t looo F. (37.8' C.) of Pipet No. 1 particles can become lodged in the tip and introduce large SAYBOLT SECONDS PIPETSGCONDS errors. With the straight-sided tip, difficulties of this kind A-Standard Oils are practically never encountered. 92.4 56.8 181.0 114.6 The jacket is a glass tube, 14 by 13/r inches, bearing stoppers 286.7 183.6 380.4 241.15 a t top and bottom. When water is used as the heating me502.0 321.9 dium, as a t 100" F. (37.8" C), the bottom stopper is rubber. B-Other Oils 20.0 In this case the heating device is an inverted test tube pro38 30.7 54 vided with an inlet tube for steam, which is controlled by a 40.75 69 quick-acting valve, with an Calibration outlet tube for steam or condensate, or an electrical The first pipet conA pipet viscometer entirely of glass has been deresistance unit controlled structed (No. 1) was caliveloped by which the Saybolt viscosity of an oil may be by a switch or adjusted with brated with five oils which determined, on small samples, with an accuracy a rheostat. Agitation is had been run with extreme higher than ordinary results from the Saybolt instruaccomplished by means of care in a Saybolt barrel ment itself. I t is inexpensive, simple to construct, paddles which may be calibrated by the Bureau easy to operate, and no measurable change in the condriven by an air-jet paddleof Standards. Three oils stants of the instrument has been found over a period wheel. Water is introduced of lower viscosity-not, of nearly two years. through a hole in the upper however, "standards," and Calibration curves, plotting Saybolt seconds versus stopper and may be drawn whose viscosity therefore pipet seconds, have been found in every case to be off by means of a stopcock had not been so accurately straight lines which intersect, when extended, in a a t the bottom. For visdetermined-were also run. point on the Saybolt axis. By reason of this fact it cosities a t 210" F. (98.9" The data are given in is possible to calibrate a pipet with one oil of known C.), it has been found, as Table I. viscosity. it was also found by Dean,* When the pipet seconds It has also been shown that a pipet may be calibrated that steam a t normal preswere plotted against the to an accuracy of about 1 per cent by means of distilled sure may be used for the Saybolt seconds (Figure 2), water alone. To this limit the pipets may be used as heating medium. This is all the points were found definite reproducible standards for the Saybolt instrudue to the fact that the to fall on a straight line. ment. rate of change of viscosity Indeed, no other curve Attempts to increase the accuracy of the water with temperature in this could be drawn through the calibration have so far been unsuccessful. If the slight range is very small. The points for the standard oils, discrepancies with water cannot be eliminated by a pipets are protected against particularly. closer study of details of construction, it is quite probthe admission of small solid able that some pure compound of somewhat higher particles by a screen of wire Accuracy and Permanence viscosity may be used. cloth. This is made byfolding the cloth over the end Table I1 presents four of a glass tube and forcing separate calibrations over a it ak&t half way throughua rubber stopper which has been period of about 21 months of pipet No. 2 which had been in drilled to fit snugly the outlet tube of the viscometer. almost constant use during the entire period. The first two calibrations are with the standards already mentioned; the Standard viscosity thermometers are used. last two are with entirely different oils, whose Saybolt visProcedure of Test cosities, as given in the table, were reported by the Bureau of In performing the test a t 100" F. (37.8" C.) the paddles Standards. are driven a t relatively high speed to insure the even distriTable 11-Calibrations a t 100' F. (37.8O C.) of Pipet No. 2 bution of heat. The temperature of the water bath is mainJANUARY 27, 1925 FEBRUARY 4 , 1926 OCTOBER 18, 1926 tained a t 100" F. (37.8" C.), * 0.1", by periodically opening Saybolt Pipet Saybolt Pipet Saybolt Pipet seconds seconds (av.) seconds seconds seconds seconds a quick-acting steam valve for a few seconds, or by adjusting 92.4 37.55 91.1 36.7 91.1 36.9 the current in the resistance unit. 286.7 120 9 36.9 37.0 502 36.9 2 1 1 . 5 36.8 The pipet is thoroughly cleaned with benzene or light 36.8 37.0 3 6 . 8 36.9 naphtha by twice drawing the material into the pipet b y Av. 3 6 . 8 Av. 3 6 . 9 means of a suction bulb and allowing it to run out. The JULY11, 1925 181 75.6 302.6 125.7 screen should always be in place when either oil or naphtha is drawn in. The pipet may be dried by forcing air through it. The temperature of the bath being regulated, the oil is drawn into the pipet. When the oil reaches a height about 302.6 125.5 378.4 157.4 126.0 157.7 0.5 inch (1.3 cm.) below the cork, the stopcock is closed, the 125.2 157.7 126.2 157.5 oil in the outlet tube (below the tip) drained out,, and the I 26 6 167.8 suction bulb removed. The temperature is maintained a t 125 3 Av. 1 5 7 . 6 125.8 100" F. (38.8" C.) for 10 minutes. The receiver being in Av. 1 2 5 . 7 place, the stopcock is fully opened. The stop watch is started when the meniscus reaches the upper mark and stopped when the meniscus reaches the lower mark. The efflux time is converted to Saybolt seconds by means of the calibration curve (discussed below), or the formula corresponding to the curve. The procedure a t 210" F. (98.9" C.) is identical except 503.5 214.4 214.2 that a sufficient quantity of steam is supplied a t the top that 214.3 214.3 it issues a t the bottom still partially uncondensed. The 214.5 sample is left in the pipet for 15 minutes instead of 10. Av. 2 1 4 . 3
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I N D U S T R I A L A 9 D ENGINEERING CHEMISTRY
It should be streesed that the check values given in Table I1 are not selected; they represent all the values obtained. It will be immediately apparent that the variations are extraordinarily small; considerably less, in fact, than the smallest variations which are normally obtained with the Saybolt instrument. While this may seem, a t first thought, surprising, there are several reasons why this result should be expected. Some of them are: 1-Very slight temperature differences seriously affect the accuracv of viscositv measGrements a t 100" F. (37.8' C.). In the relatively large Saybolt barrel thorough heating depends largely upon the stirring of the oil with the thermometer (a personal variable), and during the actual determination stirring is of course stopped. This leaves the oil surface exposed to room temperature. I n the case of a very viscous oil, considerable cooling may take place while the oil is issuing. If aperforated cap is used on the barrel, there is the danger of slight suction affecting the efflux time, and since air is drawn into the barrel the cooling effect is not entirely e l i m i n a t e d , I n the pipet, temperature control is attained without stirring the oil, and is m a i n t a i n e d throughout the period of efflux. Figure I-Construction of Pipet Viscometer 2-The efflux time of the Saybolt is read according to the volume as recorded by the receiver. This receiver is not controlled as to temperature. The actual volume, therefore, depends upon room temperature to a certain degree. The temperature of the receiving vessel has no bearing on the reading of pipet seconds. 3-The Saybolt seconds depend upon the accuracy of Calibration of the receiver. The actual volume of the pipet is, of course, a constant for any one apparatus, and it has been found that there is no necessity of making the volume of all pipets the same. 4-The end point of a Saybolt run is often difficult t o estimate, owing t o bubbles on the surface, running of the oil down the side of the "vis-glass,'' or inability to distinguish the meniscus in the case of dark oils. The end point of the pipet is perfectly distinct, and because of the much smaller diameter of the pipet stem as compared with the neck of the "vis-glass,'' the meniscus is almost entirely visible. &The method of starting the Saybolt is open to the objections of almost inevitable slight leakage before the cork is pulled and of spattering upon pulling the cork. When the watch is started with the pipet, the oil is flowing smoothly and conditions have been established.
Table I1 also shows the permanence of the pipet. If all the data for the four calibrations on two distinct sets of five oils each be plotted, it will be found that only one line can be drawn and that a straight one. In other words, there was no demonstrable change in the constants of the instrument in 21 months of constant use. C o n s t a n t s of Different P i p e t s
When the pipet viscometer was first developed, only one instrument was built and, since it was designed for approxi-
Vol. 20, s o . 9
mate determinations, little attention was paid to details such as dimensions. As its value was demonstrated by constant use, several more were built, a t widely separated intervals, and usually only one a t a time. I t cannot be too strongly emphasized that there was no attempt whatever to make them identical. For that matter, it would be clearly impossible to duplicate exactly a pipet containing several glass seals and in which the orifice is made by merely drawing down a glass tube in the flame. However, the six instruments built could easily have been made very much more nearly identical in volume and dimensions. Pipets Nos. 2 and 4, for example, varied more than 10 per cent in such measurements as the length of the large tube, the distance between the marks, the distance from lower mark to tip, and the length of the tip from the beginning of the "drawing." Each of the pipets, as they were made, was calibrated with standard oils and in every case a straight line resulted. Five of them are plotted in Figure 2. It is immediately noted that, not only are all the lines straight, but when they are extended they all intercept the Saybolt axis a t the same point. Pipet 4 was calibrated a t both 100' F. (37.8' C.) and a t 210" F. (98.9' C.). That the graphs are different a t the two temperatures is explained by the fact that the coefficient of expansion of the metal Saybolt is different from that of the glass pipet. Severtheless, the curves intercept the Saybolt axis a t the same point. The pipet viscometer described by Dean2 was very different from the present one; it consisted of a bulb of about 1.5 cc. capacity, blown in a capillary tube of about I-mm. bore. The orifice was, then, a length of capillary, having parallel sides, while the present orifice has no parallel sides. The Bureau of Mines pipet was calibrated in the same manner as that just described. The information so obtained was, however, used for the development for each pipet of a calibration curve based upon theoretical considerations. I t is interesting to note that a straight line drawn through the three points which they report fits the points considerably closer than the curve which they believed to be, theoretically, more sound. I
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The results as shown in Figure 2 seem, however, to warrant the following conclusions: 1-The calibration curve of a pipet such as described is a straight line. 2-It intercepts the Saybolt axis a t a point representing +4 Saybolt. This renders it possible to calibrate a pipet viscometer with one oil of known Saybolt viscosity, since the curve would be drawn through the point cor:esponding to this oil and the point +4.Saybolt. The point +4 Saybolt" is, of course, a purely hypothetical one, since there is no liquid which will flow through the Saybolt instrument in less than, say, 27 seconds. For the same reason "0 pipet" is also hypothetical.
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1928
No attempt has been made to justify on a theoretical basis the straight-line graphs obtained with these pipets. Neither has it been determined a-hether any of the dimensions could be so altered that the line would be curved or intercept a t another point. However. about twenty of these pipets have been made, only approximately the same, and all of them have given straight-line graphs intercepting a t "$4 Saybolt." It can be quite safely said, therefore, that any pipet constructed as described with general dimensions (excluding the orifice) within 15 per cent of those given in the drawing will have a calibration curve as described. The size of the orifice need only be kept within such limits that the eflux time is betneen half Saybolt and full Saybolt time, although again these limits have not been fully investigated to determine how wide they may safely be made. It is obviously very easy to keep within the limits just stated. The calibration curve being straight, it is of course of the form y=mx+b
where m is the slope of the line and b the intercept, or Saybolt seconds = m . pipet seconds
+4
Formulas of this type have been computed for the calibration curves in Figure 2, and are given on their respective eurves. I t will be noted that the only variable in the formulas is the value of m. This value should bear a definite relationship to the time required for any given substance to issue from the pipet. An attempt was therefore made to ealibrate a new pipet in the following manner. The efflux time of distilled water was determined in each of three pipets whose calibration curves had been determined. This value could be checked with an accuracy of * 0.1 second. These "water" times were plotted against the corresponding values of the slope m, as shown in Figure 3. The water time was then determined for pipet No. 6 and found to be 11.8 seconds. From the curve the value of the slope was found to be 2.257 and the formula of pipet No. 6 was, therefore, S = 2.257P
+4
Three standard oils were then run in this pipet and their viscosities computed by the above fokmula. The results agreed within about 1 per cent with the viscosities as reported by the Bureau of Standardq. Inasmuch as 1 per cent of water time for pipet KO.6 represented about 0.1 second, beyond which limit it is manifestly impossible to go n i t h a stop watch, the water calibration served as a satisfactory verification of the conclusions already made concerning the calibration curves of pipet viqcometers. Attempts to Increase Accuracy of Water Calibration
I t was then attempted to improve the accuracy of the nater calibration by constructing a pipet such that the n-ater time was markedly increased. To accomplish this the tips and diameter of the large tube were left approximately the same, but the volume was increased to about 12-13 cc. This, of course, resulted in an increased height. The water times were in the vicinity of 40 seconds and the oil times about double Saybolt. The accuracy of these pipets was even higher than that of the smaller ones, check determinations on oils agreeing somewhat closer than 0.1 per [cent. The calibration curves were again straight and the intercept was practically unchanged. The water times could also be accurately determined. However, slight discrepancies in n-ater times compared with oil times for different pipets precluded the possibility of setting up awater calibration curve of a degee of accuracy comparable with the accuracy of the pipet on either oils or water. Materials of very low viscosity, such as benzene, acted similarly to water. When pure compounds
977
of slightly higher viscosity, such as isobutyl alcohol, were used, the discrepancies were considerably diminished. It therefore appears that a calibration curlre for the pipet viscometers may be developed with a high degree of accuracy by the use of some definite single compound of a viscosity somewhat higher than that of water. If this mere done, pipets could of course be established as definite reproducible standards for the Saybolt instruments themselves. The slight discrepancies in the water times are not defi- 2 4 nitely explained, but they are quite prob- 2 2 ably due to turbulence, which of course is diminished with $ 2 0 compounds of higher viscosity and is practically negligible with oils in the Saybolt viscosity range. I t is quite possible that some detail of construction could be so . altered that these errors with water ,1 1 1 i j a4 I I I I I I I I I\ would be eliminated. 1 l I l 1 l l I l A compound of higher
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