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to the evolution flask, followed by 50 ml. of concentrated hydrochloric acid. Warm gently until solution is complete, and then introduce carbon dioxide into the evolution flask, either as carbonate solution or as gas, to flush out the apparatus. Acidify the contents of the absorption flask with hydrochloric acid and titrate with iodine solution in the usual manner. None of the usual impurities interferes. ACCURACY OF RESULTS
Vol. 4. No. 3
hydroxide, and then acidified with 0.1 N sulfuric acid. The zinc was then removed as zinc sulfide, dissolved in hydrochloric acid, and precipitated with sodium acid phosphate. This precipitate was ignited and weighed as Zn2P20,( I O ) . The following table gives a comparison of the results obtained: SAMPLE METHOD A R ~ E N I CSULFUR COPPERIRONLEAD % % % % % 1 Aa O,OOO;+ None 0.010 0.012 0.001 Bb .... None 0.013 0.009 None 2 A 0.001 None 0.004 0.004 0.006 B .... None 0,011 0.007 0.002 3 A 0.0001 0.014 0.004 0.008 0.055 B .... 0.006 0.003 0.007 0.059 4 A 0.0001 0.010 0.032 0.020 0.028 B .... 0.004 0.036 0.016 0.030 Proposed method. b Other method as outlined. C Arsenic determined by Gutseit method only.
ZINC % 0.004 None
0.028
0.025 The results on four samples by the above procedure were 0.016 0.019 compared with those obtained by the following methods: 0.008 Copper was deposited from a 50-gram sample in nitric acid 0.010 solution by electrolysis. The copper was redissolved and redeposited (7). Iron, A, S. T. M. method B-38-21 using 25-gram sample (4). Lead, A. S .T. M. method B-38-21 using 50-gram sample (3). It will be noted that the proposed procedure requires no Arsenic. The Gutzeit method for arsenic in small amounts filtrations and but one separation in the entire scheme of is considered standard. Hence the accuracy of this use of analysis. This is in marked contrast to the many filtrations the method was not investigated. necessary when other methods are used. Sulfur was determined by the oxidation method of the A. S. T. M. (I), except that the nitric acid used for dissolving LITERATURE CITED the metal was saturated with bromine. Twenty grams of Am. SOC.Testing Materials, Standards, 8-33-24, p. 273 (1924). sample were used. Am. SOC.Testing Materials, I b i d . , B-38-21, p. 564 (1924). Zinc. The method of the Committee on Analytical ReAm. SOC.Testing Materials, Ibid., B-38-21, p. 566 (192Q. SOCIETY (8) was altered agents of the AMERICANCHEMICAL Am. SOC.Testing Materials, Ibid., B-38-21, p. 567 (1924). to accommodate larger samples as follows: A 25-gram sample Behrens and Burgeois, “Analyse Qualitative Miorochimique,” p. 36, Dunod, Paris, 1593. was dissolved in hydrochloric acid with the aid of a platinum Behrens and Burgeois, Ibid., p. 54. catalyst. The solution was evaporated to dryness and the Classen-Hall, “Quantitative Analysis by Electrolysis,” 5th ed., excess acid driven off. The residue was then dissolved in hot Wiley, 1913. water, and sulfuric acid was added a t the rate of 3.5 ml. per Committee on Analvtioal Reagents. . IND.E N Q .CXEM..Anal. Ed.. 3, 2, 221 (1931). 100 ml. of solution. Hydrogen sulfide was then passed into Hammond, W., ChemistAnalyst, 17, 14 (1928). the solution until no further precipitation took place. The Low, A. H., “Technical Methods of Ore Analysis,” 9th ed., p. cadmium sulfide was filtered off, the filtrate was evaporated 266, Wiley, 1922. to dryness, and the excess acid again driven off. This process Lyons, E. J., J. Am. Chsm. Soc., 49, 1916 (1927). Montequi, R., Anales SOC. espafi. fis. quim., 25, 52-76, especially was repeated until no further precipitate was obtained in the 64-9 (1927). solution containing 3.5 ml. of sulfuric acid per 100 ml. of Scott, W. W., ”Standard Methods of Chemical Analysis,” Vol. solution. I, 4th ed., p. 52, Van Nostrand, 1925. The cadmium sulfide was reprecipitated t o remove occluded Scott, W. W., Ibid.,p. 197. Scott, W. W., Ibid., p. 500. zinc. The final solutions were combined and evaporated until nearly all the excess sulfuric acid had been driven off. The residue was dissolved in water, neutralized with sodium RECEIWDNovember 10, 1931. Resubmitted March 7. 1932.
Determination of Relative Humidities by Means of Thermocouples J. H. LANNING,Bakeries Service Corp., Jamaica, N. Y.
F
OR certain types of work it is necessary to have a definite relative humidity in a small space such as a
desiccator. Mixtures of sulfuric acid and water are usually relied upon RS a means of control, but if the substance under test is constantly giving off unknown quantities of moisture, then the actual relative humidity is always somewhat in doubt. For a particular piece of work this difficulty was encountered, so it was necessary t o devise a means for roughly checking the relative humidity. Wet- and dry-bulb thermometers were impractical, since the amount of moisture evaporated from the wet bulb would be sufficient to change the equilibrium between the acid solution and the air. It was found that relative-humidity determinations can be made by means of two small thermocouples, one of which has a small drop of water surrounding it. Because of the evapora-
tion of water from this drop, a difference in temperature is produced which causes the couples to exhibit a definite potential difference, which may be measured by a sensitive instrument such as a Leeds and Northrup type K potentiometer in combination with a sensitive galvanometer. This voltage difference, when calculated to degress Fahrenheit, is not the same as the temperature difference shown by wet- and dry-bulb thermometers, but is always lower. This is probably due t o the difference in transfer of heat of metals and glass, and could possibly be further minimized by the use of smaller couples than those which will be described. However, it was found that the relation between the two was practically a straight-line function, so it was a fairly simple matter to construct the curve illustrated in Figure 1 showing the relationship between millivolt differences .and wet- and
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flows out. Equilibrium is soon reached and water flows from the capillary end in the amount necessary to replenish the drop. In order to obtain the value of the voltage readings in terms of relative humidity, the following procedure was used: The thermocouples were placed in a large desiccator with a side opening and the leads brought out through this opening. A one-hole rubber stopper was inserted to separate them, and a piece of glass tubing was run through the hole t o serve as a bearing for the shaft of a small fan which served to keep the air circulating around the wet couple. The fan shaft was given a thick coating of vaseline which served as ti lubricant and also to make the apparatus more nearly air-tight. A cork was used as a pulley and a small electric motor furnished the driving power. The essential apparatus is illustrated in Figure 2. The humidity within the desiccator was controlled by means of solutions of sulfuric acid and water. Approximate amounts of acid were added to the water and the specific gravity taken. 3. lo 13 From this the exact percentage of acid in the mixture and the humidity produced was ascertained by reference to tables ( 2 ) . 7emperatwe difference in degrees Fahrenheit Both the solution and the air under observation were kept at as DIFFERENCES VS. WET- AND DRYFIGURE1. MILLIVOLT nearly the same temperature as possible, BULBTEMPERATURE DIFFERENCES The feed tube was dry-bulb temperature diff erencea a t definite relative humidi- filled with water and mounted so that it ties. would feed the d'rop The thermocouples are made by fusing a copper wire about 60 cm. in length to each end of a 15-cm. piece of constantan w h i c h w a s s u s wire. These wires should be 0.02546 cm. in diameter (No. 30 pended in the coiled B. & S. gage). This fusion is readily accomplished by hold- couple. The lid was ing the two wires in the flame of a Bunsen burner until the t h e n replaced and ends begin to melt, when they are brought together and fused t h e f a n s t a r t e d . into a small bead. The copper wires may then be covered with The apparatus was allowed to remain an insulating material and used as electrical leads. At one of the thermocouples four small loops are made by undisturbed for a winding the wire four times around a nail with two turns on period of a t least 30 each side of the point of fusion. When the nail is withdrawn, minutes, when the FIGURE 2. APPARATUS FOR VOLTAGE a small coil will remain which should be 2 or 3 mm. in diameter voltage differences READINGS developed b y t h e and which serves to hold a drop of water which will remain A . Wet thermooouple couples were deterB . Dry thermocouple suspended owing to its surface tension. C. Pulley mined with the type D. Bearing E. Rubber stopper K potentiometer. OF RELATIVE HUMIDITIES TO EQUIYATABLEI. CONVERSION F. Fanahaft G . Water-supply tube LENTS OF WET-AND DRY-BULB TEMPERATURE DIFFERENCES After another 5 or 10 H. Support for water-supply tube m i n u t e s , another RELATIVE HUMIDITY RELATIV HUMIDITY ~ I. Eleotrical leads t o potentiometer OBTAINED WITH EQUIV. IN WETAND ROOM J. Fan r e a d i n g was taken DIFFERENCE ACIDSOLNS. DRY-BCLB DIFF. TEMP. to make certain that F. Millivolts 96 1.0 75 equilibrium had been reached. This procedure was carried 0.023 94 67 1.2 0.024 out for a number of different values of relative humidity and 2.0 75 91 0.041 86 2.8 65 0.052 the results are given in Table I. The temperatures a t which 3.8 73 83 0.078 the determinations were made were observed with a mercury 5.3 76 77 0.101 thermometer. 5.2 70 76 0.089 6.0 74 74 0.117 The relative humidities indicated by the acid mixtures were 6.8 70 72 0.127 6.5 67 converted to their equivalents of wet- and dry-bulb tempera69 0.119 ture differences from Table VI of Marvin's Psychrometric 0.131 65 7.8 70 0.150 64 8.3 72 Tables ( I ) , and these results are also shown in Table I, 0.155 61 9.0 72 0.149 61 8.5 69 The wet- and dry-bulb temperature differences were plotted 0.136 61 7.8 64 against the differences in millivolts obtained between the 60 8.8 69 0.161 thermocouples, and are graphically shown in Figure 1. A 59 9.8 74 0.176 10.3 58 76 0.176 working curve is thus obtained from which millivolt differ0.172 10.0 57 72 ences can be converted to wet- and dry-bulb temperature 56 10.5 74 0.186 differences. These temperature differences can then be 0.209 51 11.5 73 0,220 48 12.5 73 directly converted into relative humidity figures from psychrometric tables. The drop may be supplied automatically by suspending a small glass tube of water above the coil in such a manner that LITERATURE CITED the water will feed into it as reauired. This tube must be (1) Marvint C. F., U. S. Dept. Agr., W. B. 235 (1915). drawn to a capillary a t one end and partially sealed a t the other. It is suspended so that the capillary end is immersed (2) R' E.s J * IND. ENG. l3, 326 (lg2')* in the drop and is set a t such an angle-that some of the water R E C B I V ~December D 28, 1931. h
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