ISDUSTRIAL AKD ENGINEERING CHEMISTRY
February, 1926
until free of soluble salts. Add enough of the lead carbonate prepared in this way to the 50-cc. sample so that some of the white lead carbonate in excess remains unused in the flask. Proposed M e t h o d
Solutions 0.1 N IODISE-This is made approximately tenth normal although the exact strength need not be known. STANDARD SODIUM THIOSULFATE SOLUTION-I cc. equals 0.2288 mg,per liter (10 grains of HCN per 100 cu. ft.) of gas on a gas sample. It is made by diluting 84.75 cc. of standard 0.1 N
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No reading need be taken a t this point. It is important to note, however, that a large over-titration a t this point tends to cause a reaction between the sodium bicarbonate present and the iodine, yielding NaIO which liberates iodine on acidification. Wash the blue solution from the Tutwiler into a receptacle, either with distilled water or with the sodium bicarbonate solution, by allowing the wash solution to enter the gas inlet of the Tutwiler and flow over the walls of the Tutwiler, leaving i t a t the bottom cock. If the blue color fades a t this point, add a small amount of dilute iodine until the color reappears. Titrate with standard solution exactly
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Special 500-Cc. T u t w i l e r B u r e t
sodium thiosulfate solution to 1 liter. This solution must be prepared daily. STARCH SOLUTION-A dilute starch solution is saturated with sodium bicarbonate. This solution must be prepared daily. ACID-A solution containing 1 part DILUTEHYDROCHLORIC of concentrated HCl to 8 parts of water.
Apparatus (1) A specially constructed Tutwiler, as shown in the accompanying drawing. Although the usual Tutwiler buret with the gas chamber enlarged t o 500 cc. could be used, this modification with a larger solution reservoir increases the volume of starch solution remaining after the sample is taken. A large volume of starch solution is desirable owing t o the large volume of gas t o be - - scrubbed. _ _ ..~ . (2) A 10- or 25-cc. buret. (3) A receptacle such as a n Erlenmeyer flask or a beaker.
Determination of Amount of Dilute HCl to Be Used to Acidiffy Fill the Tutwiler with the starch-bicarbonate solution to the 540 cc. mark. Remove the solution and titrate with the dilute hydrochloric acid solution to decided acidity, note the amount of hydrochloric acid solution used, and then use this same quantity of acid for each determination a t the point where the colorless solution is acidified. Sampling Two methods have been used for filling the modified Tutd e r with the gas sample. The first, or usual, method requires filling the Tutwiler with a starch-bicarbonate solution first and then displacing this with the gaseous mixture to be analyzed. The second method, which is preferred here, is simpler and necessitates filling the modified Tutwiler with water, displacing all the water with the gas sample and purging for a few minutes with gas. Both methods of sampling are equally good. I n the first method any absorbed hydrocyanic acid is held in the solution, which does not leave the modified Tutwiler apparatus. I n the second method there can be no loss of hydrocyanic acid, as the gas which enters the top of the apparatus has the true amount of hydrocyanic acid in i t and purges the gas which has lost hydrocyanic acid from the apparatus. The gas volume is now adjusted in the usual manner with a leveling bottle so that i t is exactly 500 cc. Procedure Add the iodine solution carefully, with violent shaking, after each addition, until the starch solution just turns blue.
to the disappearance of the blue and take the reading of the buret. Acidify with the amount of hydrochloric acid solution which was previously determined as necessary to give an acid reaction and allow the acid solution to stand for 0.5 to 1 minute. If a blue color appears, there is hydrocyanic acid present, and the solution should be titrated exactly to the disappearance of the blue color with standard solution. The difference in cubic centimeters of the two buret readings multiplied by 10 equals the grains of HCN per 100 cu. ft. when a 500-cc. sample is taken for analysis, or 1 cc. equals 0.2288 mg. of HCN per liter. The method never gives low results, but if the original titration in the Tutwiler is carried too far or if too much acid is added to the colorless solution when it is acidified, the results will be high. Although the HCN HB may be determined on one sample by titrating with standard iodine solution using starch saturated with sodium bicarbonate, the hydrogen sulfide is then determined alone by titrating, using acid starch for another sample and the hydrocyanic acid finally taken by difference, the chances of error are too great and this procedure is not recommended.
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Pipets for Potash Work' By C. M. Bible READPHOSPHATE Co.,NASHVILLE, TENN.
NALYSTS who make frequent determinations of potash A as potassium chloroplatinate find that the use of pipets transferring multiples of 19.38 ml. simplifies the calculation of per cent KzO. There is no need for reference tables, errors in calculation are minimized, and much time is saved. Three sizes give sufficient range for all potash materials encountered, and provide the individual chemist much choice in his selection of the weight of potassium chloroplatinate to be precipitated. The following figures illustrate the convenience in using the pipets: Per cent M1. . Gram To convert wt. potash in materials
up t o 4
4 t o 20 Above 20 1
solution used 58.14 38.76 19.38
Received August 29, 1923.
sample represented 0.5814 0.3876 0.1938
KtPtCls t o % KzO divide by 0.03 0.02 0.01
