402 FRASER AND WESTON: GASOMETRIC METHOD FOR THE ESTIMATION OF [VOl. 75 Gasometric Method for the Estimation of Creta Praeparata in National Flour BY J. R. FRASER AND R. E. WESTON SYNOPSIS-A method for the routine estimation of Creta praeparata in National flour has been developed on similar lines to the Chittick gasometric method for baking powder. Apparatus for multiple determinations is described, and a mathematical formula has been evolved from which the amount of Creta praeparata in a sack of flour may be calculated directly from the volume of gas evolved and from factors which can be tabulated. These factors take account of the temperature, barometric pressure and vapour pressure of the liquid under which the estimation has been carried out. THE following method has been adopted at the Goverpment Laboratory as rather more convenient for the routine examination of a large number of samples than other methods1p2 previously published for this purpose.The procedure has been developed on similar lines to the Chittick gasometric method for baking p ~ w d e r . ~ It has proved applicable to multiple simultaneous determinations, is reasonably easy to manipulate and free from hidden sources of error, whilst the operator has the advantage of a progressive visual change to observe. EXPERIMENTAL APPARATUS- An approximately 400-ml. flask with ground glass fitted head is connected with a manometer burette by means of well secured pressure tubing, as shown in the diagram (Fig. 1). The reaction flasks are mounted on a shaking machine. PROCEDURE- Introduce 2Og.of flour into a flask, add 50ml. of saturated brine, insert the stopper and shake the flask gently to mix the contents thoroughly. Wash the rubber stopper and neck of flask with a further 20ml. of brine. Then place the flask on the platform of the shaker and clip it into position. Cautiously lower the special phial containing 20 ml. of hydrochloric acid (1 + 1) so that i t stands upright in the middle of the flask. Fit the head connecting the flask with the measuring limb of the burette, and allow the apparatus to stand for a few minutes until it has reached temperature equilibrium with the surrounding atmosphere. Adjust the brine in the burette by running sufficient out of the tap provided at the U-bend so that the levels are the same in both limbs (the air within the flask is then at atmospheric pressure) and note the level (see Fig.l), and record the temperature and barometric pressure. Start the shaking device; this causes the acid phial to fall over and allows the acid to mix with the other contents of the flask. The carbon dioxide evolved from the reaction of the acid with any chalk present in the flour will cause a displacement of air in the head into the burette limb, the pressure in which will be apparent by the alteration of the levels in the two limbs. Periodic adjustment must be made during the course of the reaction so that it may proceed at atmospheric pressure all the time. Twenty minutes is sufficient to reach a final equilibrium. Stop the shaker, allow the contents of the apparatus to settle for 2 or 3 minutes, make the final adjustment of the levels, and record the increase in volume of gas in the system.Vigorous shaking is necessary to ensure rapid and efficient evolution of gas, as is demonstrated in Fig. 2. This volume, adjusted for vapoa pressure, temperature and barometric pressure to N.T.P., is directly proportional to the amount of chaik originally present. The temperature of the surroundings milst be kept as steady as possible throughout the determinations. There is a slight rise of about 0.5" C. between the initial and final temperatures of the liquor in the reaction flask. This difference is always checked as well as any differences between the air temperatures. All flasks and fittings are of standard pattern and interchangeable.August , 1950 J CRETA PRAEPARATA I N NATIONAL FLOUR 403 As all standards will experience the same minor fluctuations, the effect of these will have been taken into account and therefore may be ignored in the final calculation.n i ;I Fig. 1. Apparatus Standards can be included in a batch of determinations so that direct comparison can be made and much calculation avoided. Alternatively, it is sufficient to make one calibration from a series of standards, and this will be within the accuracy required, provided that the necessary corrections for pressure and temperature are first applied to all data. CALCULATION OF RESULTS Let V = volume increase observed, T = temperature in degrees absolute, B = barometric pressure, Aq = vapour tension of liquid at temperature T, 1-976 g.= weight of 1 litre of carbon dioxide at N.T.P.404 FRASER AND WESTON: GASOMETRIC MLETHOD FOR THE ESTIMATION OF [VOl. 75 It can be demonstrated that: Ounces of Creta praeparata per - 273 B-Aq 1.!376 100 280 453.6 -o,Fi x -- x - x - sack of 280 1b- -[ (T 760 1000 44 28.35 -%-) ] x 1406 x F -0.5 > I 273 B-Aq The factor within brackets ( r K ) can be ta.bulated for values of B, T and Aq (Fig. 4). The vapour tension of brine is approximately 0.75 of that of water at the same temperature. c - . 2 4 6 8 I0 12 I4 16 I8 M 21 24 26 ZE 30 TIME Of SHAKING, MIN Fig. 2. Effect of Shaking Fig. 3 FACTOR, K Fig. 4. Factors for values of B, T and Aq.August, 19501 CRETA PRAEPARATA IN NATIONAL FLOUR 405 It has been determined experimentally for the brine - acid mixture used and found to be 0.70 of that of water.A correction for the “blank” in Creta-free flour is necessary. This varies only slightly from an average of 1 mg. of carbon dioxide per 20 g. of flour, which may be taken as the standard deduction to be made, and this deduction is represented by the figure of 0.5 appearing as the final term in the expression above. The factor F is dependent upon the solubility of carbon dioxide and the nature of the apparatus used and must be determined experimentally. The actual value obtained was 1.14 which was found to be fairly constant throughout the working range (17” rt 5” C.). Factors found from standard runs containing known additions of Creta praeparata (0, 25, 50 and 75 g.) are as follows- Barometric Experimental Temperature pressure factor F 14 763 1-160 18 760 1.136 20 755 1.140 21 755 1-145 22 749 1.140 TST An approximate relation F = 1 + - 273H These results are shown graphically (Fig.3). may be deduced, where : ST = solubility of the gas in liquor at temperature T, and H = headspace of flask, i.e., total volume less volume of liquor. Some calculated factors obtained from this expression and recorded solubilities of carbon dioxide,4s6 taking H as 300 ml. and volume of liquor as 100 ml. are: Calculated factor F at temperatures of r 16” C. 20” c. 25” C.’ Water . . .. .. .. .. 1-36 1-31 1.28 Brine, 16 per cent. w/v of NaCl . . 1.18 1.16 (1.14) 0.5 N hydrochloric acid . . .. 1-37 - 1.30 Brine, 30 per cent. w/v of NaCl . . 1.12 1.10 (1.09) Hence, routine examination comprises the recording of volume of gas evolved, temperature and pressure and then making the calculation- where values of K and F are defined as above. Ounces per sack = (V x K x F)-0-5, We wish to thank the Government Chemist for permission to publish this work. REFERENCES 1. 2. 3. 4. 6. Greer, E. N., Mounfield, J . D., and Pringle, W. J . S., AnaZyst, 1942, 67, 352. Hartley, A. W., and Green, A., Ibid., 1943, 68, 142. Chittick apparatus, “A.O.A.C., 1945,” p. 208. Seidell, A., “Solubilities of Inorganic and Metal Organic Compounds,” van Nostrand Co., Inc., Comey, A. M., and Hahn, D. A., “Dictionary of Chemical Solubilities,” Macmillan & Co., New New York, 1940. York, 1921. GOVERNMENT LABORATORY LONDON. W.C.2 December, 1949