Physical Chemistry Laboratory Report

PHYSICAL CHEMISTRY LABORATORY REPORT (CHE 414L) Department of Chemical Engineering School of Engineering and Architecture Saint Louis University

Group No. 9 Names of Students: Michelle Marzan Judith Mercado Vhal Marquez Frederique Moralejo I. II.

Signature ________________________________ ________________________________ ________________________________ ________________________________

Determination of Viscosity of a Liquid using the Ostwald Viscometer Background of the Study Viscosity is a molecular property. The coefficient of viscosity (η) is the proportionality constant between the force that causes a laminar flow and the velocity gradient of the flow, (dv/dr), over an area, A, that is parallel to the direction of the flow. The relationship is defined by Newton’s law: Equation 3-1 Flows that obey this law are called Newtonian flows. It is visualizes that a liquid that flows in a capillary tube is made up of concentric layers, each of which has finite velocity. Near the walls of the capillary the first layer can be considered to be stationary with zero velocity. Each adjacent layer has a greater velocity as we proceed from fro the wall toward the center of the capillary. In Newtonian flow the greatest velocity is at the center. Empirically, it has been found out that for Reynolds number that is less than about 2100 the flow is laminar. Thus measurements of viscosity are made therefore at low Reynolds number. It is observed that an equation governs the flow only for small diameter tubes and flow rates. A suitable and often used apparatus for many ordinary liquids is the Ostwald Viscometer. In the Ostwald Viscometer the efflux time of a certain volume of a liquid is observed, and b y using Poiseuille’s equation the viscosity coefficient of the liquid can be evaluated. Equation 3-2 The efflux time (t), the time required for the amount of liquid to hold between points a and b. The parameters include η, the viscosity coefficient, P, the pressure acting on the capillary, L, the length of the capillary, V, the volume of the liquid, R, the radius of the capillary and t, the efflux time. Since it is difficult to measure accurately the radius of the capillary tube, unless it is supplied by the manufacturer, the best way to determine the coefficient of viscosity of liquids is to compare the efflux time of the liquid having a known viscosity coefficient to that of the liquid which it is unknown.

Equation 3-3 In this equation η is the viscosity coefficient, t is the efflux time, ρ is the density of the liquid. The subscript 1 refers to the liquid with a known viscosity and 2 the liquid under experimentation. It is a practice that the viscosity of a sample is obtained by comparison to a standard reference substance. The experiment was conducted to determine the coefficient viscosities of different sample liquids. The sample liquids include Chloroform, Methanol, benzene and an Unknown Liquid. The measurement of viscosity is a significant importance in both the industry and the academia. Accurate knowledge of viscosity is necessary for various industrial processes and estimation of viscosity must be verified using experimental data and an instrument to measure viscosity is the Capillary Viscometer. Capillary viscometer which includes the Ostwald Viscometer is widely used for measuring viscosity of Newtonian liquids. They are simple in operation; requires a small volume of the sample liquid, temperature is controlled, the instrument itself is inexpensive. The volumetric flow of the liquid flowing through the fine bore is measured usually by noting the time required for the liquid to pass through the gradation marks. Ostwald viscometer can cause a significant error in the measurement if the viscometer is not in vertical alignment. It follows that 1 degree deviation from the vetical axis will introduce a 1% error in the hydrostatic head. An other source of error os the requirement to use an exact volume of liqiud for the reference liqiud and the test liquid. This requirements creates a problem when measurements are made at dofferent temperatues. Design and Methods A.Determination of time for liquid flow The Ostwald Viscometer with its capillary was thoroughly cleaned with Hydrochloric acid solution. The acid was pumped up and down the Viscometer with the aid of the pumping system. The Viscometer was washed with distilled water after the cleansing with the Hydrochloric acid. The apparatus was dried with a blower. The dried Viscometer was allowed to be filled with the liquid and the flow time was determined. The rinsing liquid was thrown and pumped dry. Five milliliters of benzene was placed in the apparatus using the pipette. The liquid was drawn in to the bulb A until it reached above the mark a. The pipetol was released to allow the complete flow of the liquid to the capillary. Upon reaching the mark a, the stopwatch was started to note the time of flow of the liquid to point b. As the liquid passed through point y, it was drawn up again to have ten trials. The apparatus was cleaned with Hydrochloric Acid, water and the next test substance which is methanol and allowed to be dried. Following the benzene was the methanol. Five milliliters of methanol was charged to the apparatus. The liquid drawn up the bulb A and reached the mark a. The time was noted when the liquid reached mark a until

the mark b. Ten trials was made for methanol. Finishing the trials include the washing of the apparatus with Hydrochloric Acid, water and few milliliters of chloroform. The apparatus was dried carefully. Five milliliters of chloroform was subjected to the Viscometer. The liquid reached the bulb A until few centimeters above the mark a. The time was recorded from mark a to mark b with ten trials. The Viscometer was cleaned with hydrochloric Acid, water and the unknown liquid. The unknown liquid was placed to the apparatus upon its immediate drying. The liquid was allowed to flow to reach he bulb A and above the mark a. The time was recorded from mark a to mark b for ten trials.

B. Determination of Liquid Density (Westphal Balance Method) The Westphal Balance was calibrated to zero reading by the necessary adjustments in the footscrew until the pointer is exactly horizontal to the supporting beam. The cylinder was filled with methanol. The surface of the liquid was about two to three centimeters from the rim. The plummet was submerged in the cylinder and attached on the hook. The rider was calibrated to balance the apparatus to the zero reading. The cylinder was cleaned and placed with benzene. The Westphal Balance was adjusted so that the pointer is in exactly horizontal with the pointer. The plummet was submerged in the cylinder. The rider was balances until it pointed to the zero reading. After the methanol was the chloroform. The cylinder was cleaned thoroughly and filled with chloroform. The zero pointer was calibrated until it was leveled horizontally with the supporting beam. The plummet was charged to the cylinder. Necessary adjustments were made to have the density of the liquid. The counterweight was removed for the liquid exceeded the 1 gram. The density was recorded for chloroform. The cylinder was rinsed and filled with the unknown reagent filled two to three centimeters to the brim. The Westphal Balance was adjusted to have its reading level with the supporting beam. The plummet was submerged in the cylinder. The rider was moved to have the density of the unknown liquid. Computations The computations involved the determination of the test reagents such as the benzene, chloroform, methanol and the unknown liquid on different parameters. The average mean flow time was recorded. The average mean flow time was determined by: ∑ Equation 3-4 The t represents the time in seconds for the ten trials. The densities were found with its respective temperature on the experiment. The viscosity of the calibrating liquid which is benzene is computed by the formula: Equation 3-5 The parameters include the temperature of the benzene and ηcl as the viscosity in Poise. The viscosities of the other liquids were determined using Equation

3-3. The true value was determined using the Perry’s Chemical Engineers’ Handbook and the percentage accuracy was computed as follows: Equation 3-6 TV refers to true value and EV for experimental value. III.

Results and Interpretation of Data This includes the data gathered in the Determination of Viscosities of Liquids using the Ostwald Viscometer. Table 3-1. The respective temperature reading of the reagents in °Centigrade Test Reagent Temperature (°C) Benzene 26 Chloroform 25 Methanol 26 Unknown Liquid 24 Table 3-2. The Average mean time of the reagents in the experiment. Test Reagent Average Mean Time (seconds) Benzene 57.45 Chloroform 31.92 Methanol 32.23 Unknown Liquid 40.29 The average mean time for the four test reagents was computed using Equation 3-4. Table 3-3. The densities of the liquid reagents obtained using the Westphal Balance. Test Reagent Density (g/cm3) Benzene 0.8706 Chloroform 1.4653 Methanol 0.7870 Unknown Liquid 0.8902 Table 3-4. The viscosities of the test reagents (EV) Test Reagent Viscosity (Poise) Benzene 0.005932724 Chloroform 0.005547979928 Methanol 0.00291535983 Unknown Liquid 0.003400229568 The following datum for benzene was calculated using Equation 3-5. The resulting datum is in Poise. For the test liquid on chloroform to the unknown liquid, the viscosity was determined using Equation 3-3. The subscript 1 refers to the benzene as the calibrating liquid and the subscript 2 for chloroform, methanol and unknown liquid in individual computations respectively.

Table 3-5. The true value of the test reagents obtained from the Perry’s Chemical Engineers’ Handbook. Test Reagent TV(in Poise) Benzene 0.007243179994 Chloroform .009639799401 Methanol 0.0003170329 Unknown Liquid The table reveals the data on the true value of the different test reagents. The data was obtained from the Perry’s Chemical Engineers’ handbook. The viscosity is in Pascal-s. To convert to Poise, the interpolated value was divided by 0.1Pascal-s. Table 3-6. The percentage accuracy of the test reagents. Test Reagents Percentage Accuracy (%) Benzene 81.91 Chloroform 57.55 Methanol 9.31 Unknown Liquid The following data were calculated using Equation 3-6.

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Conclusions and Recommendations References Frederick A. Bettelheim (1971). Experimental Physical Chemistry. W.B Saunders Company, Philadelphia. George H. Duffey (1962). Physical Chemistry. McGraw-Hill Book Company, New York. Gordon M Barrow (1961). Physical Chemistry. McGraw-Hill Book Company, Mew York.

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