Solubility of Oxalic Acid

 

Prof. Dr Syed Tasleem Hussain*, Gul Abad Khan and Muhammad Shabeer

Department of Chemistry, Kohat University of Science and Technology, Kohat, Pakistan.

 

ABSTRACT:

 

 

INTRODUCTION:

Solubility is the ability of a material to dissolve in water or any other solvent. The solubility is increase with increase with temperature of organic compounds. For the purification of solid the technique are used recrystallization which depends on solute’s different solubilities in solvent. When a substance is dissolved in any solvent heat is evolve or absorbed, the amount of heat depends upon the nature and the quantity or amount of solvent used. When one mole of solute is dissolved in a solvent the heat is evolved or absorbed is called heat of solution. The quantitative aspect of interpreting the solubility data and temperature with heat of solution is shown by Van’t Hoff equation. ^[4]

 

dlnS/dt = ∆H/RT² …………………………..(1)

 

In this equation S is the solubility of the salt in mole per kg of the solvent and ∆H is the of solution per mole. When we integrate the equation (1) then we get the following equation.

 

logS = -∆H/2.303RT + constant ……………..(2)

 

If ∆H is independent of temperture then the curve be a straight line of smooth curve, the slop of straight line as -∆H/2.303RT, the ∆H can be calculated as

 

logS2 – logS1 = ∆H/ 2.303R [1/T2 -1/T1]……..(3) [1]

 

The main factors that have an effect on solubility are:

 

The nature of the solute and solvent --  While only 1 gram of lead (II) chloride can be dissolved in 100 grams of water at room temperature, 200 grams of zinc chloride can be dissolved.  The great difference in the solubilities of the of these two substances is the the result of differences in their natures.

 

Temperature -- Generally, an increase in the temperature of the solution increases the solubility of a solid solute.  A few solid solutes, however, are less soluble in warmer solutions. For all gases, solubility decreases as the temperature of the solution rises.

 

Pressure -- For solids and liquid solutes, changes in pressure have practically no effect on solubility. For gaseous solutes, an increase in pressure increases solubility and a decrease in pressure decreases solubility. (When the cap on a bottle of soda pop is removed, pressure is released, and the gaseous solute bubbles out of solution.  This escape of a gas from solution is called effervescence.)

 

The rate of solution is a measure of how fast a substance dissolves. Some of the factors determining the rate of solution are:

 

Size of the particles -- When a solute dissolves, the action takes place only at the surface of each particle. When the total surface area of the solute particles is increased, the solute dissolves more rapidly. Breaking a solute into smaller pieces increases its surface area and hence its rate of solution. (Sample problem: a cube with sides 1.0 cm long is cut in half, producing two pieces with dimensions of 1.0 cm x 1.0 cm x 0.50 cm. How much greater than the surface area of the original cube is the combined surface areas of the two pieces? 2.0 cm²

 

Stirring -- With liquid and solid solutes, stirring brings fresh portions of the solvent in contact with the solute, thereby increasing the rate of solution.

 

Amount of solute already dissolved -- When there is little solute already in solution, dissolving takes place relatively rapidly.  As the solution approaches the point where no solute can be dissolved, dissolving takes place more slowly.

 

Temperature -- For liquids and solid solutes, increasing the temperature not only increases the amount of solute that will dissolve but also increases the rate at which the solute will dissolve.  For gases, the reverse is true. An increase in temperature decreases both solubility and rate of solution.

 

In order for a solvent to dissolve a solute, the particles of the solvent must be able to separate the particles of the solute and occupy the intervening spaces. Polar solvent molecules can effectively separate the molecules of other polar substances. This happens when the positive end of a solvent molecule approaches the negative end of a solute molecule. A force of attraction then exists between the two molecules. The solute molecule is pulled into solution when the force overcomes the attractive force between the solute molecule and its neighboring solute molecule. Ethyl alcohol and water are examples of polar substances that readily dissolve in each other.

 

Ammonia, water, and other polar substances do not dissolve in solvent whose molecules are nonpolar.  The nonpolar molecules have no attraction for polar molecules and exert no force that can separate them.  However, nonpolar substance such as fat will dissolve in nonpolar solvents.

 

Polar solvents can generally dissolve solutes that are ionic. The negative ion of the substance being dissolved is attracted to the positive end of a neighboring solvent molecules. The positive ion of the solute is attracted to the negative end of the solvent molecule. Dissolving takes place when the solvent is able to pull ions out of their crystal lattice or structure. The separation of ions by the action of a solvent is called dissociation. When you sprinkle table salt (NaCl) in water and stir, the grains of salt disappear.  From what you have just read (on solubility), you have a model to explain what actually happens to the salt.  Sodium chloride, an ionic compound, is made of sodium ions and chloride ions.  The slightly charged ends of water molecules attract these ions. As a result the ions are dissociated, or separated by the water molecules and spread evenly throughout the solution.

 

Oxalic acid

Oxalic acid was discovered by Scheele, in 1776, and it is found in the organic as well as in the inorganic kingdoms. In plants it is generally met in combination with calcium or potassium; rhubarb, Rumex acetosa, Oxalis acetosella, phytolacca, belladonna, etc., contain the acid or bin-oxalate of potassium; rhubarb also contains oxalate of calcium, as likewise do many lichens, and in the human body it forms the mulberry calculus, a frequent form of gravel. In combination with calcium it is found in ginger, orris-root, squill, valerian, curcuma, quassia, and other drugs. As an ammonium oxalate it is present in the fertilizer guano. In the Cicer arietinum or chick pea it occurs in a free condition. It may also be formed artificially by the action of nitric acid on sugar, molasses, rice, starch, gum, wool, silk, hair, and many other organic compounds, which are free from nitrogen. Berthelot has obtained it synthetically from acetylene, ethylene, propylene, and allylene, by oxidation with permanganate of potassium.

 

Description:

Oxalic acid crystallizes in colorless, transparent, oblique, quadrilateral prisms with two-sided summits. The crystals are inodorous, have a strongly acid taste, faintly effloresce in a dry atmosphere, redden litmus paper, and when pure are completely volatilized by heat, and without becoming blackened. They dissolve in from 8 to 11 parts of water at 15.5° C. (60° F.), in their own weight of water at 100° C. (212° F.), and in 4 parts of alcohol; the addition of a small quantity of nitric acid to the water causes them to dissolve more readily. Nearly all the oxalates are insoluble in water, excepting the alkaline. Oxalate of calcium is insoluble, and hence oxalic acid is useful as a test for calcium, and is usually employed in the form of oxalate of ammonium; if the liquor to be examined contains any free acid, this must first be neutralized, as the oxalate can only detect calcium in neutral or alkaline fluids. Oxalic acid reduced by hydrogen is converted into glycolic and acetic acids, and if the action be kept up sufficiently long the glycolic becomes wholly formed into acetic acid.

 

Oxalic acid may be detected in any solution, by being entirely volatilized by heat; by yielding a white precipitate with nitrate of silver, soluble in nitric acid; and by giving a white precipitate with lime water, which is insoluble in water, readily soluble in nitric acid, insoluble in acetic acid, and which, when dried and heated to low redness, is converted, without blackening, into carbonate of calcium. Solution of sulphate of calcium produces a bluish-white precipitate with oxalic acid. Oxalic acid is sometimes contaminated with nitric acid, which gives a faint odor to it, and stains the cork of the bottle in which it is kept, yellow. If a very dilute solution of sulphate of indigo, containing the impure crystals, be boiled, the nitric acid present will decolorize the solution. On account of the resemblance between crystals of this acid and of magnesium sulphate, the latter has been used as an adulterant. This resemblance has also led to cases of poisoning, the person believing the acid to be Epsom salts. The acid may likewise be used for removing iron-rust and ink-stains from linen, and is employed in calico printing as a bleaching and discharge agent.

 

Action and Toxicology:

Oxalic acid and the oxalates poison the nervous system and the blood, producing, as well, gastro-intestinal lesions. A dose of 60 grains killed a boy (Taylor). Again, by prompt treatment, two cases recovered after a half ounce had been swallowed. Death takes place in varying lengths of time, a circumstance that can not readily be accounted for. Some cases die in from 10 minutes to an hour. The above-mentioned boy died in 8 hours. The symptoms are an intensely pure, acid taste, burning of the parts over which the poison passes, intense pain, vomiting, especially a bloody material, an extremely feeble pulse, an inability to assume the upright posture, collapse, and stupor. These symptoms, with the rapidity with which death takes place, will point to oxalic acid as the cause. Still, persons have been known to live for 22 days, death being produced by a slow poisoning. The post-mortem changes are a whitened oesophageo-gastric tract, though the stomach may contain a dark, gelatinous liquid, appearing like disorganized blood. The mucous coats are softened and loosened, but rarely perforated. The blood is excessively red, and in some instances oxalates have been found in the tubuli uriniferi of the kidneys. Koch regards it as a heart poison. The same lethal symptoms may be produced from salt of sorrel. Poisoning by oxalic acid, oxalate of ammonium, or oxalate of potassium, is best remedied by the speedy administration of chalk, suspended in water; when chalk cannot be had, magnesia may be used; either of these forms insoluble oxalates.

 

Experimental work:

The reagents used throughout the work were dehydrated oxalic acid. For making various solutions conductivity and solubility 15 different solvents were used throughout the experiment. Oxalic acid dehydrated used for the determination of solubility in water, acetone, methanol, ethanol, chloroform and different percentage of this solvent in water.

 

Phenolphthalein solution was used as indicator in titration process before titration the stock solution of approximately 2 M of oxalic acid dehydrated was prepared and kept open atmosphere for a week to allow precipitation and determine it standardize against different solutions.

 

Solubility of the oxalic acid (dihydrated):

From the titration data the normality of oxalic acid solution was calculated. Thus the gm of oxalic acid per liter of solution were determined by

 

Gm of solute per liter = N × eq.wt. of  oxalic acid

 

The weight of the solvent was calculated by finding the difference in the weights of the saturated solutions in the solute dissolve in it.

 

Weight of the solvent  =wt of solution – wt of solute

 

As such the amount of solute present in 100 ml of the solvent was calculated gram of oxalic acid per 100 ml = wt of solute / wt of solvent × 100.

 

Heat of solutions:

Log S was plotted against 1/T , and slop measured .

Heat of solution was determined by the following formula:  Slop = H / 2.303 R

 

RESULTS :

The solubility of Oxalic acid in 15 different solvents at different temperature (C^o).

 

1. Solubility of Oxalic acid in solvent 1(pure water)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution(ml)
1.	14	0.3	10
2.	30	0.6	Do
3.	40	0.75	Do
4.	65	6.89	Do
5.	79	9.6	Do
6.	95	12.44	Do

 

2: solubility of oxalic acid in solvent 2 (chloroform)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	17	0	10
2.	25	0	Do
3.	33	0.0006	Do
4.	39	0.0008	Do
5.	45	0.001	Do
6.	60	0.01	Do

 

3: Solubility of oxalic acid in solvent No 3 (Acetone)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	17	0.7	10
2.	23	1.69	Do
3.	27	1.87	Do
4.	34	2.01	Do
5.	38	2.16	Do
6.	43	2.66	Do

 

4 : Solubility of oxalic acid in solvent No 4 (Ethanol)

S#	Temperature (Co)	Oxalic acid Dissolved(gm)	Volume of solution (ml)
1.	17	0.2	10
2.	24	0.22	Do
3.	28	0.25	Do
4.	32	0.29	Do
5.	36	0.33	Do
6.	40	0.35	Do

 

5 : Solubility of oxalic acid in solvent No 5 ( methanol)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	10	0.19	10
2.	17	0.27	Do
3.	21	0.36	Do
4.	25	0.4	Do
5.	28	0.51	Do
6.	30	0.62	Do

 

6 : Solubility of oxalic acid in solvent No 6 ( 50% water + 50% methanol)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	12	0.29	10
2.	16	0.43	Do
3.	22	0.6	Do
4.	27	0.8	Do
5.	33	0.92	Do
6.	40	1.01	Do

 

7 : Solubility of oxalic acid in solvent No 7 (50% water + 50% Ethanol)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	10	1.02	10
2.	18	1.28	Do
3.	25	1.5	Do
4.	31	1.6	Do
5.	35	1.69	Do
6.	40	1.87	Do

 

8 : Solubility of oxalic acid in solvent No 8 (70% water + 30% Methanol)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	17	0.4	10
2.	25	0.52	Do
3.	30	0.58	Do
4.	35	0.79	Do
5.	39	0.88	Do
6.	43	0.98	Do

 

9 : Solubility of oxalic acid in solvent No 9 (90% water + 10% Methanol)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	17	0.49	10
2.	25	0.71	Do
3.	34	0.98	Do
4.	46	1.48	Do
5.	53	1.75	Do
6.	64	2.05	Do

 

10 : Solubility of oxalic acid in solvent No 10(95% Water + 5% Methanol)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	17	0.24	10
2.	23	0.4	Do
3.	28	0.65	Do
4.	36	0.98	Do
5.	42	1.33	Do
6.	49	1.61	Do

 

11 : Solubility of oxalic acid in solvent No 11 ( 50% Water + 50% chloroform)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	16	0.45	10
2.	24	0.7	Do
3.	31	0.91	Do
4.	38	1.17	Do
5.	44	1.35	Do
6.	50	1.76	Do

 

12 : Solubility iof oxalic acid in solvent No 12 ( 80% Water + 20% Chloroform)

S#	Temperature (Co)	Oxalic acid Dissolved(gm)	Volume of solution(ml)
1.	16	0.39	10
2.	22	0.65	Do
3.	27	0.87	Do
4.	33	1.09	Do
5.	38	1.32	Do
6.	43	1.51	Do

 

13 : Solubility of oxalic acid in solvent No 13 ( 90% Water + 10% Chloroform)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	17	0.26	10
2.	23	0.38	Do
3.	28	0.69	Do
4.	36	1.1	Do
5.	42	1.28	Do
6.	49	1.67	Do

 

 

14 : Solubility of oxalic acid in solvent No 14 ( 50% Water + 50% Acetone)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	17	0.26	10
2.	23	0.38	Do
3.	28	0.38	Do
4.	36	0.69	Do
5.	42	1.1	Do
6.	49	1.67	Do

 

15 : Solubility of oxalic acid in solvent No 15 ( 90% Water + 10% acetone)

S#	Temperature (Co)	Oxalic acid Dissolved (gm)	Volume of solution (ml)
1.	17	0.09	10
2.	25	0.38	Do
3.	34	0.78	Do
4.	43	1.23	Do
5.	52	1.61	Do
6.	59	1.87	Do

 

DISCUSSION:

From figure 1-15 and table 1-15, it show that the solubility of oxalic acid increases with increase in temperature. This increase in the solubility with temperature is due to the dissociation of oxalic acid which is in accordance to the Le chlatelier’s principle which state that the solubility increases with the increase in temperature. The value of solubility of oxalic acid at room temperature is similar to the reported in the literature. The small difference should be negligible ^[2].

 

From chart.1 oxalic acid was found to be more soluble in hot water as within the temperature range of measurement the solubility of oxalic acid in water increase linearly with increase in temperature. Our result were found to have a good one with slight disagreement is shown in chart.1, this show that oxalic acid is soluble in water due to the hydrogen bond formation and it increasing solubility with temperature is due to the fact that we increase in temperature, the hydrogen bond in water is weaken and it formed hydrogen bond forces with oxalic acid at a particular temperature. It was observed that in all these solvents, oxalic acid show the same increasing trend of solubility in temperature range (17 to 50 C^o).

 

The solubility of oxalic acid was high in water as compared to other solvents like chloroform, acetone, methanol and ethanol due to the hydrogen bonding and polarity. Solubility of oxalic acid was less found in acetone as compared to other solvents.

 

The solubility data obtained from oxalic acid in term of molality (M) in pure water and all alcohol + water mixture was converted to mole fraction (x) by an equation as bellow: ^[3]

 

X = 0.018M / 1 + 0.018M.

 

The solubility of oxalic acid in water in term of mole fraction (x) was correlated with temperature using a straight line equation ^{[4] [5]}.

 

lnx = A + B (T / K)

 

The oxalate coprecipitation is an appropriate method for the synthesis of precursors for superconducting materials. There are some difficulties to control the quantitative coprecipitation in the Bi-Sr-Ca-Cu oxalate system. The oxalate solubility study shows that Cu-oxalate displays the highest solubility, because Cu²⁺ is rapidly complexed by increasing pH. The precipitation of Ca²⁺, Sr²⁺ and Ba²⁺ ions is not quantitative at the lower pH (pH < 3). The presence of other cations in solution displaces the solubility equilibrium of Cu oxalate towards lower solubility.

 

The optimum pH value for the quantitative oxalate coprecipitation in the Bi-Sr-Ca-Cu- system was established at 3,0 - 3,5, at the optimum oxalic acid concentration of 0,1M. Using this method, the superconducting compounds of the Bi-Sr-Ca-Cu-O type were obtained, with T_c = 85 K. ^[6-26].

 

Generally the solubility of Oxalic Acid are increase with increase with temperature. It is clear and prove from chart 1 to chart15; Figures 2-4 present the calculated solubility curves for a value range of the pH from 0 to 6, and for oxalic acid excess concentrations of 0.05, 0.1, 0.2 and 1 mol∙l^-1.

 

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ethanol# ixzz1GJrwYQXm

 

 

Received on 13.09.2012        Modified on 18.09.2012

Accepted on 27.09.2012        © AJRC All right reserved