Removal of Hardness of Water Using Precipitation and Complexation Methods
KWAME NKRUMAH UNIVERSITY OF SCIENCE AND TECHNOLOGY COLLEGE OF TITLE: REMOVAL OF HARDNESS OF WATER USING PRECIPITATION AND COMPLEXATION METHODS. NAME: KWARTENG YAW PRINCE COURSE: BSC. ENVIRONMENTAL SCIENCE YEAR: FIRST YEAR EXPERIMENT NO. : A. 1. 1. 3. T. A. : BRIGHT KOFI LEONARD DATE: 7TH NOVEMBER, 2007. Aims and Objectives: 1. To describe water hardness. 2. To soften hard water in terms of the species involved and the reactions they undergo. 3. To test the effectiveness of the methods. INTRODUCTION The experiment seeks to test the effectiveness of the two methods (precipitation and complexation) of separating hard water.
Water containing Ca2+ and/or Mg2+ ions is called hard water, and water that is mostly free of these ions is called soft water. These ions do not pose any health threat, but they can engage in reactions that leave insoluble mineral deposits. These deposits can make hard water unsuitable for many uses, and so a variety of means have been developed to “soften” hard water; i. e. , remove the calcium and magnesium ions. Water can be softened in a number of ways. An automatic water softener connected to water supply pipes removes magnesium and calcium from water and replaces them with sodium.
Sodium does not react with soap or detergents. If you don’t have an automatic water softener, you can still soften laundry water by adding softeners directly to the wash water. These softeners combine with calcium and magnesium, preventing the minerals from forming a soap scum. Precipitation: One common type of reaction of reaction that occurs in aqueous solution is the precipitation reaction, which results in the formation of an insoluble product, or precipitate. A precipitate is an insoluble solid that separates from the solution. Precipitation reaction usually involves ionic compounds.
Hard water cations Ca2+, Mg2+ and Fe3+ which precipitates soap from aqueous solution and cannot be removed by heating if bicarbonate is absent. Such solutions which cannot be softened by boiling are referred to as permanent hard water. Permanent hard water can be softened only by removing the offending cations. For example: 2CH3 (CH2) 16COONa(s) + Ca2+ > [CH3 (CH2) 16COO] 2Ca(s) + 2Na+ Soap curd/scum For large-scale municipal operations, a process known as the “lime-soda process” or precipitation is used to remove Ca2+ and Mg2+ from the water supply.
Ion-exchange reactions, similar to those you performed in this experiment, which result in the formation of an insoluble precipitate, are the basis of this process. The water is treated with a combination of slaked lime, Ca(OH)2, and soda ash, Na2CO3. Calcium precipitates as CaCO3, and magnesium precipitates as Mg(OH)2. These solids can be collected, thus removing the scale-forming cations from the water supply. In this experiment, calcium carbonate can be precipitated from a hard water sample containing Ca2+ simply by adding a soluble carbonate salt such as Na2CO3. 2Na (aq) + CO2-(aq) + Ca2+ > CaCO3(s) + 2Na+(aq)
A significant disadvantage of sodium carbonate as water softener is that it leaves a suspension of CaCO3(s) which dulls the colours of many laundered fabrics. Complexation: It is the . A complex ion is an ion containing a central metal cation bounded to one or more molecules or ions. EDTA is a complex and as such has the ability to hold metal ions/atoms (both Ca2+ and Mg2+) like a claw, thus complexing the cations. In the second phase of the experiment, a small amount of EDTA will be added to the water after which some soap solution will be added to the solution and then shaken. Calculations for water hardness
The concentration of EDTA solution is found by: M EDTA = (wt. of CaCO3)(V10/100 mL) ((Mol wt CaCO3))(mL EDTA/1000) where V10 is the calibrated volume of the 10 mL pipette. Water hardness, expressed in parts per million or milligrams per liter of CaCO3,11 can be calculated by Hardness in ppm = (mL EDTA/1000)(M EDTA)(mol wt CaCO3)(1000 mg/g) (vol of sample in liters) The hardness in ppm value represents the CaCO3 concentration in the solution prepared from your unknown sample. Report the result as ppm of CaCO3 in the solid unknown sample that was supplied to you.
CHEMICALS AND EQUIPMENT Funnel Na2CO3 solution Graduated cylinder Soap solution Test tube Distilled water A wash bottleEthylenediamminetetraacetic acid (EDTA) solution Hard water PROCEDURE |Procedure |Observation | |A. HEATING | | |25ml permanent hard water was heated. Boiled within a very short time with no change in colour ad no | | |formation of precipitate. | |5ml of soap solution to heated permanent hard water and shaken. |Formation of curd. | | |No formation of lather upon stirring or shaking. | |B. Na2CO3 | |10ml of permanent hard water was placed in a test tube and 2ml of |No change in colour and state of the solution. | |0. M Na2CO3 solution was added and the mixture shaken | | |5ml of the soap solution was added and then shaken |Large amount of curds were formed after a little shaking and after a | | |while, lather was formed with continuous shaking. | |C. EDTA | |10ml of permanent hard water was placed in a test tube and 10ml of |A clear solution was maintained. | |0. 1M disodium EDTA solution was added. The mixture was then | | |shaken. | | |5ml of the soap solution was added and then shaken. |Small amounts of curd were formed. | | |Lather forms easily after a little shaking and stirring. | POST LAB a) Temporary hardness of water will be removed Procedure 1- heating water: Temporary hardness will be removed because temporary hard water contains HCO3- ions.
Under heat and pressure, the offending cations (Mg2+ and Ca2+) will react with the bicarbonate to form a salt, hence removing hardness. For instance: Ca2+(aq) + 2HCO3-(aq) > CaCO3(s) + CO2(aq) + H2O(l) and gaseous carbon dioxide is driven off: Procedure 2 – Addition of Na2CO3: Addition of Na2CO3 causes the cations to react with the CO3- ion. This reaction leads to the formation of a carbonate salt. For instance Ca2+ ion reacts with the CO3- ion to precipitate CaCO3 from the hard water. 2Na (aq) + CO2-(aq) + Ca2+ > CaCO3(s) + 2Na+(aq)
Hence, temporary hardness will be removed. Procedure 3 – Addition of EDTA: Ethylenediamminetetraacetic acid (EDTA) is known to isolate ions in solution. EDTA will isolate the ions, thus reducing their concentration. b) Procedure 1- heating Advantages 1) No chemicals were used. 2) Least expensive method used to remove water hardness. Disadvantages 1) This method is more time consuming. 2) There is also a risk of injury by the hot water. Procedure 2 – Permanent hard water + Na2CO3 Advantages 1) It takes a relatively short time. 2) It is more efficient.
Disadvantages 1) It is more time consuming since one has to first prepare the carbonate salt. 2) There is a probability of the chemical not being 100% pure. Procedure 3 – Permanent hard water + EDTA Advantages 1) It leaves no precipitate. 2) It takes a very short time. Disadvantages 1) It is capital intensive since the complexing agent would be bought. 2) The chemical may not be 100% pure. DISCUSSION Permanent hard water did not lather when heated and tested with soap. Thus implies that hardness was not removed. Instead, it formed a curd with the soap solution. CH3 (CH2) 16COONa(s) + Ca2+ > [CH3 (CH2) 16COO] 2Ca(s) + 2Na+ Soap + cation > curd/scum This goes to justify the fact that permanent hard water cannot be softened by boiling because before this method can be effective, bicarbonate ions should be present in the water. On the other hand, temporary hard water when heated and tested with soap solution, formed lather with the soap. This can be explained by the fact that it contained bicarbonate ions which reacted with offending cations to precipitate carbonate salts, thus removing the hardness from the water.
This could be seen as the white precipitate formed at the upper layer on the boiling water. Ca2+(aq) + 2HCO3-(aq) > CaCO3(s) + CO2(aq) + H2O(l) ? heat The addition of Na2CO3 to permanent hard water removed its hardness. This is because For large-scale municipal operations, a process known as the “lime-soda process” is used to remove Ca2+ and Mg2+ from the water supply. Ion-exchange reactions, similar to those you in this experiment, which result in the formation of an insoluble precipitate, are the basis of this process. The water is treated with a combination of slaked lime, Ca(OH)2, and soda ash, Na2CO3.
Calcium precipitates as CaCO3, and magnesium precipitates as Mg(OH)2. These solids can be collected, thus removing the scale-forming cations from the water supply. To see this process in more detail, let us consider the reaction for the precipitation of Mg(OH)2. Consultation of the solubility guidelines in the experiment reveals that the Ca(OH)2 of slaked lime is moderately soluble in water. Hence, it can dissociate in water to give one Ca2+ ion and two OH- ions for each unit of Ca(OH)2 that dissolves. The OH- ions react with Mg2+ ions in the water to form the insoluble precipitate.
The Ca2+ ions are unaffected by this reaction, and so we do not include them in the net ionic reaction. They are removed by the separate reaction with CO32- ions from the soda ash. But in this experimental method, only soda was used, meaning that more of the Ca2+ reacted with the carbonate ion as compared to the Mg2+. The Mg2+ that did not react accounted for the large amount of curd that was formed subsequently causing the water to lather considerably when soap was added. The solution EDTA lathered easily with after the soap solution was added. This is because EDTA is a complex and as such has the ability to hold metal ons/atoms (both Ca2+ and Mg2+) like a claw is thus called a chelating agent. Six donor atoms enable EDTA to form a very stable complex with Ca2+ and Mg2+. This consequently led to the formation of a small amount of curd. Calcium and magnesium in water interfere with the cleaning action of soap and detergent. They do this by combining with soap or detergent and forming a scum that does not dissolve in water. Because they react with soap and detergent, they remove the soap and detergent, thereby reducing the effectiveness of these cleaning agents.
This could be overcome by adding more soap or detergent. However, the scum that is formed can adhere to what is being washed, making it appear dingy. CONCLUSION Hard water can be softened by various methods but depending on the cations present, not all the methods will be effective. From the experiment, it can be concluded that heating softens temporary hard water provided there is the presence of carbonate ions. It can also be concluded that EDTA is a better softener than the sodium carbonate since it reacts more with cations than the carbonate ion o for the precipitate. PRECAUTIONS . Reading of the volumes was taken from the bottom of the meniscus. 2. All glassware was washed with distilled water before use. REFERNECES 1. Chemistry Laboratory Manual, KNUST, page 33, 34. 2. Introduction to Chemistry by Chopper and Johnson, pages 40-41. 3. Essential Chemistry (Second Edition) – Raymond Chang – pages 150-151, 118. STANDARD DEVIATION The standard deviation of a set of data gives an indication of the amount of dispersion, or the scatter, of members of the set from the measure of the central tendency. Its value is the root mean square value of the members of the set.
Generally, the more data you record the more extreme your highs and lows will be. That is to say, there is a common range of variation even as larger data sets produce rare “outliers” with ever more extreme deviation. Estimates of the range of variation seek to put a number to this common range of variation that doesn’t depend on sample size. The most common way to describe the range of variation is standard deviation (usually denoted by the Greek letter sigma: [pic]). The standard deviation is simply the square root of the variance.
To obtain the variance, start by subtracting the average from each data item. Since there will be about as many items above average as below average, the resulting list of numbers will have about as many positive values as negative values. (In fact this list of deviations-from-average must itself average to zero! ) Square each deviation, and proceed to find the average of the squared-deviations. However, in finding the average squared-deviation, divide by N-1 rather than N. The result is the variance; take its square root to get the standard deviation.
An important measure of reproducibility is the standard deviation in the measurements, or the standard deviation for short. [pic] The formula for the standard deviation looks kind of like the formula for an average; there is a fraction with a summation in the numerator. However, you don’t divide by n, you divide by n-1. And you must take the square root of the fraction. Notice that the standard deviation is linked to the average. The numerator is called the sum of the squared errors; the “error” is the difference between each value and the mean value. Those errors are squared and then totaled.
In some circumstances (namely, where the overall number of counts isn’t fixed), it is appropriate to use n as the degrees of freedom rather than n-1. For example, you go out to the field and count bird species for one hour or one day or something. Your data is the number of each species. You can test whether that fits your expectation using n degrees of freedom. WATER HARDNESS SCALE |Description |Hardness in mg/l as calcium |Hardness in mg/l as calcium carbonate | |Soft |0 – 20 0 – 50 | |Moderately soft |20 – 40 |50 – 100 | |Slightly hard |40 – 60 |100 – 150 | |Moderately hard |60 – 80 |150 – 200 | |Hard |80 – 120 |200 – 300 | |Very hard |Over 120 |Over 300 | Key: mg/l = milligrammes per litre = 1 part per million [pic]