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Precipitation (chemistry)

2008-12-04T20:40:32Z

142.33.132.119: /* Applications */

[[Image:Chemical precipitation diagram.png|thumb|right|Chemical Precipitation]]
{{redirect|Precipitate|the album by Interpol|Precipitate EP}}
'''Precipitation''' is the formation of a [[solid]] in a [[solution]] during a [[chemical reaction]]. When the reaction occurs, the solid formed is called the '''precipitate''', and the liquid remaining above the solid is called the '''supernate'''.

==Applications==
[[Image:Tetratolylporphyrin.jpg|thumb|right|Crystals of ''meso''-tetratolylporphyrin from refluxing [[propionic acid]] precipitate on cooling]]
Precipitation reactions can be used for making [[pigments]], removing [[salts]] from water in [[water treatment]], and in [[classical qualitative inorganic analysis]].

Precipitation is also useful to isolate the products of a reaction during [[workup (chemistry)|workup]]. Ideally, the product of the reaction is insoluble in the reaction solvent. Thus, it precipitates as it is formed, preferably [[crystallization|forming pure crystals]]. An example of this would be the synthesis of [[porphyrin]]s in refluxing [[propionic acid]]. By cooling the reaction mixture to room temperature, crystals of the porphyrin precipitate, and are collected by filtration:<ref>{{cite journal | author = A. D. Adler; F. R. Longo; J. D. Finarelli; J. Goldmacher; J. Assour; L. Korsakoff | title = A simplified synthesis for meso-tetraphenylporphine | year = 1967 | journal = [[J. Org. Chem.]] | volume = 32 | issue = 2 | pages = 476–476 | doi = 10.1021/jo01288a053 }}</ref>

:[[Image:H2TPPsyn.png|400px]]
Precipitation may also occur when an ''antisolvent'' (a solvent in which the product is insoluble) is added, drastically reducing the solubility of the desired product. Thereafter, the precipitate may easily be separated by [[filtration]], [[decanting]], or [[centrifugation]]). An example would be the synthesis of chromic tetraphenylporphyrin chloride: water is added to the DMF reaction solution, and the product precipitates.<ref>{{cite journal | doi = 10.1016/0022-1902(70)80535-8 | title = On the preparation of metalloporphyrins | year = 1970 | author = Alan D. Adler; Frederick R. Longo; Frank Kampas; Jean Kim| journal = Journal of Inorganic and Nuclear Chemistry | volume = 32 | pages = 2443 }}</ref> Precipitation is also useful in purifying products: crude bmim-Cl is taken up in [[acetonitrile]], and dropped into ethyl acetate, where it precipitates.<ref>{{OrgSynth | author = Dupont, J., Consorti, C., Suarez, P., de Souza, R. | title = Preparation of 1-Butyl-3-methyl imidazolium-based Room Temperature Ionic Liquids | collvol = 10 | collvolpages = 184 | year = 2004 | prep = v79p0236}}</ref>

In [[metallurgy]], precipitation from a [[solid solution]] is also a useful way to [[precipitation strengthening|strengthen alloy]]s; this process is known as [[solid solution strengthening]].

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== Mechanism ==
Precipitation can occur when an [[solubility|insoluble]] substance is formed in the [[solution (chemistry)|solution]] due to a [[chemical reaction]] or when the solution has been [[supersaturated]] by a [[compound (chemistry)|compound]]. The formation of a precipitate is a sign of a [[chemical change]]. In most situations, the solid forms ("falls") out of the solute phase, and sinks to the bottom of the solution (though it will float if it is less dense than the solvent, or form a [[suspension (chemistry)|suspension]]).

The solid may reach the bottom of a container by means of [[settling]], [[sediment]]ation, or [[centrifugation]].

An important stage of the precipitation process is the onset of [[nucleation]]. The creation of a hypothetical solid [[particle]] includes the formation of an [[interface (chemistry)|interface]], which requires some [[energy (chemistry)|energy]] based on the relative [[surface energy]] of the solid and the solution. If this energy is not available, and no suitable nucleation surface is available, [[supersaturation]] occurs.

== Representation using chemical equations ==
An example of a precipitation reaction: Aqueous [[silver nitrate]] (AgNO3) is added to a solution containing [[potassium chloride]] (KCl) and the precipitation of a white solid, [[silver chloride]] is observed. (Zumdahl, 2005)

:AgNO3 (aq) + KCl (aq) → AgCl (s) + KNO3 (aq)

The silver chloride(AgCl) has formed a solid, which is observed as a precipitate.

This reaction can be written emphasizing the [[dissociation|dissociated]] [[ions]] in a combined solution. This is known as the [[ionic equation]].
:Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq) → AgCl (s) + K+ (aq) + NO3- (aq)

A final way to represent a precipitate reaction is known as a ''net ionic reaction''. In this case, any [[spectator ions]] (those which do not contribute to the reaction) are left out of the formula completely. This simplifies the above equations to the following:
:Ag+ (aq) + Cl- (aq) → AgCl (s)

==Cation sensitivity==
Precipitate formation is useful in the detection of the type of [[cation]] in [[salt]]. To do this, an [[alkali]] first reacts with the unknown salt to produce a precipitate which is the [[hydroxide]] of the unknown salt. To identify the cation, the color of the precipitate and its solubility in excess are noted. Similar processes are often used to separate chemically similar elements, such as the [[Alkali earth metals]].

==Digestion==
Digestion, or ''precipitate ageing'', happens when a freshly-formed precipitate is left, usually at a higher [[temperature]], in the solution from which it is precipitated. It results in cleaner and bigger particles.<ref>{{cite web |url=http://www.ktf-split.hr/glossary/en_o.php?def=digestion |title=Chemical dictionary definition |accessdate=2008-02-26}}</ref> The physico-chemical process underlying digestion is called [[Ostwald ripening]].

==Coprecipitation==
{{main|coprecipitation}}
Coprecipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed. It is an important issue in chemical analysis, where it is often undesirable, but in some cases it can be exploited. In [[gravimetric analysis]], it is a problem because undesired impurities often coprecipitate with the analyte, resulting in excess mass. On the other hand, in the analysis of trace elements, as is often the case in [[radiochemistry]], coprecipitation is often the only way of separating an element.

==References==
*Zumdahl, Steven S. ''Chemical Principles.'' 4th ed. New York: Houghton Mifflin Company, 2005.
*Mark Smith ''Principles of Science'' 1993
{{reflist}}
{{refimprove|date=February 2008}}

==Further reading==
*Banert, T., Brenner, G., Peuker, U. A. (2006), Operating parameters of a continuous sono-chemical precipitation reactor, Proc. 5. WCPT, Orlando Fl., 23.-27. April 2006.

==External links==
{{Commonscat|Solid precipitation}}
* [http://www.hielscher.com/ultrasonics/precipitation_01.htm Continuous Precipitation Using Ultrasonication (e.g. for nano-size magnetite particles)]
* http://www.answers.com/supernatant

[[Category:Separation processes]]
[[Category:Chemical processes]]

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[[it:Precipitazione (chimica)]]
[[he:תגובת שיקוע]]
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JavaScript/Basics

2007-11-28T18:52:37Z

142.33.163.2: /* String Operator */

''[[Basic JavaScript]] is a course under the [[Topic:Javascript | topic of JavaScript]]''.

What you should know - HTML/XHTML

Basics -
*JavaScript is the most popular web page scripting language and is suppported by almost every browser.
*It adds interactivity to your web pages.
*You can use only the tools already on your computer without the need to download any software.
*It is easy and fun

== First Lesson ==

Open up your text editor of choice and create a basic page. Now I am going to teach you how to write on your webpage using JavaScript.

<html>
<body>
<script type="text/javascript">
document.write("Hello World!")
</script>
</body>
</html>

As you can see, JavaScript is added directly into your HTML by adding the <script> tag. You don't have to understand exactly how that worked, I'm about to explain.

To start off with, the <code><script type="text/javascript"></code> tag tells the browser that whatever comes between that tag and the coming </script> tag is script, and the <code>type="text/javascript"<code> tells it that it is JavaScript. Similarly, to use [[wikipedia:VBScript | VBScript]], you might use <code>type="text/vbscript"</code> instead of <code>"text/javascript"</code>. For the purposes of this course, though, you should only ever need to use the JavaScript tag. 
 
It is also important to note that the <code>script</code> tag is case sensitive. What this means is if you use lowercase letters in the tag, all the letters in the tag must be lowercase, and uppercase if you use any uppercase. For example, if you wrote
<ScRiPt TyPe="TeXt/JaVaScRiPt">
not only would you waste a lot of time typing, but you would also confuse your browser.
 
You are probably wondering when I'm going to explain the rest of what you did. Well, <code>document.write()</code> is the JavaScript standard for writing text to the browser window. The 'document' clause refers to the HTML webpage (termed a document) as a whole; what follows ('<code>.write()</code>') is a command for the document object to carry out. In this case, you told the page to write the classic "<code>Hello World</code>" to the screen.
 
These are the absolute essentials to understand for starting JavaScript.
 
--[[User:Xora K Joken|Xora K Joken]] 19:18, 3 April 2007 (UTC)

==Second Lesson==

This second lesson will introduce the topics of variables and operators.

===Variables===

One of the things JavaScript is used for is the manipulation of data. In JavaScript, pieces of data can be stored either in variables or arrays.

Variables are declared using the ''var'' keyword:
var numValue;
var textValue;
var binaryValue;
and can be given values when they are created:
var numValue = 3;
var textValue = "This is text.";
var binaryValue = true;
Multiple variables can even be declared at the same time:
var numValue = 3, textValue = "This is text.", binaryValue = true;
Variables in JavaScript are weakly typed, meaning that the types of individual variables are not determined by the programmer. Unlike many languages, JavaScript only provides a generic ''var'' rather than separate types for integers, floating point numbers, characters, and strings. For example, in the statement
var x = 0;
it is fairly obvious to any individual reading the code that <code>x</code> is a number. However, if the statement
x = x + "This is text that is probably not a number";
were to appear later in the code, JavaScript would change the variable to a text variable. This is one of the reasons that JavaScript is referred to as weakly typed; at any particular point in time, it is impossible to tell exactly what type a particular variable is.

====Arrays====

Arrays in most languages are a means of collecting variables so that they can be referred to by an index. This is usually a numerical index beginning at 0(i.e., 0,1,2,...), but does not necessarily have to be a number. Arrays are declared in several ways; they can be declared either explicitly, indicating the number of variables in the array:

var colors = new Array(7);

or not:

var colors = new Array();

In the last case, the programmer leaves the number of colors open-ended. This may allow him to be more flexible in the long run.

To use a particular variable within the array, simply indicate the index of the element using brackets:

colors[2] = "orange";
x = colors[7] ;
z = "The book's color is " + colors[5] + ".";

It was mentioned before that arrays can use indexes other than numbers. One example of this might be using an index based on text instead of numbers. We might wish to index on the names of various fruits to get their colors:

colors["lemon"] = "yellow";
colors["lime"] = "green";
colors["cherry"] = "red";

Programs can be made significantly more readable using this feature.

document.write("The color of a cherry is " + colors[cherry]
+ ", but the color of a lime is " + colors[lime]);

===Operators===
----
==== Arithmatic Operators ====
{| border=1 cellspacing=0 cellpadding=5
| '''Operator'''
| '''Description'''
| '''Example'''
| '''Result'''
|-
| +
| Addition
| x = 2
y = 2

x + y
| 4
|-
| -
| Subtraction
| x = 5
y = 2

x - y
| 3
|-
| *
| Multiplication
| x = 5
y = 4

x * y
| 20
|-
| /
| Division
| 15 / 5
5 / 2
| 3
2.5
|-
| %
| Modulus (division remainder)
| 5 % 2
10 % 8

10 % 2
| 1
2

0
|-
| ++
| Increment
| x = 5
x++
| 6
|-
| --
| Decrement
| x = 5
x--
| 4
|}

==== Assignment Operators ====
{| border=1 cellspacing=0 cellpadding=5
| '''Operator'''
| '''Example'''
| '''Is The Same As'''
|-
| =
| x = y
| x = y
|-
| +=
| x += y
| x = x + y
|-
| -=
| x -= y
| x = x - y
|-
| *=
| x *= y
| x = x * y
|-
| /=
| x /= y
| x = x / y
|-
| %=
| x %= y
| x = x % y
|}

==== Comparison Operators ====
{| border=1 cellspacing=0 cellpadding=5
| '''Operator'''
| '''Description'''
| '''Example'''
|-
| ==
| is equal to
| 5 == 8 returns false
|-
| ===
| is equal to (checks for both value and type)
| x = 5
y = "5"

x == y returns true

x === y returns false
|-
| !=
| is not equal
| 5 != 8 returns true
|-
| >
| is greater than
| 5 > 8 returns false
|-
| <
| is less than
| 5 < 8 returns true
|-
| >=
| is greater than or equal to
| 5 >= 8 returns false
|-
| <=
| is less than or equal to
| 5 <= 8 returns true
|}

==== Logical Operators ====
{| border=1 cellspacing=0 cellpadding=5
| '''Operator'''
| '''Description'''
| '''Example'''
|-
| &&
| and
| x = 6
y = 3

(x < 10 && y > 1) returns true
|-
| <nowiki>||</nowiki>
| or
| x = 6
y = 3

(x == 5 || y == 5) returns false
|-
| !
| not
| x = 6
y = 3

<nowiki>!(x == y) returns true</nowiki> 
|}

==== String Operator ====
The string operator is used to [[w:concatenate | concatenate]] two strings together. For example:

<pre> var x = "Hello " + "world!" </pre>

The above example would result in the variable x equaling "Hello world!".

==== Conditional Operator ====
JavaScript also contains a conditional operator that assigns a value to a variable based on some condition.

The syntax would be:

<pre> variablename = ( condition ) ? value1 : value2 </pre>

For example:

<pre> greeting = ( visitor == "PRES" ) ? "Dear President " : "Dear " </pre>

[[Category:JavaScript]]
[[Category:Basics]]