Advice 1: How to find the ph of a solution

There are two ways of determining pH of solutions – potentiometric (by using pH meter) and colorimetric (using chemical indicators). First thus more accurate and allows to determine the acidity in any medium, of any composition, color and texture, while the second method is suitable for clear aqueous solutions. This method of determining pH of solutions based on the use of acid-base indicators, the colour of which varies with changes in the environmental acidity.
The colour change of the indicator
You will need
  • 1. A mixture of equal volumes of 0.12 n of acids: phosphoric, acetic, boric.
  • 2. Caustic soda NaOH and 0.2 n.
  • 3. Indicators:
  • Tropeolin 00, 0.1% aqueous solution.
  • Methyl orange, 0.1% aqueous solution.
  • Methyl red, 0.1% solution in 60% alcohol.
  • Bronchiology blue, 0.05% solution in 20% alcohol.
  • Crazyby red, 0.04 percent aqueous solution.
  • Phenolphthalein, 0.1 percent alcoholic solution.
  • Thymolphthalein, 0.1 percent alcoholic solution.
  • The transition table of svetvincenat
  • Table of acid-base indicators.
Guide the preliminary determination of the pH of the test solution by using universal indicator or universal indicator paper. The degree of acidity of the aqueous solution is expressed by hydrogen index (number of hydrogen ions), the value of which ranges from 0 (very high acidity) to 14 (very high alkalinity). The change in hydrogen ion concentration corresponds to 10 times pH changes by one unit. A neutral environment has a ph equal to 7 (at room temperature). Methyl orange at pH < 3,1 is red, and at pH > 4,4 – yellow; litmus at a pH of < 5 is red and at pH > 8 blue. For example, you have litmus paper is red color therefore, the test solution has high acidity, pH value less than 5.
Find in the table the indicators that more accurately diagnose a region of acidity, have some universal indicator. Ie look at what the indicator in the observed values of pH from 0 to 5 – methylene red, methylorange and tropeolin 00.
Prepare standard buffer solutions, covering this area of pH. To do this, take a calibrated dry test tubes, place them in 5 ml of a mixture of acids, caustic soda put into each tube, respectively table. Number (or otherwise label) them. Stir the solution and with a pipette remove the excess, bringing the volume of solution in each test tube until exactly 5 ml.
In a separate clean tube (if you check several indicators, take the vial according to the number of indicators) dial 5 ml of test solution. Add 2 drops of the indicator and compare color of the test solution with the color of the standard solutions series.

Advice 2 : How to find the concentration of acid

The concentration of the acid – value that indicates what proportion of this substance is a certain quantity or volume of its solution. It can be expressed in different ways: in the form of a mass fraction, molarity, molality, etc. In laboratory practice, it is often necessary to determine the concentration of acid.
How to find the concentration of acid
You will need
  • - graduated measuring Cup;
  • - laboratory scales;
  • - glass pipette;
  • - litmus;
  • the alkali solution.
Suppose you have a container label H2SO4. That is, once it becomes clear that it contains sulfuric acid. But no more information there. How do you determine its concentration? You can do this by using the table of densities of solutions. There are many directories where the values of the density of the solution of chemicals depending on their concentration.
Take a graduated measuring Cup, weigh it on a laboratory scale. Denote the mass of the empty Cup as m1. Using a glass pipette add it some volume V of sulfuric acid. Again weigh the Cup, indicate its mass as m2. The density of the acid is given by: (m2 – m1)/V.
According to the table of densities, set the concentration of the solution. Assume that during the described experience, you calculated the density of sulfuric acid: 1,303 grams/milliliter. It corresponds to 40% concentration.
As determined by the concentration of the acid? There are sensitive and very accurate method called direct titration. It is based on the neutralization of the acid by the alkali solution whose concentration is known. For example, in the case of sulfuric acid: H2SO4 + 2NaOH = Na2SO4 + 2H2O.
The reaction scheme shows that to neutralize one mole of acid will need two moles of sodium hydroxide. On this basis, knowing the volume of the investigated solution of acidvolume of alkali used for its neutralization and the concentration of the alkali, we can calculate the concentration of acid.
But how to determine the exact amount of alkali needed to neutralize the acid? With the indicator changing color. For example, litmus. The experience is so. Over a vessel with known volume of acid (which also add a few drops of indicator), attach a graduated burette with the alkali solution.
Record the reading of the upper level of alkali, then carefully loosening the tap of the burette, drop by drop start to add it to the acid. Your task is to turn off the tap in the moment when the red indicator will disappear. Record the reported lower level of alkali and calculate what its volume left for the neutralization of the acid.
And then, knowing the magnitude of the volume and the precise concentration of lye is easy to calculate how many moles of alkali are reacted. Accordingly, the number of moles of acid was 2 times less. Knowing the initial volume of the acid, you will find its molar concentration.
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