method principle

a battery was composed of glass electrode as indicator electrode and saturated calomel electrode as reference electrode. Under the ideal condition of 25℃, the hydrogen ion activity changes by 1 times, which makes the electromotive force shift by 59.16mv. Many pH meters are equipped with temperature compensation devices to correct the temperature difference, which can be accurately and accurately reproduced to .1pH unit when used for routine water sample monitoring. More sophisticated instruments can be accurate to .1 pH. In order to improve the accuracy of the determination, the pH value of the standard buffer solution used in calibrating the instrument is close to that of the water sample.

instrument

various types of pH meters or ion activity meters.

glass electrode.

calomel electrode or silver-silver chloride electrode.

magnetic stirrer.

5ml beaker, preferably polyethylene or polytetrafluoroethylene beaker.

reagent

for the standard buffer solution used for calibrating the instrument, weigh the reagent according to the quantity specified in the following table, dissolve it in water at 25℃, and fix the volume to 1 ml in a volumetric flask. The conductivity of water should be lower than 2? S/cm, boil for several minutes before use, drive off carbon dioxide, and cool. Take 5 ml of cooled water and add 1 drop of saturated potassium chloride solution. If the pH is between 6 and 7, it can be used to prepare various standard buffer solutions.

preparation of ph standard solution

standard substance ph (25℃) per 1ml aqueous solution (25℃)

basic standard

potassium hydrogen tartrate (saturated at 25℃) 3.557 6.4 gkhc4h4o6 (1).

potassium dihydrogen citrate 3.776 11.41 gkh2c6h5o7

potassium dihydrogen phthalate 4.8 1.12 gkhc8h4o4

potassium dihydrogen phosphate+disodium hydrogen phosphate 6.865 3.388 gkh2po4 (2)+3.533 gna2hpo4 (2, 3)

potassium dihydrogen phosphate+disodium hydrogen phosphate 7.413 1.179 gkh2po4 (2)+4.32 gna2hpo4 (2, 3)

sodium tetraborate 9.18 3.8gNa2B4O7·1H2O(3)

sodium bicarbonate+sodium carbonate 1.12 2.92 gnahco3+2.64gna2co3

auxiliary standard

potassium tetroxalate dihydrate 1.679 12.61gKH3C4O8·2H2O(4)

Steps

Prepare according to the instruction manual of the instrument.

adjust the water sample and standard solution to the same temperature, record the measured temperature, and adjust the instrument compensation knob to this temperature. Select a standard solution with a difference of no more than 2 pH units from the pH value of the water sample to calibrate the instrument. Take out two electrodes from the first standard solution, rinse them thoroughly and blot them with filter paper. Then immersed in the second standard solution, its pH value is about 3 pH units different from the previous one. If the difference between the measured value and the pH value of the second standard solution is greater than .1pH value, it is necessary to check whether there is any problem with the instrument, electrode or standard solution. Water samples can only be determined when there is no abnormality in the three.

water sample determination: firstly, wash the two electrodes carefully with water, then wash them with water sample, then immerse the electrodes in the water sample, carefully stir or shake them to make them uniform, and record the pH value after the reading is stable.

Precautions

The glass electrode should be soaked in distilled water for more than 24 hours before use. After use, rinse and soak in water.

When measuring, the bulb of the glass electrode should be completely immersed in the solution, so that it is slightly higher than the ceramic core end of the calomel electrode, so as to avoid being broken when stirring.

there should be no bubbles between the inner electrode of the glass electrode and the bulb, and between the inner electrode of the calomel electrode and the ceramic core, so as to prevent the circuit from being broken.

the saturated potassium chloride level of calomel electrode must be higher than that of mercury, and there should be a proper amount of potassium chloride crystals to ensure the saturation of potassium chloride solution. Before use, the upper hole rubber plug must be unplugged.

in order to prevent carbon dioxide from dissolving in the air or escaping from the water sample, it is not advisable to open the bottle stopper of the water sample in advance before determination.

When the glass electrode bulb is polluted, dilute hydrochloric acid can be used to dissolve the inorganic salt and scale, and acetone can be used to remove the oil stain (but anhydrous ethanol cannot be used). Electrodes treated according to the above method should be soaked in water for one day and night before use.

pay attention to the factory date of the electrode, and the performance of the electrode will deteriorate if it is stored for too long.

2. colorimetric method

method principle

The acid-base indicator produces different colors in its aqueous solution with a specific pH range. Adding the indicator to the standard buffer solution, the generated color is used as a standard colorimetric tube, and the color is visually compared with the water sample colorimetric tube with the same indicator, so that the pH value of the water sample can be measured. This method is suitable for natural water and drinking water with low chroma and turbidity. If the water sample is colored and turbid, or contains high free residual chlorine, oxidant and reducing agent, it will interfere with the determination.

instrument

a set of colorimeter for hydrogen ion concentration determination. It can be made by ourselves, and the standard colorimetric solution can be encapsulated in an ampoule tube with a height of 6 mm drawn from a hard test tube with an inner diameter of 15 mm.

colorimetric tube: a hard test tube with an inner diameter of 15 mm and a height of 6 mm, and its glass quality and thickness are consistent with those of ampoules.

agate or porcelain mortar.

reagent

the following reagents are all prepared with newly boiled and chilled water.

indicator solution of chlorophenol red: weigh .1g of chlorophenol red, put it in a mortar, grind it, add 23.6ml of .1mol/L sodium hydroxide solution, and grind it until it is completely dissolved. Pour into a 15 ml beaker, then transfer into a 25 ml volumetric flask and dilute with water to the marking line. The suitable pH range is 4.8-6.4.

bromothymol blue indicator solution: weigh .1g bromothymol blue, put it in a mortar, grind it, add 16.ml of .1mol/L sodium hydroxide solution, and operate according to the above method until the volume reaches 25 ml. The suitable pH range is 6.-7.6.

indicator solution of phenol red: weigh .1g of phenol red, put it in a mortar, grind it fine, add 28.2ml of .1mol/L sodium hydroxide solution, and operate according to the above method until the volume reaches 25 ml. The suitable pH range is 6.8-8.4.

thymol blue indicator solution: weigh .1g thymol blue, put it in a mortar, grind it, add 21.5ml of .1mol/L sodium hydroxide solution, follow the above method, and adjust the volume to 25 ml. The suitable pH range is 8.-9.6.

.1mol/L sodium hydroxide standard solution.

.1mol/L potassium hydrogen phthalate solution: weigh 2.41g of potassium hydrogen phthalate (KHC8H4 O4) which has been dried at 15℃ for 2 hours in advance, dissolve it in water, transfer it into a volumetric flask, and fix the volume to 1ml.

.1mol/L potassium dihydrogen phosphate solution: weigh 13.616g of potassium dihydrogen phosphate (KH2PO4) which has been dried at 15℃ for 2 hours in advance, dissolve it in water, transfer it into a volumetric flask, and fix the volume to 1ml.

.1mol/L boric acid and .1mol/L potassium chloride solution: 6.22g boric acid (H3BO3) which is ground and put in a silica gel dryer for 24 hours is weighed; In addition, weigh KCl)7.456g of dry potassium chloride (KCl), dissolve it in water, transfer it into a volumetric flask, and fix the volume to 1ml.

step by step

preparation of ph standard colorimetric series

prepare standard buffer solution with ph of 4.8-9.6 according to the dosage of various solutions in the following three tables.

standard buffer solution (ph 4.8-5.8)

.1mol/l potassium hydrogen phthalate solution (ml) .1mol/ L sodium hydroxide standard solution (ml) is added with water to constant volume (ml)

4.8 5 16.5 1

5. 5 22.6 1

5.2 5 28.8 1

5.4 5 34.1 1

5.6 5 38.8 1 < .)

.1mol/l potassium dihydrogen phosphate solution (ml) .1mol/ L sodium hydroxide standard solution (ml) is added with water to constant volume (ml)

6. 5 5.6 1

6.2 5 8.1 1

6.4 5 11.6 1

6.6 5 16.4 1

6.8 5 22.4 1

7. P > 7.4 5 39.1 1

7.6 5 42.4 1

7.8 5 44.5 1

8. 5 46.1 1

standard buffer solution (pH 8.-9.6)

.1mol/L boric acid and .1 mol/L potassium chloride solution. L sodium hydroxide standard solution (ml) is added with water to constant volume (ml)

8. 5 3.9 1

8.2 5 6. 1

8.4 5 8.6 1

8.6 5 11.8 1

8.8 5 15.8 1

9.. > 9.4 5 32.1 1

9.6 5 36.9 1

Suck 1. ml of various pH standard buffer solutions and inject them into clean, dried hard ampoules with the same inner diameter. Add .5 ml of chlorophenol red indicator (1) to the standard buffer solution with pH of 4.8-6.4; Add .5 ml bromothymol blue indicator (2) to standard buffer solution with pH of 6.-7.6; Add .5 ml of phenol red indicator (3) to the standard buffer solution with pH of 6.8-8.4; Add .5 ml thymol blue indicator (4) to the standard buffer solution with pH of 8.-9.6. Then, seal it quickly with a blowtorch. Put the tightly sealed pH colorimetric ampoule tube in a wire basket and sterilize it in an open boiling water bath for 3 minutes, once every 24 hours and three times, and store it in the dark, which can be used for nearly 1 years.

determination of water sample

suck 1 ml of clear water sample into a colorimetric tube, add .5 ml of indicator liquid (for example, bromothymol blue), mix evenly, visually compare the color with the standard colorimetric tube, record the pH value of the standard tube similar to the color of the water sample, and estimate it to .1pH.

Precautions

If the water sample has slight chroma or turbidity, a water sample tube without indicator can be placed behind the standard tube and a distilled water tube can be placed behind the chromogenic water sample tube for color comparison compensation.