Some standard content:
National Metrology Verification Regulation of the People's Republic of China JJG119—2005
Laboratory pH (Acidity) Meter
Laboratory pH Meters
Promulgated on September 5, 2005
Implementation on March 5, 2006
Promulgated by the General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China JJG119—2005
Verification Regulation of
Laboratory pH Meters
JJG119—2005
Replaces JJG119—1984
This regulation was approved by the General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China on September 5, 2005, and came into effect on March 5, 2006.
Responsible unit: National Technical Committee for Physical and Chemical Metrology Main drafting unit: National Research Center for Standard Materials Participating drafting unit: Beijing Institute of Metrology and Testing Science This regulation is entrusted to the National Technical Committee for Physical and Chemical Metrology to be responsible for the interpretation of this regulation Main drafter:
Xiu Hongyu
Participating drafter:
Gu Jiayu
JJG119—2005
(National Research Center for Standard Materials)
(Beijing Institute of Metrology and Testing Science Institute) (Beijing Institute of Metrology and Testing Science) 2
Scope·
References
Overview·
Metering performance requirements
Electrometer indication error
Electrometer input current
JJG119—2005
Electrometer indication error caused by input impedance·Electrometer indication error caused by temperature compensatorElectrometer indication repeatability
Total error of instrument indication·
Instrument indication repeatability
General Technical requirements
Appearance·
Glass electrode
Reference electrode·
Measuring instrument control
Verification conditions
Verification items and verification methods
Processing of verification results
Verification cycle·
Appendix A
Appendix B
Appendix C
Appendix D
Preparation and storage of standard solution
Value of (0~100)℃（k=2.30259R T/F) Original verification record format
Verification certificate and verification result notification format
(2)
(2)
(3)
1 Scope
JJG119—2005
Verification procedure for laboratory pH (acidity) meter
This procedure is applicable to the initial verification, subsequent verification and in-use inspection of pH (acidity) meter and laboratory general ion meter (hereinafter referred to as instrument) that can be used as pH (acidity) meter. 2 References
This procedure refers to the following documents:
JJF1001—1998 "General metrological terms and definitions" OIML Recommendation R54 (1980): pH Scale for Aqueous Solutions When using this procedure, attention should be paid to the use of the current valid versions of the above-mentioned references. 3Overview
Laboratory pH (acidity) meter is an electrochemical analysis instrument, mainly used to measure the pH value of aqueous solution. The instrument mainly consists of two parts: measuring electrode and electrometer. The electrometer consists of impedance converter, amplifier, function regulator and display. Measuring electrodes include indicator electrode and reference electrode. Common indicator electrodes include glass electrode, hydrogen electrode, hydrogen aldehyde electrode, antimony electrode, etc. Reference electrode mainly refers to external reference electrode, and the most commonly used external reference electrodes are silver/silver chloride electrode, calomel electrode, etc. When using pH (acidity) meter to measure the pH value of solution, the comparison method is used. First, the indicator electrode, reference electrode and pH standard buffer solution form a battery, and its electromotive force is input into the electrometer to "calibrate" the instrument. Then the measured solution and the same pair of electrodes form a battery, and the battery electromotive force is also input into the electrometer. After comparison, the value displayed by the electrometer is the pH value of the measured solution. 4.1 Meter indication error
The indication error caused by graduation and nonlinearity shall not exceed the provisions of Table 1 at any point within the range. 4.2 Meter input current
The input current of the meter shall not exceed the provisions of Table 1. 4.3 Indication error caused by meter input impedance When a potential equivalent to 3 pH units is input to the meter, the change in meter indication produced when the resistor R is connected in series or not in series at the meter input terminal shall not exceed the provisions of Table 1. 4.4 Indication error caused by meter temperature compensator At any compensation temperature, when a potential equivalent to 3 pH units at the compensation temperature is input to the meter, the difference between the meter indication and the actual value shall not exceed the provisions of Table 1. 4.5 Meter indication repeatability
The meter indication repeatability (standard deviation of a single measurement) shall not exceed the provisions of Table 1. 4.6 Total error of instrument indication
JJG119—2005
The total error of the indication of the instrument shall not exceed the provisions of Table 1. Instrument indication repeatability
The repeatability of the indication of the instrument (standard deviation of a single measurement) shall not exceed the provisions of Table 1. Table 1 Metrological performance requirements
Instrument level
Metrological performance
Division value or minimum display value (pH))
Electrometer indication error
pH (pH)
Input current/A
Electrometer inspection room
Indication error caused by input impedance (pH)
Approximate equivalent input impedance/Q
Temperature compensator error (pH)
Electrometer indication repeatability (pH)
Temperature Temperature measurement error of temperature probe/℃
Instrument indication error (pH)
Instrument indication repeatability (pH)
±2%FS
3×10″
±1%FS
1×10-1
3×10″
1:×10-12
1×10*2
1×10~12
1×10/2
Note: The maximum allowable error of the digital display instrument is the pH value given in the table ± the minimum display value. 5 General technical requirements
5.1 Appearance
±0.03%FS
1×10-12
3×1012
5.1.1 The appearance of the instrument should be smooth and flat, with uniform color. Each function key of the instrument should be able to work normally, and each fastener should not be loose, and the niel should be clear and complete.
5.1.2 The nameplate of the instrument should indicate its manufacturer's name, trademark, name, model, specification, factory number and factory date, and the nameplate should be clear.
5.2 Glass electrode
Glass electrode should be free of cracks and bursts. The electrode plug should be clean and dry. Reference electrode
The reference electrode should be filled with solution, and there should be no adsorbed impurities at the liquid junction. The electrolyte solution can leak normally (it can be wiped with filter paper or crystals can be precipitated at the salt bridge mouth within a certain period of time). 6 Measuring instrument control
Measuring instrument control includes initial verification, subsequent verification and in-use inspection. 2
6.1 Verification conditions
JJG119—2005
6.1.1 The environmental conditions for verification should comply with the provisions of Table 2. 6.1.2 pH standard solution should use certified pH standard material approved by the government metrology administration department. The preparation method and pH value of the standard solution can be found in the corresponding standard material certificate. 0.001-level instruments should use first-level pH standard materials, and other-level instruments can use second-level standard materials. 6.1.3 The accuracy of standard DC potential signal sources such as pH (acidity) meter calibrators (hereinafter referred to as calibrators) or DC potentiometers should be (3~5) times higher than the measurement accuracy of the meter being tested. 0.001-level instruments should use 0.0006-level calibrators, and other-level instruments can use 0.003-level calibrators. 6.1.4 During the calibration process, high-insulation output connectors, shielded wires, etc. should be used. The temperature range of the thermometer is (0~60)℃, and the temperature measurement error should not be greater than 0.1℃. 6.1.5
6.1.6 For the calibration of instruments of level 0.1 and below, the resistance value of the high resistor R is 300MQ, and for the calibration of instruments above level 0.1, the resistance value of R is 1000MQ.
6.2 Verification items and verification methods
The various verification items specified in this regulation are listed in Table 3. Table 2 Verification environmental conditions
Instrument level
Honey temperature/℃
23 ± 10
23 ±10
23±15
23 ±15
Relative lakeness/%
Temperature stability of standard solution and electrode system
/%
Note: When using a DC potentiometer to calibrate a 0.001-level instrument, the room temperature is required to be (20±3)℃. Table 3 List of calibration items
Calibration items
Appearance inspection
Electrode inspection
Electrometer indication error
Electrometer input current
Electrometer input impedance
Temperature compensator
Electrometer indication repeatability
Instrument indication error
Instrument indication repeatability
First calibration
Subsequent calibration
“+” indicates, and “_” indicates items that need to be inspected. Note: All items to be inspected are used for interference factors, especially strong mechanical vibration and electromagnetic interference near the instrument. Inspection during use. 6.2.1 Appearance inspection. JJG119-2005. According to the requirements of Chapter 5, inspect the appearance by visual inspection and touch. 6.2.2 Verification of the indication error of the meter. 6.2.2.1 Verification of the indication error of pH. Connect the circuit according to Figure 1, turn on switch K, and short-circuit high resistance R. Adjust the temperature compensator of the instrument to 25℃ (or a certain middle temperature point of the temperature compensator). Calibrate the instrument according to the instrument manual. Then use the calibration instrument to input the standard signal pH input to the meter, and record the meter indication pH value respectively. Repeat the measurement twice (one time each by input increase and decrease), take the average value pH value, and calculate the meter indication error according to formula (1). ApH indication = pH indication - pH input
For pointer instruments, within the pH (7~8) or pH (7~6) range, one point should be calibrated every 0.2pH, and within other ranges, one point should be calibrated every 1pH. For digital instruments, within the full range, one point should be calibrated every 1pH. For instruments with multiple ranges, each range should be calibrated according to the corresponding instrument level requirements. When the level is the same, for the same value, the change in the indication error calibrated under different ranges should not be greater than the repeatability of the meter at that level. Switch K
pH (acidity) meter calibration instrument
Resistance R
Figure! Calibration principle diagram
pH (acidity) meter
6.2.2.2 Calibration of mV indication error
Connect the circuit according to Figure 1, turn on switch K, and short-circuit high resistance R. Set the "pH-mV" selection switch of the meter to the mV position. Adjust the calibration instrument to output millivolt potential signals, input the instrument, measure and record the meter readings. The millivolt indication calibration points are: ±1, ±2, ±4, ±10, ±40, ±80, ±100, ±200, ±300, ±400, ±500, ±600, ±700, ±800, ±900, ±mV range (do it twice according to the method of increasing input and decreasing input respectively). Calculate the difference AmV between the meter indication (the average value of the two readings is taken as the average value) and the corresponding input value mV.
AmV=mV indication-mV input
×100% shall not exceed the provisions of Table 1.
mV full scale
6.2.3 Verification of input current of electric meter
Put the temperature compensator of the instrument at 25℃ (or a certain middle temperature point of the temperature compensator), adjust the verification instrument so that its output signal is pH7 (or the equal potential pH value of the electric meter), record the change of the electric meter indication when the high resistance switch K is turned on or off, repeat the measurement three times, take the average value, and calculate the input current according to the following formula. 1=1ApH1·h
×10-3
JJG119—2005
Where: IApH current 1 is the average value of the change of the electric meter indication error caused by the input current measured three times, and the absolute value is taken: k---the theoretical slope of the glass electrode (see Appendix B); R--the resistance value of the series resistor, α.
6.2.4 Verification of indication error caused by input impedance of electric meter Connect the circuit according to Figure 1, turn on switch K, and short-circuit high resistance R. The temperature compensator of the instrument is placed at 25°C (or a certain middle temperature point of the temperature compensator), and the calibration instrument is adjusted to make it output a signal equivalent to the pH isopotential value + 6 pH units, and the meter indication pH, is recorded. Switch K is disconnected and high resistance R is connected, and the calibration instrument is adjusted to make its output signal pH7 (or the meter isopotential value), and the instrument is adjusted to make its indication pH7 (or the meter isopotential value). Then adjust the calibration instrument to make it output a signal equivalent to the pH isopotential value + 6 pH units, and record the meter indication pHz. Repeat the above operation three times, take the average value, and calculate the error caused by input impedance.
ApHau=（pH,-pH,）
Where: ApHal-indication error caused by meter input impedance; pH,, pH,-are the average values of meter indications respectively (4)
When the input pH isopotential value of -6 pH units is calibrated by the sampling method, the error caused by input impedance ApH is taken as the larger of ApHal and △pH'. 6.2.5 Verification of indication error caused by temperature compensator of electric meter 6.2.5.1 Verification of indication error caused by manual temperature compensator of electric meter Connect the circuit according to Figure 1, turn on switch K, and short-circuit high resistance R. Set the temperature compensator to the scale other than 25℃ (select at least 5 nominal scale points including both ends of the temperature compensator as needed), input the signal equivalent to the net pH value + 6pH unit at the temperature at each verification point, record the indication of the electric meter, repeat the measurement twice, and take the average value. Convert the difference between the average value and the nominal value of the electric meter (ApH degree) into ApHu per 3pH unit: ApHu degree = ×ApHau
6.2.5.2 Verification of indication error caused by automatic temperature compensator of electric meter (5)
Connect the circuit according to Figure 1, turn on switch K, and short-circuit high resistance R. Place the temperature probe in a constant temperature water bath, adjust the constant temperature water bath to a temperature other than 25°C (select at least 5 nominal scale points including both ends of the temperature compensator as needed), input the signal of the temperature equivalent to the pH isopotential value + 6pH units at each calibration point, and record the meter indication. Repeat the measurement twice and take the average value. Convert the difference between the average value and the nominal value of the meter (ApH degrees) into the ApH temperature per 3pH units:
ApHu degrees = ×ApHm degrees
The temperature measurement error of the temperature probe of pH (acidity) meter of different levels shall meet the requirements of Table 1. 6.2.6 Verification of the repeatability of the meter indication
Connect the circuit according to Figure 1, disconnect the switch K, connect the high resistance R, set the temperature compensator to 25°C, adjust the calibration instrument so that it inputs the signal of the pH isopotential value + 3pH units to the meter, and record the meter indication pH;. Repeat the above operation 6 times, and express the repeatability with the standard deviation of a single measurement. 5
Where: S—standard deviation of a single measurement; JJG119—2005
Z(pH,-pH)2
pH is the meter indication of the ith measurement;
pH is the average value of 6 measurements of pH,.
6.2.7 Verification of instrument indication error
When the value of the solution to be measured is within the pH range (3-10), under normal working conditions of the instrument, select 35 bath solutions from the B34B4, B6, B9 solutions specified in Table 1 of Appendix A. After the instrument is calibrated with one standard solution (for instruments with two-point calibration and dual-point calibration, two or more solutions should be used for calibration, and the pH difference between the calibration solution and the measurement solution should not exceed 3 pH units), measure another standard solution. Repeat the "calibration" and "measurement" operations three times, and take the average value as the instrument indication. The difference between this indication and the standard value of the solution at the measurement temperature is the instrument indication error ApH.
ApH=pH-pH
6.2.8 Verification of instrument indication repeatability
After the instrument is calibrated with a standard solution, measure another standard solution, repeat the "calibration" and "measurement" operations 6 times, and use the standard deviation of a single measurement to represent repeatability. The calculation formula is the same as formula (7). (This item can be combined with the verification of the instrument indication error).
6.3 Processing of verification results
6.3.1 A verification certificate shall be issued to qualified instruments. The verification certificate shall give the verification results and instrument level. Newly produced instruments must fully comply with the provisions of Table 1 to be qualified instruments. 6.3.2 For instruments in use or after repair, when the meter calibration complies with the provisions of this regulation, the meter is qualified: if the matching calibration of the original electrode of the instrument exceeds the provisions of this regulation, the calibration unit can select other qualified electrodes to re-calibrate the matching. After the replacement of the electrode, the instrument that passes the matching calibration is still a qualified instrument and a calibration certificate is issued, but the calibration result of the original electrode of the instrument should be notified to the inspection unit. 6.3.3 The instrument can be calibrated in pH or mV gear according to the user's requirements, or both gears can be calibrated, and it should be noted in the calibration certificate.
6.3.4 Instruments that are judged as unqualified according to the calibration results are allowed to be downgraded. When downgraded to the next level, it must meet the various requirements of the instrument at that level; for instruments that do not meet the requirements, a calibration result notice will be issued, and the unqualified items will be noted. 6.4 Calibration cycle
The calibration cycle is generally not more than 1 year.
Appendix A
JJG119-2005
Preparation and preservation of standard solutions
Standard solutions are divided into two levels, prepared with primary and secondary standard substances respectively. Primary standard solutions are used for the calibration of 0.001-level instruments, and secondary standard solutions are used for the calibration of other levels of instruments. A.1 Preparation of standard solutionsbZxz.net
A.1.1 There are 7 kinds of standard solutions, and their composition and the mass of standard substances required for the preparation of 11.0 solution and 1 kg of solution are listed in Table A.1.
A.1.2 Double distilled water or deionized water must be used to prepare standard solutions, and its conductivity should be less than 2×10-°S/cm. If used for 0.1-level instruments, ordinary distilled water can be used. A.1.3 The preparation method of standard solution is as follows: Bl0.05mol-kg potassium tetraoxalate solution: weigh 12.61 of potassium tetraoxalate that has been dried at (54±3)℃ for ten (4~5) hours (no need to dry if used for 0.1 grade instrument), dissolve in distilled water, and dilute to 11
in a volumetric flask at 25℃. B325 saturated potassium hydrogen tartrate solution: fill a milled glass bottle with distilled water and an excess of potassium hydrogen tartrate powder (7g/L), control the temperature at (25±3)℃, shake vigorously for (20~30) minutes, and after the solution is clarified, use the decanting method to take the clear solution for later use.
B40.05mol·kg potassium hydrogen phthalate solution: weigh 10.12g of potassium hydrogen phthalate dried at (115±5)℃ for (2~3)h (if used for 0.1 level instruments, drying is not necessary), dissolve in distilled water, and dilute to 1L in a volumetric flask at 25%.
B60.025mol·kg-disodium hydrogen phosphate and 0.025mol·kg-potassium dihydrogen phosphate mixed solution: weigh 3.533g of disodium hydrogen phosphate and 3.387g of potassium dihydrogen phosphate dried at (115±5)℃ for (2~3)h, respectively, dissolve in distilled water, and dilute to 1L in a volumetric flask at 25℃ (if used for instruments above 0.02 level, the distilled water used to prepare the solution should be boiled in advance for (15~30)min or an inert gas should be passed through to remove dissolved carbon dioxide). B70.03043mol-kg disodium hydrogen phosphate and 0.008695mol·kg potassium dihydrogen phosphate solution: Weigh 4.303g disodium hydrogen phosphate and 1.179g potassium dihydrogen phosphate that have been dried at (115±5)℃ for (2~3)h, respectively, and dissolve in distilled water. Dilute to 1L in a volumetric flask at 25℃ (If used for instruments above 0.02 level, the distilled water used to prepare the solution should be pre-boiled for (15~30)min or inert gas should be passed through to remove dissolved carbon dioxide). B90.01lmol·kg borax solution: Weigh 3.80g borax (note! Do not bake), dissolve in distilled water, and dilute to 1L in a volumetric flask at 25℃ (If used for instruments above 0.02 level, the distilled water used to prepare the solution should be pre-boiled for (15~30)min or inert gas should be passed through to remove dissolved carbon dioxide). B1225℃ saturated calcium hydroxide solution: add distilled water and excess calcium hydroxide powder (about 2g/L) to a glass ground-mouth bottle or polyethylene plastic bottle, control the temperature at (25±3)℃, shake vigorously for (20~30)min, filter quickly, and take the clear solution for use [when preparing the primary standard solution, the temperature should be controlled at (25±1)℃]. The pH values of the 7 standard solutions at (0~95)℃ are listed in Table A.2. A.2 Storage of standard solutions
B9, B12 alkaline solutions should be stored in polyethylene bottles and sealed. B3 Potassium hydrogen tartrate solution can be added with thymol to prevent mold, the dosage is about 1g per liter of solution. The standard solution is generally stored in a refrigerator at 4℃ for (2~3)7
JJG119—2005
But if turbidity, mold or precipitation is found, it cannot be used. Months,
Table A.1 Composition of standard solution
Name of standard substance
Potassium tetraoxalate
Potassium hydrogen tartrate
Potassium hydrogen phthalate
Disodium hydrogen phosphate
Potassium dihydrogen phosphate
Disodium hydrogen phosphate
Potassium dihydrogen phosphate
Calcium hydroxide
Molecular formula
KH(C,0.)22H,0
Na,HPO,
Na,HPO
Na,B,,10H,0
Ca (OH)
Mass in air.
Concentration of standard solution
Imol·kg!
Saturated at 25℃
About 0.034
Saturated at 25℃
About 0.020
Mass of standard substances required to prepare 1L of standard solution
/g
pH value of 7 standard solutions
Temperature/℃
Mass of standard substances required to prepare 1kg of standard solution
/g008695mol·kg potassium dihydrogen phosphate solution: Weigh 4.303g of disodium hydrogen phosphate and 1.179g of potassium dihydrogen phosphate that have been dried at (115±5)℃ for (2~3)h, respectively, and dissolve them in distilled water. Dilute to 1L in a volumetric flask at 25℃ (If used for instruments above level 0.02, the distilled water used to prepare the solution should be pre-boiled for (15~30)min or inert gas should be passed through to remove dissolved carbon dioxide). B90.01lmol·kg borax solution: Weigh 3.80g of borax (note! Do not bake), dissolve it in distilled water, and dilute to 1L in a volumetric flask at 25℃ (If used for instruments above level 0.02, the distilled water used to prepare the solution should be pre-boiled for (15~30)min or inert gas should be passed through to remove dissolved carbon dioxide). B1225℃ saturated calcium hydroxide solution: add distilled water and excess calcium hydroxide powder (about 2g/L) to a glass ground-mouth bottle or polyethylene plastic bottle, control the temperature at (25±3)℃, shake vigorously for (20~30)min, filter quickly, and take the clear solution for use [when preparing the primary standard solution, the temperature should be controlled at (25±1)℃]. The pH values of the 7 standard solutions at (0~95)℃ are listed in Table A.2. A.2 Storage of standard solutions
B9, B12 alkaline solutions should be stored in polyethylene bottles and sealed. B3 Potassium hydrogen tartrate solution can be added with thymol to prevent mold, the dosage is about 1g per liter of solution. The standard solution is generally stored in a refrigerator at 4℃ for (2~3)7
JJG119—2005
But if turbidity, mold or precipitation is found, it cannot be used. Months,
Table A.1 Composition of standard solution
Name of standard substance
Potassium tetraoxalate
Potassium hydrogen tartrate
Potassium hydrogen phthalate
Disodium hydrogen phosphate
Potassium dihydrogen phosphate
Disodium hydrogen phosphate
Potassium dihydrogen phosphate
Calcium hydroxide
Molecular formula
KH(C,0.)22H,0
Na,HPO,
Na,HPO
Na,B,,10H,0
Ca (OH)
Mass in air.
Concentration of standard solution
Imol·kg!
Saturated at 25℃
About 0.034
Saturated at 25℃
About 0.020
Mass of standard substances required to prepare 1L of standard solution
/g
pH value of 7 standard solutions
Temperature/℃
Mass of standard substances required to prepare 1kg of standard solution
/g008695mol·kg potassium dihydrogen phosphate solution: Weigh 4.303g of disodium hydrogen phosphate and 1.179g of potassium dihydrogen phosphate that have been dried at (115±5)℃ for (2~3)h, respectively, and dissolve them in distilled water. Dilute to 1L in a volumetric flask at 25℃ (If used for instruments above level 0.02, the distilled water used to prepare the solution should be pre-boiled for (15~30)min or inert gas should be passed through to remove dissolved carbon dioxide). B90.01lmol·kg borax solution: Weigh 3.80g of borax (note! Do not bake), dissolve it in distilled water, and dilute to 1L in a volumetric flask at 25℃ (If used for instruments above level 0.02, the distilled water used to prepare the solution should be pre-boiled for (15~30)min or inert gas should be passed through to remove dissolved carbon dioxide). B1225℃ saturated calcium hydroxide solution: add distilled water and excess calcium hydroxide powder (about 2g/L) to a glass ground-mouth bottle or polyethylene plastic bottle, control the temperature at (25±3)℃, shake vigorously for (20~30)min, filter quickly, and take the clear solution for use [when preparing the primary standard solution, the temperature should be controlled at (25±1)℃]. The pH values of the 7 standard solutions at (0~95)℃ are listed in Table A.2. A.2 Storage of standard solutions
B9, B12 alkaline solutions should be stored in polyethylene bottles and sealed. B3 Potassium hydrogen tartrate solution can be added with thymol to prevent mold, the dosage is about 1g per liter of solution. The standard solution is generally stored in a refrigerator at 4℃ for (2~3)7
JJG119—2005
But if turbidity, mold or precipitation is found, it cannot be used. Months,
Table A.1 Composition of standard solution
Name of standard substance
Potassium tetraoxalate
Potassium hydrogen tartrate
Potassium hydrogen phthalate
Disodium hydrogen phosphate
Potassium dihydrogen phosphate
Disodium hydrogen phosphate
Potassium dihydrogen phosphate
Calcium hydroxide
Molecular formula
KH(C,0.)22H,0
Na,HPO,
Na,HPO
Na,B,,10H,0
Ca (OH)
Mass in air.
Concentration of standard solution
Imol·kg!
Saturated at 25℃
About 0.034
Saturated at 25℃
About 0.020
Mass of standard substances required to prepare 1L of standard solution
/g
pH value of 7 standard solutions
Temperature/℃
Mass of standard substances required to prepare 1kg of standard solution
/g
Tip: This standard content only shows part of the intercepted content of the complete standard. If you need the complete standard, please go to the top to download the complete standard document for free.